Chapter 15: Tables
15.7. Blank entries

We are allowed to leave certain entries blank (perhaps to be filled in later, perhaps not) by writing "--" instead of the relevant value:

Table 2.1 - Selected Elements
Element   Symbol   Atomic number   Atomic weight   
"Hydrogen"   "H"   1   1   
"Iron"   "Fe"   --   56   
"Zinc"   --   30   65   
"Uranium"   "U"   92   238   

In effect, blank entries don't exist. "--" is not a value, but only a hole where a value might be. It can be useful to check for this:

if there is (a table entry):

This condition is true if the entry referred to exists, that is, that is, the space for it in the table is not blank. Examples:

if there is a symbol corresponding to an atomic number of 30 in the Table of Standard Elements ...
if there is an atomic number in row 2 of the Table of Standard Elements ...

if there is no (a table entry):

This condition is true if the entry referred to does not exist, that is, the space for it in the table is blank. Examples:

if there is no symbol corresponding to an atomic number of 30 in the Table of Standard Elements ...
if there is no atomic number in row 2 of the Table of Standard Elements ...


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