15.7. Blank entries

| 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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