ยง16.7. 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:
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 ...