[] (subscript brackets)

[] are used to access subscripted values.

This article describes this function, but [] can also be used as ordinary parentheses and to denote a closed interval of real numbers.

Syntax

A[x, ...]
- x, ... are indices

The returned value is an lvalue if A is.

Description

[] are used to access subscripted values.

For linear containers, such as vectors, matrices (considered as linear containers in row-major order), strings, lists, structures, and binary data objects, A[n] returns the nth value (number, character, object, value, byte, etc) of A; it can also be used as an lvalue. Typically, negative indices count from the end of A: while A[1] is the first element in A, A[−1] is the last element.

For planar containers, such as matrices, pixmaps, and tables, A[i, j] returns the index on row i and column j.

For structures, it is possible to access values both by position and by name: A[1] or A["FirstName"]. In the latter case, however, it is more idiomatic to use the member access operator: A.FirstName.

Notes

When [] are used to access subscripted values, they are implemented by the subscript function.

In addition to being used to access subscripted values, [] can also be used as ordinary parentheses, just like ().

Finally, [a, b] can also be used to denote a closed interval of real numbers.

Examples

v ≔ ❨1, 2, 3❩; s ≔ "ball of light";

A ≔ RandomIntMatrix(3, 0, 10)

⎛8  9  4⎞
⎜3  5  3⎟
⎝4  5  6⎠

'(v[1], s[−1], A[3], A[2, 3])

v[2] ≔ 3; v

 ⎛1⎞
e⎜3⎟
 ⎝3⎠

D ≔ date()

year: 2020
month: September
day: 29

'(D[1], D["month"], D[−1], D.day)

2020  September  29  29