Matlab: Update elements in array by identifier

Question

I have a large array A with ~500.000 rows of the form

[ id1  id2  value1  value2  zero  zero ]

and another, smaller Array B (~20.000 rows) with rows with some of the identifiers from A

[ id1  id2  value3  value4 ]

All the pairs of IDs in B exist in A. I want to update the values of B into A at the positions where respectively the values of id1 and id2 match. The (row-)order of the new array may be arbitraty.

An example:

A = 1 1  3 5 0 0
    1 2  6 4 0 0
    1 3  3 1 0 0
    2 1  3 8 0 0
    3 4  0 2 0 0

B = 2 1  7 4
    1 1  2 1

should yield

C = 1 1  3 5 2 1
    1 2  6 4 0 0
    1 3  3 1 0 0
    2 1  3 8 7 4
    3 4  0 2 0 0

Iterating through A for each element in B with for loops takes incredibly long. I hope there is a faster way.

Eitan T · Accepted Answer · 2013-05-06 08:06:54Z

3

You can use ismember to obtain the indices of the rows where "id1" and "id2" match, and then update the last two columns with the corresponding values from B:

C = A;
[tf, loc] = ismember(B(:, 1:2), A(:, 1:2), 'rows');
C(loc, 5:6) = B(:, 3:4);

answered May 6, 2013 at 8:06

Eitan T

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