@c Copyright (C) 1993, 1994, 1995 John W. Eaton
@c This is part of the Octave manual.
@c For copying conditions, see the file gpl.texi.

@node Input and Output, Special Matrices, Image Processing, Top
@chapter Input and Output

There are two distinct classes of input and output functions.  The first
set are modeled after the functions available in @sc{Matlab}.  The
second set are modeled after the standard I/O library used by the C
programming language.  The C-style I/O functions offer more flexibility
and control over the output, but are not quite as easy to use as the
simpler @sc{Matlab}-style I/O functions.

When running interactively, Octave normally sends any output intended
for your terminal that is more than one screen long to a paging program,
such as @code{less} or @code{more}.  This avoids the problem of having
a large volume of output stream by before you can read it.  With
@code{less} (and some versions of @code{more}) it also allows you to
scan forward and backward, and search for specific items.

@findex fflush
No output is displayed by the pager until just before Octave is ready to
print the top level prompt, or read from the standard input using the
@code{fscanf} or @code{scanf} functions.  This means that there may be
some delay before any output appears on your screen if you have asked
Octave to perform a significant amount of work with a single command
statement.  The function @code{fflush} may be used to force output to be
sent to the pager immediately.  @xref{C-Style I/O Functions}.

You can select the program to run as the pager by setting the variable
@code{PAGER}, and you can turn paging off by setting the value of the
variable @code{page_screen_output} to the string @samp{"false"}.
@xref{User Preferences}.

@menu
* Basic Input and Output::      
* C-Style I/O Functions::       
@end menu

@node Basic Input and Output, C-Style I/O Functions, Input and Output, Input and Output
@section Basic Input and Output

Since Octave normally prints the value of an expression as soon as it
has been evaluated, the simplest of all I/O functions is a simple
expression.  For example, the following expression will display the
value of pi

@example
octave:13> pi
pi = 3.1416
@end example

This works well as long as it is acceptable to have the name of the
variable (or @samp{ans}) printed along with the value.  To print the
value of a variable without printing its name, use the function
@code{disp}.  For example, the following expression

@findex disp

@example
disp ("The value of pi is:"), disp (pi)
@end example

@noindent
will print

@example
The value of pi is:
3.1416
@end example

@noindent
Note that the output from @code{disp} always ends with a newline.

A simple way to control the output format is with the @code{format}
statement.  For example, to print more digits for pi you can use the
command

@example
format long
@end example

@noindent
Then the expression above will print

@example
The value of pi is:
3.14159265358979
@end example

@findex format

Here is a summary of the options for @code{format}:

@table @code
@item short
This is the default format.  Octave will try to print numbers with at
least 5 significant figures within a field that is a maximum of 10
characters wide.

If Octave is unable to format a matrix so that columns line up on the
decimal point and all the numbers fit within the maximum field width,
it switches to an @samp{e} format.

@item long
Octave will try to print numbers with at least 15 significant figures
within a field that is a maximum of 24 characters wide.

As will the @samp{short} format, Octave will switch to an @samp{e}
format if it is unable to format a matrix so that columns line up on the
decimal point and all the numbers fit within the maximum field width.

@item long e
@itemx short e
The same as @samp{format long} or @samp{format short} but always display
output with an @samp{e} format.  For example, with the @samp{short e}
format, pi is displayed as

@example
 3.14e+00
@end example

@item long E
@itemx short E
The same as @samp{format long e} or @samp{format short e} but always
display output with an uppercase @samp{E} format.  For example, with
the @samp{long E} format, pi is displayed as

@example
 3.14159265358979E+00
@end example

@item free
@itemx none
Print output in free format, without trying to line up columns of
matrices on the decimal point.  This also causes complex numbers to be
formatted like this @samp{(0.604194, 0.607088)} instead of like this
@samp{0.60419 + 0.60709i}.

@item bank
Print in a fixed format with two places to the right of the decimal
point.

@item +
Print a @samp{+} symbol for nonzero matrix elements and a space for zero
matrix elements.  This format can be very useful for examining the
structure of a large matrix.
@end table

The @code{input} function may be used for prompting the user for a
value and storing the result in a variable.  For example,
@findex input

@example
input ("Pick a number, any number! ")
@end example

@noindent
prints the prompt

@example
Pick a number, any number!
@end example

@noindent
and waits for the user to enter a value.  The string entered by the user
is evaluated as an expression, so it may be a literal constant, a
variable name, or any other valid expression.

Currently, @code{input} only returns one value, regardless of the number
of values produced by the evaluation of the expression.

If you are only interested in getting a literal string value, you can
call @code{input} with the character string @samp{s} as the second
argument.  This tells Octave to return the string entered by the user
directly, without evaluating it first.

Because there may be output waiting to be displayed by the pager, it is
a good idea to always call @code{fflush (stdout)} before calling
@code{input}.  This will ensure that all pending output is written to
the screen before your prompt.  @xref{C-Style I/O Functions}.

@findex keyboard

The second input function, @code{keyboard}, is normally used for simple
debugging.  Using @code{keyboard}, it is possible to examine the values
of variables within a function, and to assign newassign new variables
Like @code{input}, it prompts the user for input, but no value is
returned, and it continues to prompt for input until the user types
@samp{quit}, or @samp{exit}.

If @code{keyboard} is invoked without any arguments, a default prompt of
@samp{debug> } is used.

For both of these functions, the normal command line history and editing
functions are available at the prompt.

@findex save
To save variables in a file, use the @code{save} command.  For example,
the command

@example
save data a b c
@end example

@noindent
saves the variables @samp{a}, @samp{b}, and @samp{c} in the file
@file{data}.

@vindex default_save_format
The @var{save} command can read files in Octave's text and binary
formats as well as @sc{Matlab}'s binary format.  You can specify the default
format with the built-in variable @var{default_save_format} using one of
the following values: @code{"binary"} or @code{"mat-binary"}.  The
initial default save format is Octave's text format.

@vindex save_precision
You can use the built-in variable @code{save_precision} to specify the
number of digits to keep when saving data in text format.

The list of variables to save may include wildcard patterns containing
the following special characters:
@table @code
@item ?
Match any single character.

@item *
Match zero or more characters.

@item [ @var{list} ]
Match the list of characters specified by @var{list}.  If the first
character is @code{!} or @code{^}, match all characters except those
specified by @var{list}.  For example, the pattern @samp{[a-zA-Z]} will
match all lower and upper case alphabetic characters. 
@end table

The following options may be specified for @code{save}.

@ftable @code
@item -ascii
Save the data in Octave's text data format.  Using this flag overrides
the value of the built-in variable @code{default_save_format}.

@item -binary
Save the data in Octave's binary data format.  Using this flag overrides
the value of the built-in variable @code{default_save_format}.

@item -float-binary
Save the data in Octave's binary data format but only using single
precision.  Using this flag overrides the value of the built-in variable
@code{default_save_precision}.  You should use this format only if you
know that all the values to be saved can be represented in single
precision.

@item -mat-binary
Save the data in @sc{Matlab}'s binary data format.  Using this flag overrides
the value of the built-in variable @code{default_save_format}.

@item -save-builtins
Force Octave to save the values of built-in variables too.  By default,
Octave does not save built-in variables.
@end ftable

@findex load
Saving global variables also saves the global status of the variable, so
that if it is restored at a later time using @samp{load}, it will be
restored as a global variable.

To restore the values from a file, use the @code{load} command.  For
example, to restore the variables saved in the file @file{data}, use the
command

@example
load data
@end example

Octave will refuse to overwrite existing variables unless you use the
option @samp{-force}.

If a variable that is not marked as global is loaded from a file when a
global symbol with the same name already exists, it is loaded in the
global symbol table.  Also, if a variable is marked as global in a file
and a local symbol exists, the local symbol is moved to the global
symbol table and given the value from the file.  Since it seems that
both of these cases are likely to be the result of some sort of error,
they will generate warnings.

As with @code{save}, you may specify a list of variables and @code{load}
will only extract those variables with names that match.

The @code{load} command can read data stored in Octave's text and
binary formats, and @sc{Matlab}'s binary format.  It will automatically
detect the type of file and do conversion from different floating point
formats (currently only IEEE big and little endian, though other formats
may added in the future).

The following options may be specified for @code{save}.

@ftable @code
@item -force
Force variables currently in memory to be overwritten by variables with
the same name found in the file.

@item -ascii
Force Octave to assume the file is in Octave's text format.

@item -binary
Force Octave to assume the file is in Octave's binary format.

@item -mat-binary
Force Octave to assume the file is in @sc{Matlab}'s binary format.
@end ftable

@node C-Style I/O Functions,  , Basic Input and Output, Input and Output
@section C-Style I/O Functions

The C-style input and output functions provide most of the functionality
of the C programming language's standard I/O library.  The argument
lists for some of the input functions are slightly different, however,
because Octave has no way of passing arguments by reference.

In the following, @var{file} refers either to an integer file number
(as returned by @file{fopen}) or a file name.

There are three files that are always available:

@table @code
@item stdin
The standard input stream (file number 0).  When Octave is used
interactively, this is filtered through the command line editing
functions.

@item stdout
The standard output stream (file number 1).  Data written to the
standard output is normally filtered through the pager.

@item stderr
The standard error stream (file number 2).  Even if paging is turned on,
the standard error is not sent to the pager.  It is useful for error
messages and prompts.
@end table

You should always use the symbolic names given in the table above,
rather than referring to these files by number, since it will make
your programs clearer.

@menu
* Opening and Closing Files::   
* Formatted Output::            
* Output Conversion Syntax::    
* Table of Output Conversions::  
* Integer Conversions::         
* Floating-Point Conversions::  
* Other Output Conversions::    
* Formatted Input::             
* Input Conversion Syntax::     
* Table of Input Conversions::  
* Numeric Input Conversions::   
* String Input Conversions::    
* Binary I/O::                  
* Other I/O Functions::         
@end menu

@node Opening and Closing Files, Formatted Output, C-Style I/O Functions, C-Style I/O Functions
@subsection Opening and Closing Files

@findex fopen
To open a file, use the function @code{fopen (name, mode)}.  It returns
an integer value that may be used to refer to the file later.  The
second argument is a one or two character string that specifies whether
the file is to be opened for reading, writing, or both.

For example,

@example
myfile = fopen ("splat.dat", "r");
@end example

@noindent
opens the file @file{splat.dat} for reading.  Opening a file that is
already open has no effect.

@c XXX FIXME XXX -- There still seem to be some minor bugs with fopen.
@c You should probably get an error if you try to reopen a file with a
@c different mode.  You should probably get a warning if the file is
@c already open, etc.


The possible values @samp{mode} may have are

@table @asis
@item @samp{r}
Open a file for reading.

@item @samp{w}
Open a file for writing.  The previous contents are discared.

@item @samp{a}
Open or create a file for writing at the end of the file.

@item @samp{r+}
Open an existing file for reading and writing.

@item @samp{w+}
Open a file for reading or writing.  The previous contents are
discared.

@item @samp{a+}
Open or create a file for reading or writing at the end of the
file.
@end table

@findex fclose
To close a file once you are finished with it, use the function
@code{fclose (@var{file})}.  If an error is encountered while trying to close
the file, an error message is printed and @code{fclose} returns 0.
Otherwise, it returns 1.

@node Formatted Output, Output Conversion Syntax, Opening and Closing Files, C-Style I/O Functions
@subsection Formatted Output

This section describes how to call @code{printf} and related functions.

The following functions are available for formatted output.  They are
modelled after the C language functions of the same name.

@ftable @code
@item printf (@var{template}, ...)
The @code{printf} function prints the optional arguments under the
control of the template string @var{template} to the stream
@code{stdout}.

@item fprintf (@var{file}, @var{template}, ...)
This function is just like @code{printf}, except that the output is
written to the stream @var{file} instead of @code{stdout}.

@item sprintf (@var{template}, ...)
This is like @code{printf}, except that the output is written to a
string.  Unlike the C library function, which requires you to provide a
suitably sized string as an argument, Octave's @code{sprintf} function
returns the string, automatically sized to hold all of the items
converted.
@end ftable

The @code{printf} function can be used to print any number of arguments.
The template string argument you supply in a call provides
information not only about the number of additional arguments, but also
about their types and what style should be used for printing them.

Ordinary characters in the template string are simply written to the
output stream as-is, while @dfn{conversion specifications} introduced by
a @samp{%} character in the template cause subsequent arguments to be
formatted and written to the output stream.  For example,
@cindex conversion specifications (@code{printf})

@smallexample
pct = 37;
filename = "foo.txt";
printf ("Processing of `%s' is %d%% finished.\nPlease be patient.\n",
        filename, pct);
@end smallexample

@noindent
produces output like

@smallexample
Processing of `foo.txt' is 37% finished.
Please be patient.
@end smallexample

This example shows the use of the @samp{%d} conversion to specify that a
scalar argument should be printed in decimal notation, the @samp{%s}
conversion to specify printing of a string argument, and the @samp{%%}
conversion to print a literal @samp{%} character.

There are also conversions for printing an integer argument as an
unsigned value in octal, decimal, or hexadecimal radix (@samp{%o},
@samp{%u}, or @samp{%x}, respectively); or as a character value
(@samp{%c}).

Floating-point numbers can be printed in normal, fixed-point notation
using the @samp{%f} conversion or in exponential notation using the
@samp{%e} conversion.  The @samp{%g} conversion uses either @samp{%e}
or @samp{%f} format, depending on what is more appropriate for the
magnitude of the particular number.

You can control formatting more precisely by writing @dfn{modifiers}
between the @samp{%} and the character that indicates which conversion
to apply.  These slightly alter the ordinary behavior of the conversion.
For example, most conversion specifications permit you to specify a
minimum field width and a flag indicating whether you want the result
left- or right-justified within the field.

The specific flags and modifiers that are permitted and their
interpretation vary depending on the particular conversion.  They're all
described in more detail in the following sections.

@node Output Conversion Syntax, Table of Output Conversions, Formatted Output, C-Style I/O Functions
@subsection Output Conversion Syntax

This section provides details about the precise syntax of conversion
specifications that can appear in a @code{printf} template
string.

Characters in the template string that are not part of a
conversion specification are printed as-is to the output stream.

The conversion specifications in a @code{printf} template string have
the general form:

@smallexample
% @var{flags} @var{width} @r{[} . @var{precision} @r{]} @var{type} @var{conversion}
@end smallexample

For example, in the conversion specifier @samp{%-10.8ld}, the @samp{-}
is a flag, @samp{10} specifies the field width, the precision is
@samp{8}, the letter @samp{l} is a type modifier, and @samp{d} specifies
the conversion style.  (This particular type specifier says to print a
numeric argument in decimal notation, with a minimum of 8 digits
left-justified in a field at least 10 characters wide.)

In more detail, output conversion specifications consist of an
initial @samp{%} character followed in sequence by:

@itemize @bullet
@item 
Zero or more @dfn{flag characters} that modify the normal behavior of
the conversion specification.
@cindex flag character (@code{printf})

@item 
An optional decimal integer specifying the @dfn{minimum field width}.
If the normal conversion produces fewer characters than this, the field
is padded with spaces to the specified width.  This is a @emph{minimum}
value; if the normal conversion produces more characters than this, the
field is @emph{not} truncated.  Normally, the output is right-justified
within the field.
@cindex minimum field width (@code{printf})

You can also specify a field width of @samp{*}.  This means that the
next argument in the argument list (before the actual value to be
printed) is used as the field width.  The value is rounded to the
nearest integer.  If the value is negative, this means to set the
@samp{-} flag (see below) and to use the absolute value as the field
width.

@item 
An optional @dfn{precision} to specify the number of digits to be
written for the numeric conversions.  If the precision is specified, it
consists of a period (@samp{.}) followed optionally by a decimal integer
(which defaults to zero if omitted).
@cindex precision (@code{printf})

You can also specify a precision of @samp{*}.  This means that the next
argument in the argument list (before the actual value to be printed) is
used as the precision.  The value must be an integer, and is ignored
if it is negative.

@item
An optional @dfn{type modifier character}.  This character is ignored by
Octave's @code{printf} function, but is recognized to provide
compatibility with the C language @code{printf}.

@item
A character that specifies the conversion to be applied.
@end itemize

The exact options that are permitted and how they are interpreted vary 
between the different conversion specifiers.  See the descriptions of the
individual conversions for information about the particular options that
they use.

@node Table of Output Conversions, Integer Conversions, Output Conversion Syntax, C-Style I/O Functions
@subsection Table of Output Conversions
@cindex output conversions, for @code{printf}

Here is a table summarizing what all the different conversions do:

@table @asis
@item @samp{%d}, @samp{%i}
Print an integer as a signed decimal number.  @xref{Integer
Conversions}, for details.  @samp{%d} and @samp{%i} are synonymous for
output, but are different when used with @code{scanf} for input
(@pxref{Table of Input Conversions}).

@item @samp{%o}
Print an integer as an unsigned octal number.  @xref{Integer
Conversions}, for details.

@item @samp{%u}
Print an integer as an unsigned decimal number.  @xref{Integer
Conversions}, for details.

@item @samp{%x}, @samp{%X}
Print an integer as an unsigned hexadecimal number.  @samp{%x} uses
lower-case letters and @samp{%X} uses upper-case.  @xref{Integer
Conversions}, for details.

@item @samp{%f}
Print a floating-point number in normal (fixed-point) notation.
@xref{Floating-Point Conversions}, for details.

@item @samp{%e}, @samp{%E}
Print a floating-point number in exponential notation.  @samp{%e} uses
lower-case letters and @samp{%E} uses upper-case.  @xref{Floating-Point
Conversions}, for details.

@item @samp{%g}, @samp{%G}
Print a floating-point number in either normal or exponential notation,
whichever is more appropriate for its magnitude.  @samp{%g} uses
lower-case letters and @samp{%G} uses upper-case.  @xref{Floating-Point
Conversions}, for details.

@item @samp{%c}
Print a single character.  @xref{Other Output Conversions}.

@item @samp{%s}
Print a string.  @xref{Other Output Conversions}.

@item @samp{%%}
Print a literal @samp{%} character.  @xref{Other Output Conversions}.
@end table

If the syntax of a conversion specification is invalid, unpredictable
things will happen, so don't do this.  If there aren't enough function
arguments provided to supply values for all the conversion
specifications in the template string, or if the arguments are not of
the correct types, the results are unpredictable.  If you supply more
arguments than conversion specifications, the extra argument values are
simply ignored; this is sometimes useful.

@node Integer Conversions, Floating-Point Conversions, Table of Output Conversions, C-Style I/O Functions
@subsection Integer Conversions

This section describes the options for the @samp{%d}, @samp{%i},
@samp{%o}, @samp{%u}, @samp{%x}, and @samp{%X} conversion
specifications.  These conversions print integers in various formats.

The @samp{%d} and @samp{%i} conversion specifications both print an
numeric argument as a signed decimal number; while @samp{%o},
@samp{%u}, and @samp{%x} print the argument as an unsigned octal,
decimal, or hexadecimal number (respectively).  The @samp{%X} conversion
specification is just like @samp{%x} except that it uses the characters
@samp{ABCDEF} as digits instead of @samp{abcdef}.

The following flags are meaningful:

@table @asis
@item @samp{-}
Left-justify the result in the field (instead of the normal
right-justification).

@item @samp{+}
For the signed @samp{%d} and @samp{%i} conversions, print a
plus sign if the value is positive.

@item @samp{ }
For the signed @samp{%d} and @samp{%i} conversions, if the result
doesn't start with a plus or minus sign, prefix it with a space
character instead.  Since the @samp{+} flag ensures that the result
includes a sign, this flag is ignored if you supply both of them.

@item @samp{#}
For the @samp{%o} conversion, this forces the leading digit to be
@samp{0}, as if by increasing the precision.  For @samp{%x} or
@samp{%X}, this prefixes a leading @samp{0x} or @samp{0X} (respectively)
to the result.  This doesn't do anything useful for the @samp{%d},
@samp{%i}, or @samp{%u} conversions.

@item @samp{0}
Pad the field with zeros instead of spaces.  The zeros are placed after
any indication of sign or base.  This flag is ignored if the @samp{-}
flag is also specified, or if a precision is specified.
@end table

If a precision is supplied, it specifies the minimum number of digits to
appear; leading zeros are produced if necessary.  If you don't specify a
precision, the number is printed with as many digits as it needs.  If
you convert a value of zero with an explicit precision of zero, then no
characters at all are produced.

@node Floating-Point Conversions, Other Output Conversions, Integer Conversions, C-Style I/O Functions
@subsection Floating-Point Conversions

This section discusses the conversion specifications for floating-point
numbers: the @samp{%f}, @samp{%e}, @samp{%E}, @samp{%g}, and @samp{%G}
conversions.

The @samp{%f} conversion prints its argument in fixed-point notation,
producing output of the form
@w{[@code{-}]@var{ddd}@code{.}@var{ddd}},
where the number of digits following the decimal point is controlled
by the precision you specify.

The @samp{%e} conversion prints its argument in exponential notation,
producing output of the form
@w{[@code{-}]@var{d}@code{.}@var{ddd}@code{e}[@code{+}|@code{-}]@var{dd}}.
Again, the number of digits following the decimal point is controlled by
the precision.  The exponent always contains at least two digits.  The
@samp{%E} conversion is similar but the exponent is marked with the letter
@samp{E} instead of @samp{e}.

The @samp{%g} and @samp{%G} conversions print the argument in the style
of @samp{%e} or @samp{%E} (respectively) if the exponent would be less
than -4 or greater than or equal to the precision; otherwise they use the
@samp{%f} style.  Trailing zeros are removed from the fractional portion
of the result and a decimal-point character appears only if it is
followed by a digit.

The following flags can be used to modify the behavior:

@c Not @samp so we can have ` ' as an item.
@table @asis
@item @samp{-}
Left-justify the result in the field.  Normally the result is
right-justified.

@item @samp{+}
Always include a plus or minus sign in the result.

@item @samp{ }
If the result doesn't start with a plus or minus sign, prefix it with a
space instead.  Since the @samp{+} flag ensures that the result includes
a sign, this flag is ignored if you supply both of them.

@item @samp{#}
Specifies that the result should always include a decimal point, even
if no digits follow it.  For the @samp{%g} and @samp{%G} conversions,
this also forces trailing zeros after the decimal point to be left
in place where they would otherwise be removed.

@item @samp{0}
Pad the field with zeros instead of spaces; the zeros are placed
after any sign.  This flag is ignored if the @samp{-} flag is also
specified.
@end table

The precision specifies how many digits follow the decimal-point
character for the @samp{%f}, @samp{%e}, and @samp{%E} conversions.  For
these conversions, the default precision is @code{6}.  If the precision
is explicitly @code{0}, this suppresses the decimal point character
entirely.  For the @samp{%g} and @samp{%G} conversions, the precision
specifies how many significant digits to print.  Significant digits are
the first digit before the decimal point, and all the digits after it.
If the precision is @code{0} or not specified for @samp{%g} or
@samp{%G}, it is treated like a value of @code{1}.  If the value being
printed cannot be expressed precisely in the specified number of digits,
the value is rounded to the nearest number that fits.

@node Other Output Conversions, Formatted Input, Floating-Point Conversions, C-Style I/O Functions
@subsection Other Output Conversions

This section describes miscellaneous conversions for @code{printf}.

The @samp{%c} conversion prints a single character.  The @samp{-} 
flag can be used to specify left-justification in the field, but no
other flags are defined, and no precision or type modifier can be given.
For example:

@smallexample
printf ("%c%c%c%c%c", "h", "e", "l", "l", "o");
@end smallexample

@noindent
prints @samp{hello}.

The @samp{%s} conversion prints a string.  The corresponding argument
must be a string.  A precision can be specified to indicate the maximum
number of characters to write; otherwise characters in the string up to
but not including the terminating null character are written to the
output stream.  The @samp{-} flag can be used to specify
left-justification in the field, but no other flags or type modifiers
are defined for this conversion.  For example:

@smallexample
printf ("%3s%-6s", "no", "where");
@end smallexample

@noindent
prints @samp{ nowhere }.

@node Formatted Input, Input Conversion Syntax, Other Output Conversions, C-Style I/O Functions
@subsection Formatted Input

Here are the descriptions of the functions for performing formatted
input.

@ftable @code
@item scanf (@var{template})
The @code{scanf} function reads formatted input from the stream
@code{stdin} under the control of the template string @var{template}.
The resulting values are returned.

@item fscanf (@var{file}, @var{template})
This function is just like @code{scanf}, except that the input is read
from the stream @var{file} instead of @code{stdin}.

@item sscanf (@var{string}, @var{template})
This is like @code{scanf}, except that the characters are taken from the
string @var{string} instead of from a stream.  Reaching the end of the
string is treated as an end-of-file condition.
@end ftable

Calls to @code{scanf} are superficially similar to calls to
@code{printf} in that arbitrary arguments are read under the control of
a template string.  While the syntax of the conversion specifications in
the template is very similar to that for @code{printf}, the
interpretation of the template is oriented more towards free-format
input and simple pattern matching, rather than fixed-field formatting.
For example, most @code{scanf} conversions skip over any amount of
``white space'' (including spaces, tabs, and newlines) in the input
file, and there is no concept of precision for the numeric input
conversions as there is for the corresponding output conversions.
Ordinarily, non-whitespace characters in the template are expected to
match characters in the input stream exactly.
@cindex conversion specifications (@code{scanf})

When a @dfn{matching failure} occurs, @code{scanf} returns immediately,
leaving the first non-matching character as the next character to be
read from the stream, and @code{scanf} returns all the items that were
successfully converted.
@cindex matching failure, in @code{scanf}

The formatted input functions are not used as frequently as the
formatted output functions.  Partly, this is because it takes some care
to use them properly.  Another reason is that it is difficult to recover
from a matching error.

@node Input Conversion Syntax, Table of Input Conversions, Formatted Input, C-Style I/O Functions
@subsection Input Conversion Syntax

A @code{scanf} template string is a string that contains ordinary
multibyte characters interspersed with conversion specifications that
start with @samp{%}.

Any whitespace character in the template causes any number of whitespace
characters in the input stream to be read and discarded.  The whitespace
characters that are matched need not be exactly the same whitespace
characters that appear in the template string.  For example, write
@samp{ , } in the template to recognize a comma with optional whitespace
before and after.

Other characters in the template string that are not part of conversion
specifications must match characters in the input stream exactly; if
this is not the case, a matching failure occurs.

The conversion specifications in a @code{scanf} template string
have the general form:

@smallexample
% @var{flags} @var{width} @var{type} @var{conversion}
@end smallexample

In more detail, an input conversion specification consists of an initial
@samp{%} character followed in sequence by:

@itemize @bullet
@item
An optional @dfn{flag character} @samp{*}, which says to ignore the text
read for this specification.  When @code{scanf} finds a conversion
specification that uses this flag, it reads input as directed by the
rest of the conversion specification, but it discards this input, does
not use a pointer argument, and does not increment the count of
successful assignments.
@cindex flag character (@code{scanf})

@item
An optional decimal integer that specifies the @dfn{maximum field
width}.  Reading of characters from the input stream stops either when
this maximum is reached or when a non-matching character is found,
whichever happens first.  Most conversions discard initial whitespace
characters (those that don't are explicitly documented), and these
discarded characters don't count towards the maximum field width.
@cindex maximum field width (@code{scanf})

@item
An optional type modifier character.  This character is ignored by
Octave's @code{scanf} function, but is recognized to provide
compatibility with the C language @code{scanf}.

@item
A character that specifies the conversion to be applied.
@end itemize

The exact options that are permitted and how they are interpreted vary 
between the different conversion specifiers.  See the descriptions of the
individual conversions for information about the particular options that
they allow.

@node Table of Input Conversions, Numeric Input Conversions, Input Conversion Syntax, C-Style I/O Functions
@subsection Table of Input Conversions
@cindex input conversions, for @code{scanf}

Here is a table that summarizes the various conversion specifications:

@table @asis
@item @samp{%d}
Matches an optionally signed integer written in decimal.  @xref{Numeric
Input Conversions}.

@item @samp{%i}
Matches an optionally signed integer in any of the formats that the C
language defines for specifying an integer constant.  @xref{Numeric
Input Conversions}.

@item @samp{%o}
Matches an unsigned integer written in octal radix.
@xref{Numeric Input Conversions}.

@item @samp{%u}
Matches an unsigned integer written in decimal radix.
@xref{Numeric Input Conversions}.

@item @samp{%x}, @samp{%X}
Matches an unsigned integer written in hexadecimal radix.
@xref{Numeric Input Conversions}.

@item @samp{%e}, @samp{%f}, @samp{%g}, @samp{%E}, @samp{%G}
Matches an optionally signed floating-point number.  @xref{Numeric Input
Conversions}.

@item @samp{%s}
Matches a string containing only non-whitespace characters.
@xref{String Input Conversions}.

@item @samp{%c}
Matches a string of one or more characters; the number of characters
read is controlled by the maximum field width given for the conversion.
@xref{String Input Conversions}.

@item @samp{%%}
This matches a literal @samp{%} character in the input stream.  No
corresponding argument is used.
@end table

If the syntax of a conversion specification is invalid, the behavior is
undefined.  If there aren't enough function arguments provided to supply
addresses for all the conversion specifications in the template strings
that perform assignments, or if the arguments are not of the correct
types, the behavior is also undefined.  On the other hand, extra
arguments are simply ignored.

@node Numeric Input Conversions, String Input Conversions, Table of Input Conversions, C-Style I/O Functions
@subsection Numeric Input Conversions

This section describes the @code{scanf} conversions for reading numeric
values.

The @samp{%d} conversion matches an optionally signed integer in decimal
radix.

The @samp{%i} conversion matches an optionally signed integer in any of
the formats that the C language defines for specifying an integer
constant.

For example, any of the strings @samp{10}, @samp{0xa}, or @samp{012}
could be read in as integers under the @samp{%i} conversion.  Each of
these specifies a number with decimal value @code{10}.

The @samp{%o}, @samp{%u}, and @samp{%x} conversions match unsigned
integers in octal, decimal, and hexadecimal radices, respectively.

The @samp{%X} conversion is identical to the @samp{%x} conversion.  They
both permit either uppercase or lowercase letters to be used as digits.

Unlike the C language @code{scanf}, Octave ignores the @samp{h},
@samp{l}, and @samp{L} modifiers.

@node String Input Conversions, Binary I/O, Numeric Input Conversions, C-Style I/O Functions
@subsection String Input Conversions

This section describes the @code{scanf} input conversions for reading
string and character values: @samp{%s} and @samp{%c}.  

The @samp{%c} conversion is the simplest: it matches a fixed number of
characters, always.  The maximum field with says how many characters to
read; if you don't specify the maximum, the default is 1.  This
conversion does not skip over initial whitespace characters.  It reads
precisely the next @var{n} characters, and fails if it cannot get that
many.

The @samp{%s} conversion matches a string of non-whitespace characters.
It skips and discards initial whitespace, but stops when it encounters
more whitespace after having read something.

For example, reading the input:

@smallexample
 hello, world
@end smallexample

@noindent
with the conversion @samp{%10c} produces @code{" hello, wo"}, but
reading the same input with the conversion @samp{%10s} produces
@code{"hello,"}.

@node Binary I/O, Other I/O Functions, String Input Conversions, C-Style I/O Functions
@subsection Binary I/O

Octave has to C-style functions for reading and writing binary data.
They are @code{fread} and @code{fwrite} and are patterned after the
standard C functions with the same names.

@ftable @code
@item fread (@var{file}, @var{size}, @var{precision})
This function reads data in binary form of type @var{precision} from the
specified @var{file}, which may be either a file name, or a file number
as returned from @code{fopen}.

The argument @var{size} specifies the size of the matrix to return.  It
may be a scalar or a two-element vector.  If it is a scalar,
@code{fread} returns a column vector of the specified length.  If it is
a two-element vector, it specifies the number of rows and columns of the
result matrix, and @code{fread} fills the elements of the matrix in
column-major order.

The argument @var{precision} is a string specifying the type of data to
read and may be one of @code{"char"}, @code{"schar"}, @code{"short"},
@code{"int"}, @code{"long"}, @code{"float"}, @code{"double"},
@code{"uchar"}, @code{"ushort"}, @code{"uint"}, or @code{"ulong"}.  The
default precision is @code{"uchar"}.

The @code{fread} function returns two values, @code{data}, which is the
data read from the file, and @code{count}, which is the number of
elements read.

@item fwrite (@var{file}, @var{data}, @var{precision})
This function writes data in binary form of type @var{precision} to the
specified @var{file}, which may be either a file name, or a file number
as returned from @code{fopen}.

The argument @var{data} is a matrix of values that are to be written to
the file.  The values are extracted in column-major order.

The argument @var{precision} is a string specifying the type of data to
read and may be one of @code{"char"}, @code{"schar"}, @code{"short"},
@code{"int"}, @code{"long"}, @code{"float"}, @code{"double"},
@code{"uchar"}, @code{"ushort"}, @code{"uint"}, or @code{"ulong"}.  The
default precision is @code{"uchar"}.

The @code{fwrite} function returns the number of elements written.

The behavior of @code{fwrite} is undefined if the values in @var{data}
are too large to fit in the specified precision.
@end ftable

@node Other I/O Functions,  , Binary I/O, C-Style I/O Functions
@subsection Other I/O Functions

@findex fgets
@example
fgets (@var{file}, len)
@end example
Read @samp{len} characters from a file.

To flush output to a stream, use the function @code{fflush (@var{file})}.
This is useful for ensuring that all pending output makes it to the
screen before some other event occurs.  For example, it is always a good
idea to flush the standard output stream before calling @code{input}.

Three functions are available for setting and determining the position of
the file pointer for a given file.

@findex ftell
The position of the file pointer (as the number of characters from the
beginning of the file) can be obtained using the the function
@code{ftell (@var{file})}.

@findex fseek
To set the file pointer to any location within the file, use the
function @code{fseek (@var{file}, offset, origin)}.  The pointer is placed
@code{offset} characters from the @code{origin}, which may be one of the
predefined variables @code{SEEK_CUR} (current position), @code{SEEK_SET}
(beginning), or @code{SEEK_END} (end of file). If @code{origin} is
omitted, @code{SEEK_SET} is assumed.  The offset must be zero, or a
value returned by @code{ftell} (in which case @code{origin} must be
@code{SEEK_SET}.  @xref{Predefined Constants}.

@findex frewind
The function @code{frewind (@var{file})} moves the file pointer to the
beginning of a file, returning 1 for success, and 0 if an error was
encountered.  It is equivalent to @code{fseek (@var{file}, 0, SEEK_SET)}.

The following example stores the current file position in the variable
@samp{marker}, moves the pointer to the beginning of the file, reads
four characters, and then returns to the original position.

@example
marker = ftell (myfile);
frewind (myfile);
fourch = fgets (myfile, 4);
fseek (myfile, marker, SEEK_SET);
@end example

@findex feof
The function @code{feof (@var{file})} allows you to find out if an
end-of-file condition has been encountered for a given file.  Note that
it will only return 1 if the end of the file has already been
encountered, not if the next read operation will result in an
end-of-file condition.

@findex ferror
Similarly, the function @code{ferror (@var{file})} allows you to find
out if an error condition has been encountered for a given file.  Note
that it will only return 1 if an error has already been encountered, not
if the next operation will result in an error condition.

@findex kbhit
The function @code{kbhit} may be usd to read a single keystroke from the
keyboard.  For example,

@example
x = kbhit ();
@end example

@noindent
will set @var{x} to the next character typed at the keyboard, without
requiring a carriage return to be typed.

@findex freport
Finally, it is often useful to know exactly which files have been
opened, and whether they are open for reading, writing, or both.  The
command @code{freport} prints this information for all open files.  For
example,

@example
@group
octave:13> freport

 number  mode  name

      0     r  stdin
      1     w  stdout
      2     w  stderr
      3     r  myfile
@end group
@end example
