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{{Primitive|⎕IO|Index origin}} is the number used for the first [[index]] along each [[axis]] of an array. Many APLs allow the user to configure index origin using the [[system variable]] <source lang=apl inline>⎕IO</source>. Even array languages which do not have such a configuration parameter must make a choice of index origin; for example, [[J]] | {{Primitive|⎕IO|Index origin}} is the number used for the first [[index]] along each [[axis]] of an array. Many APLs allow the user to configure index origin using the [[system variable]] <source lang=apl inline>⎕IO</source>. Even array languages which do not have such a configuration parameter must make a choice of index origin; for example, [[A+]] and [[J]] use a non-configurable index origin of 0. The [https://en.wikipedia.org/wiki/English_language English language] uses an index origin of one: thus the element with index <source lang=apl inline>⎕IO</source> in APL is referred to in English as the "first" element. | ||
The index origin affects both the handling of index arguments to functions (and other functionality like [[axis specification]]) and their results. For example, [[Iota]] (or "Index Generator") creates results which begin with <source lang=apl inline>⎕IO</source>. | The index origin affects both the handling of index arguments to functions (and other functionality like [[axis specification]]) and their results. For example, [[Iota]] (or "Index Generator") creates results which begin with <source lang=apl inline>⎕IO</source>. | ||
< | <source lang=apl> | ||
⎕IO←0 | ⎕IO←0 | ||
⍳4 | ⍳4 | ||
| Line 9: | Line 9: | ||
⍳4 | ⍳4 | ||
1 2 3 4 | 1 2 3 4 | ||
</ | </source> | ||
Any number could consistently be used for the index origin, but almost all APLs restrict the choice to 0 or 1. [[ngn/apl]] supports <source lang=apl inline>⎕IO</source> but only allows it to have the value 0. Attempting to assign an unsupported value to <source lang=apl inline>⎕IO</source> typically results in a [[DOMAIN ERROR]]. | Any number could consistently be used for the index origin, but almost all APLs restrict the choice to 0 or 1. [[ngn/apl]] supports <source lang=apl inline>⎕IO</source> but only allows it to have the value 0. Attempting to assign an unsupported value to <source lang=apl inline>⎕IO</source> typically results in a [[DOMAIN ERROR]]. | ||