Editing Control flow (section)

=== Labels ===
{{main article|Label (computer science)}}
A [[Label (programming language)|label]] is an explicit name or number assigned to a fixed position within the [[source code]], and which may be referenced by control flow statements appearing elsewhere in the source code. A label marks a position within source code and has no other effect.

[[Line number]]s are an alternative to a named label used in some languages (such as [[BASIC]]). They are [[Natural number|whole numbers]] placed at the start of each line of text in the source code. Languages which use these often impose the constraint that the line numbers must increase in value in each following line, but may not require that they be consecutive. For example, in BASIC:

<syntaxhighlight lang="qbasic">
10 LET X = 3
20 PRINT X
</syntaxhighlight>

In other languages such as [[C (programming language)|C]] and [[Ada (programming language)|Ada]], a label is an [[identifier]], usually appearing at the start of a line and immediately followed by a colon. For example, in C:

<syntaxhighlight lang="c">
Success: printf("The operation was successful.\n");
</syntaxhighlight>

The language [[ALGOL 60]] allowed both whole numbers and identifiers as labels (both linked by colons to the following statement), but few if any other [[ALGOL]] variants allowed whole numbers. Early [[Fortran]] compilers only allowed whole numbers as labels. Beginning with Fortran-90, alphanumeric labels have also been allowed.