Editing Off-by-one error (section)

===Looping over arrays===
Consider an [[array data type|array]] of items, and items ''m'' through ''n'' (inclusive) are to be processed. How many items are there? An intuitive answer may be {{nowrap|''n'' − ''m''}}, but that is off by one, exhibiting a [[#Fencepost error|fencepost error]]; the correct answer is {{nowrap|''n'' − ''m'' + 1}}.

For this reason, ranges in computing are often represented by [[half-open interval]]s; the range from ''m'' to ''n'' (inclusive) is represented by the range from ''m'' (inclusive) to {{nowrap|''n'' + 1}} (exclusive) to avoid fencepost errors. For example, a [[program loop|loop]] that iterates five times (from 0 to 4 inclusive) can be written as a half-open interval from 0 to&nbsp;5:

<syntaxhighlight lang="c">
for (index = 0; index < 5; index++) 
{
    /* Body of the loop */
}
</syntaxhighlight>

The loop body is executed first of all with {{mono|index}} equal to 0; {{mono|index}} then becomes 1, 2, 3, and finally 4 on successive iterations. At that point, {{mono|index}} becomes 5, so {{mono|index < 5}} is false and the loop ends. However, if the comparison used were <= (less than or equal to), the loop would be carried out six times: {{mono|index}} takes the values 0, 1, 2, 3, 4, and 5. Likewise, if {{mono|index}} were initialized to 1 rather than 0, there would only be four iterations: {{mono|index}} takes the values 1, 2, 3, and 4. Both of these alternatives can cause off-by-one errors.

Another such error can occur if a [[do-while loop]] is used in place of a [[while loop]] (or vice versa.) A do-while loop is guaranteed to run at least once.

Array-related confusion may also result from differences in programming languages. Numbering from 0 is most common, but some languages start array numbering with 1. [[Pascal (programming language)|Pascal]] has arrays with user-defined indices. This makes it possible to model the array indices after the problem domain.