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[apl/en] Add a short APL introduction (#722)
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⍝ These examples can be tested here: | ||
⍝ http://ngn.github.io/apl/web/index.html | ||
⍝ | ||
⍝ Comments in APL are prefixed by ⍝. | ||
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⍝ A list of numbers. (¯ is negative) | ||
2 3e7 ¯4 50.3 | ||
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⍝ An expression, showing some functions. In APL, there's | ||
⍝ no order of operations: everything is parsed right-to- | ||
⍝ left. This is equal to 5 + (4 × (2 ÷ (5 - 3))) = 9: | ||
5 + 4 × 2 ÷ 5 - 3 ⍝ 9 | ||
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⍝ These functions work on lists, too: | ||
1 2 3 4 × 5 ⍝ 5 10 15 20 | ||
1 2 3 4 × 5 6 7 8 ⍝ 5 12 21 32 | ||
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⍝ All functions have single-argument and dual-argument | ||
⍝ meanings. For example, "×" applied to two arguments | ||
⍝ means multiply, but when applied to only a right-hand | ||
⍝ side, it returns the sign: | ||
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× ¯4 ¯2 0 2 4 ⍝ ¯1 ¯1 0 1 1 | ||
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⍝ Values can be compared using these operators (1 means | ||
⍝ "true", 0 means "false"): | ||
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10 20 30 = 10 20 99 ⍝ 1 1 0 | ||
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10 20 30 < 10 20 99 ⍝ 0 0 1 | ||
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⍝ "⍳n" returns a vector containing the first n naturals. | ||
⍝ Matrices can be constructed using ⍴ (reshape): | ||
4 3 ⍴ ⍳5 ⍝ 0 1 2 | ||
⍝ 3 4 0 | ||
⍝ 1 2 3 | ||
⍝ 4 0 1 | ||
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⍝ Single-argument ⍴ gives you the dimensions back: | ||
⍴ 4 3 ⍴ ⍳5 ⍝ 4 3 | ||
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⍝ Values can be stored using ←. Let's calculate the mean | ||
⍝ value of a vector of numbers: | ||
A ← 10 60 55 23 | ||
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⍝ Sum of elements of A (/ is reduce): | ||
+/A ⍝ 148 | ||
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⍝ Length of A: | ||
⍴A ⍝ 4 | ||
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⍝ Mean: | ||
(+/A) ÷ (⍴A) ⍝ 37 | ||
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⍝ We can define this as a function using {} and ⍵: | ||
mean ← {(+/⍵)÷⍴⍵} | ||
mean A ⍝ 37 |