How to add up numbers with same value in matlab

Question

So if i have an number array:

a     b
1     2.5 
1     1.2 
3     2.5
4     0.4
6     3
3     1.2

i want to sum up numbers in column a with

Answer 1

Assuming A to be the input array, you have two approaches to play with here.

Approach #1

A combination of accumarray and unique -

[unqcol2,~,idx] = unique(A(:,2),'stable')
[accumarray(idx,A(:,1)) unqcol2]

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Approach #2

With bsxfun -

[unqcol2,~,idx] = unique(A(:,2),'stable')
[sum(bsxfun(@times,bsxfun(@eq,idx,1:max(idx)),A(:,1)),1).' unqcol2 ]