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---
language: APL
contributors:
- ["nooodl", "https://github.com/nooodl"]
filename: learnapl.apl
---
```apl
⍝ Comments in APL are prefixed by ⍝.
⍝ A list of numbers. (¯ is negative)
2 3e7 ¯4 50.3
⍝ 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
⍝ 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
⍝ 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:
× ¯4 ¯2 0 2 4 ⍝ ¯1 ¯1 0 1 1
⍝ Values can be compared using these operators (1 means
⍝ "true", 0 means "false"):
10 20 30 = 10 20 99 ⍝ 1 1 0
10 20 30 < 10 20 99 ⍝ 0 0 1
⍝ "⍳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
⍝ Single-argument ⍴ gives you the dimensions back:
⍴ 4 3 ⍴ ⍳5 ⍝ 4 3
⍝ Values can be stored using ←. Let's calculate the mean
⍝ value of a vector of numbers:
A ← 10 60 55 23
⍝ Sum of elements of A (/ is reduce):
+/A ⍝ 148
⍝ Length of A:
⍴A ⍝ 4
⍝ Mean:
(+/A) ÷ (⍴A) ⍝ 37
⍝ We can define this as a function using {} and ⍵:
mean ← {(+/⍵)÷⍴⍵}
mean A ⍝ 37
```
## Further Reading
- [APL Wiki](https://aplwiki.com/)
- An older version of APL book by the creator: [Kenneth Iverson - A Programming Language](https://www.softwarepreservation.org/projects/apl/Books/APROGRAMMING%20LANGUAGE/view)
- Additional Books: [APL Books](https://aplwiki.com/wiki/Books)
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