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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 which begin with <source lang=apl inline>⎕IO</source>. | ||
<source lang=apl> | <source lang=apl> | ||
⎕IO←0 | ⎕IO←0 | ||
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! [[Monadic function]]s !! [[Dyadic function]]s !! [[Operator]]s | ! [[Monadic function]]s !! [[Dyadic function]]s !! [[Operator]]s | ||
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| [[ | | [[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> | ||