Nested indexing in a loop

Question

I have a for loop performing the current operation:

T = [1,1,1,4,5,6,3];
A = [20,15,4,21,14,3];
l = length(T);

% how can we vectorize this?
for(i = 1:l)
    A(T(i)) = A(T(i)) + 1;
end

Simply put, it uses the T vector as a list of indices to increment on the A array in a particular order. For instance, the first element in array A will be incremented 3 times (corresponding to the 3 ones in T), while the rest get incremented once, 2 doesn't get incremented. The resulting changes to A are thus:

A = [23,15,5,22,15,4];

However, ideally I would like to avoid a for loop here. Before I tried:

A(T) = A(T) + 1;

This didn't work; MATLAB just ignores the repeated indices. Is there someway I can perform the operation in the for loop by vectorisation or some other means?

O'Neil · Accepted Answer · 2016-11-01 14:03:57Z

3

With accumarray:

T = [1,1,1,4,5,6,3];
A = [20,15,4,21,14,3];
res = A + accumarray(T(:), 1, [numel(A) 1]).';

edited Nov 1, 2016 at 14:03

answered Nov 1, 2016 at 13:57

O'Neil

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2 Comments

user2261062 Over a year ago

A + accumarray(T(:), 1, [numel(A) 1])' otherwise Matlab complains: When SUBS is a column vector, the third input SZ must be of the form [N 1].

Luis Mendo Over a year ago

Also s = accumarray(T(:),1).'; w = 1:numel(s); A(w) = A(w)+s;

score 0 · Accepted Answer · 2016-11-01 13:46:35Z

0

[q,w] = hist(T,0:numel(A));
A(w(q>0)) = A(w(q>0)) + q(q>0);


A =

23    15     5    22    15     4

Explanation:

You can use hist to know how many time each index occurs in T. Since indexes are integers, you can specify the ranges from 0 to the number of elements of A.

Hist will then return two vectors:

q is the number of occurrences of each index:

q =

0     3     0     1     1     1     1     0     0     0     0

w are the indexes themselves:

w =

0     1     2     3     4     5     6     7     8     9    10

Then you just take the indexes that occur at least once w(q>0) as the indexes of A to be updated, and you update those indexes by the number of occurrences q(q>0)

edited Nov 1, 2016 at 13:46

answered Nov 1, 2016 at 13:40

user2261062

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