idx = sub2allind(sz, sub1, sub2, sub3, ... )

Like Matlab's sub2ind, sub2allind computes the equivalent linear indices for
given subscripts of an array with size SZ.
Unlike sub2ind, it computes a field of all combinations of
subscripts. So, instead of calling A( 2:3, 1, 4:11) you might
use
linIdx = sub2allind( size(A), 2:3, 1, 4:11 );

and then call A(linIdx) or A(linIdx(:)) or you reshape linIdx the way you need.

This is usefull whenever you want to access a subset of an array and compare it with a differently shaped thing, like

A(linIdx(:)) = A(linIdx(:)) + <1D-Vector>

Using the naked colon operator is allowed to reference an entire dimension:

linIdx = sub2allind( sz, :, sub2 );