Adding a row vector to multiple rows of a matrix, with duplicate row index

Suppose we have a matrix

    A = [1,2;3,4;5,6] 
    1 2
    3 4
    5 6

I know that matlab allows adding vector to multiple rows, for example,

    A([1,2],:) = A([1,2],:) + [1,1];

then

    A =
        2 3
        4 5
        5 6

However, A([1,2,1],:) + [1,1] gives the same result

    A([1,2,1],:) = A([1,2,1],:) + [1,1];

then

    A =
        2 3
        4 5
        5 6

This is NOT what I want. The *desired effect* is adding [1,1] to the first row twice, and the expected result is,

    A([1,2,1],:) = A([1,2,1],:) + [1,1];

and A should be

    A =
        3 4
        4 5
        5 6

How do I achieve this? thanks!

Answer 1

[mizuki]

What version do you use? The results of the second and the third examples on my R2017a were:

>> A([1,2],:) + [1,1] 
ans =
     2     3
     4     5

>> A+1
ans =
     2     3
     4     5
     6     7

If you are using R2016a or previous version, use REPMAT as

A + repmat(1,size(A))

or with different elements for each column:

A + [repmat(1,[size(A,1) 1]) repmat(2,[size(A,1) 1])]

Answer 2

[Andrei Bobrov]

ii = [1;2;1];

a = unique(ii);
A(a,:) = A(a,:) + accumarray(ii,1)*[1, 1]