APL

 1⍝ Comments in APL are prefixed by ⍝.
 2
 3⍝ A list of numbers. (¯ is negative)
 42 3e7 ¯4 50.3
 5
 6⍝ An expression, showing some functions. In APL, there's
 7⍝ no order of operations: everything is parsed right-to-
 8⍝ left. This is equal to 5 + (4 × (2 ÷ (5 - 3))) = 9:
 95 + 4 × 2 ÷ 5 - 3        ⍝ 9
10
11⍝ These functions work on lists, too:
121 2 3 4 × 5              ⍝ 5 10 15 20
131 2 3 4 × 5 6 7 8        ⍝ 5 12 21 32
14
15⍝ All functions have single-argument and dual-argument
16⍝ meanings. For example, "×" applied to two arguments
17⍝ means multiply, but when applied to only a right-hand
18⍝ side, it returns the sign:
19
20× ¯4 ¯2 0 2 4            ⍝ ¯1 ¯1 0 1 1
21
22⍝ Values can be compared using these operators (1 means
23⍝ "true", 0 means "false"):
24
2510 20 30 = 10 20 99      ⍝ 1 1 0
26
2710 20 30 < 10 20 99      ⍝ 0 0 1
28
29⍝ "⍳n" returns a vector containing the first n naturals.
30⍝ Matrices can be constructed using ⍴ (reshape):
314 3 ⍴ ⍳5                 ⍝ 0 1 2
32                         ⍝ 3 4 0
33                         ⍝ 1 2 3
34                         ⍝ 4 0 1
35
36⍝ Single-argument ⍴ gives you the dimensions back:
37⍴ 4 3 ⍴ ⍳5               ⍝ 4 3
38
39⍝ Values can be stored using ←. Let's calculate the mean
40⍝ value of a vector of numbers:
41A ← 10 60 55 23
42
43⍝ Sum of elements of A (/ is reduce):
44+/A                      ⍝ 148
45
46⍝ Length of A:
47⍴A                       ⍝ 4
48
49⍝ Mean:
50(+/A) ÷ (⍴A)             ⍝ 37
51
52⍝ We can define this as a function using {} and ⍵:
53mean ← {(+/⍵)÷⍴⍵}
54mean A                   ⍝ 37
Further Reading ¶
