Size of the unique elements and size of the output of accumarray

Why are the sizes of the two arrays, unique and accumarray, not the same?

size(unique(MAT(:,1)))
size(accumarray(MAT(:,1),MAT(:,2)))

Answer 1

[Matt J]

They will only be the same size if unique(MAT(:,1)) consists of consecutive integers starting from 1. Otherwise, accumarray must leave certain spots blank. A simpler example:

accumarray([2;4],1)

ans = 4×1

0 1 0 1

[Snoopy]

Thank you.