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C++ API

: [s1, s2, …, sN] = ind2sub (dims, ind)

Convert linear indices to subscripts.

The input dims is a dimension vector where each element is the size of the array in the respective dimension (see ‘size’). The second input ind contains linear indies to be converted.

The outputs s1, …, sN contain the converted subscripts.

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

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

The following example shows how to convert the linear indices 2 and 8 in a 3-by-3 matrix into subscripts. 

ind = [2, 8];
[r, c] = ind2sub ([3, 3], ind)
   ⇒ r =  2   2
   ⇒ c =  1   3

If the number of output subscripts exceeds the number of dimensions, the exceeded dimensions are set to 1. On the other hand, if fewer subscripts than dimensions are provided, the exceeding dimensions are merged into the final requested dimension. For clarity, consider the following examples: 

ind  = [2, 8];
dims = [3, 3];
## same as dims = [3, 3, 1]
[r, c, s] = ind2sub (dims, ind)
   ⇒ r =  2   2
   ⇒ c =  1   3
   ⇒ s =  1   1
## same as dims = [9]
r = ind2sub (dims, ind)
   ⇒ r =  2   8

See also: ind2sub, size.

Package: octave