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C++ API

: ind = sub2ind (dims, i, j)
: ind = sub2ind (dims, s1, s2, …, sN)

Convert subscripts to linear indices.

The input dims is a dimension vector where each element is the size of the array in the respective dimension (see ‘size’). The remaining inputs are scalars or vectors of subscripts to be converted.

The output vector ind contains the converted linear indices.

Background: Array elements can be specified either by a linear index which starts at 1 and runs through the number of elements in the array, or they may be specified with subscripts for the row, column, page, etc. The functions ind2sub and sub2ind interconvert between the two forms.

The linear index traverses dimension 1 (rows), then dimension 2 (columns), then dimension 3 (pages), etc. until it has numbered all of the elements. Consider the following 3-by-3 matrices: 

[(1,1), (1,2), (1,3)]     [1, 4, 7]
[(2,1), (2,2), (2,3)] ==> [2, 5, 8]
[(3,1), (3,2), (3,3)]     [3, 6, 9]

The left matrix contains the subscript tuples for each matrix element. The right matrix shows the linear indices for the same matrix.

The following example shows how to convert the two-dimensional indices (2,1) and (2,3) of a 3-by-3 matrix to linear indices with a single call to sub2ind. 

s1 = [2, 2];
s2 = [1, 3];
ind = sub2ind ([3, 3], s1, s2)
   ⇒ ind =  2   8

See also: ind2sub, size.

Package: octave