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{{Built-in|Index origin|⎕IO}} 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 [[wikipedia: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. | {{Built-in|Index origin|⎕IO}} 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 [[wikipedia: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, [[ | The index origin affects both the handling of index arguments to functions (and other functionality like [[axis specification]]) and their results. For example, [[Index Generator]] creates results that begin with <source lang=apl inline>⎕IO</source>. | ||
<source lang=apl> | <source lang=apl> | ||
⎕IO←0 | ⎕IO←0 | ||
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{| class="wikitable c" style="margin:1em auto" | {| class="wikitable c" style="margin:1em auto" | ||
! [[Monadic | ! [[Monadic function]]s !! [[Dyadic function]]s !! [[Operator]]s | ||
|- | |- | ||
| [[ | | [[Index Generator]] (<source lang=apl inline>⍳</source>) || [[Index-Of]] (<source lang=apl inline>⍳</source>) || [[Key]] (<source lang=apl inline>⌸</source>) argument to <source lang=apl inline>⍺⍺</source> | ||
|- | |- | ||
| [[Roll]] (<source lang=apl inline>?</source>) || [[Deal]] (<source lang=apl inline>?</source>) || [[At]] (<source lang=apl inline>@</source>) result of <source lang=apl inline>⍵⍵</source> | | [[Roll]] (<source lang=apl inline>?</source>) || [[Deal]] (<source lang=apl inline>?</source>) || [[At]] (<source lang=apl inline>@</source>) result of <source lang=apl inline>⍵⍵</source> | ||
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|} | |} | ||
Additionally, [[indexing]] with brackets and [[axis specification]] depend on index origin. | Additionally, [[Bracket indexing|indexing]] with brackets and [[axis specification]] depend on index origin. | ||
== External links == | == External links == | ||
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=== Articles === | === Articles === | ||
* [ | * [[wikipedia:Array data type#Index origin|Index origin]] | ||
* [https://www.jsoftware.com/papers/indexorigin.htm Is Index Origin 0 a Hindrance?] | * [https://www.jsoftware.com/papers/indexorigin.htm Is Index Origin 0 a Hindrance?] | ||
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* [https://help.dyalog.com/latest/index.htm#Language/System%20Functions/io.htm Dyalog] | * [https://help.dyalog.com/latest/index.htm#Language/System%20Functions/io.htm Dyalog] | ||
* [http://microapl.com/apl_help/ch_020_070_380.htm APLX] | * [http://microapl.com/apl_help/ch_020_070_380.htm APLX] | ||
{{APL built-ins}} | {{APL built-ins}}[[Category:System constants]] | ||