2024-05-12 13:38:39 +03:00
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---
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language: APL
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contributors:
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- ["nooodl", "https://github.com/nooodl"]
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filename: learnapl.apl
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---
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```apl
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2024-05-12 13:34:52 +03:00
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⍝ Comments in APL are prefixed by ⍝.
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⍝ A list of numbers. (¯ is negative)
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2 3e7 ¯4 50.3
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⍝ An expression, showing some functions. In APL, there's
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⍝ no order of operations: everything is parsed right-to-
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⍝ left. This is equal to 5 + (4 × (2 ÷ (5 - 3))) = 9:
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5 + 4 × 2 ÷ 5 - 3 ⍝ 9
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⍝ These functions work on lists, too:
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1 2 3 4 × 5 ⍝ 5 10 15 20
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1 2 3 4 × 5 6 7 8 ⍝ 5 12 21 32
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⍝ All functions have single-argument and dual-argument
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⍝ meanings. For example, "×" applied to two arguments
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2024-05-12 13:38:39 +03:00
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⍝ means multiply, but when applied to only a right-hand
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2024-05-12 13:34:52 +03:00
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⍝ 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
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⍝ "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.
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⍝ Matrices can be constructed using ⍴ (reshape):
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4 3 ⍴ ⍳5 ⍝ 0 1 2
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⍝ 3 4 0
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⍝ 1 2 3
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⍝ 4 0 1
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⍝ Single-argument ⍴ gives you the dimensions back:
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⍴ 4 3 ⍴ ⍳5 ⍝ 4 3
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⍝ Values can be stored using ←. Let's calculate the mean
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⍝ value of a vector of numbers:
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A ← 10 60 55 23
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⍝ Sum of elements of A (/ is reduce):
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+/A ⍝ 148
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⍝ Length of A:
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⍴A ⍝ 4
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⍝ Mean:
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(+/A) ÷ (⍴A) ⍝ 37
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⍝ We can define this as a function using {} and ⍵:
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mean ← {(+/⍵)÷⍴⍵}
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mean A ⍝ 37
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2024-05-12 13:38:39 +03:00
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```
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