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How to replace data in a matrix by comparing from another matrix?

Asked by suchismita on 5 May 2015
Latest activity Commented on by suchismita on 5 May 2015
I have a matrix A,
[1 5
2 6
3 6
4 7
5 6]
and I have another matrix B as
[1 2
1 3
1 4
1 5
2 3
2 4
3 4
3 5
4 5]
now i want to compare B with a and i want to replace 2nd column of B with A data for example,
[1 5
1 5
1 5
1 5
2 6
2 6
3 6
3 6
4 7]
in first column when there is 1 it will take the value of matrix A's second column data as 5 and when first column is having 2 it will take the value of 2 from matrix A.
please please help me.

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3 Answers

Answer by David Sanchez
on 5 May 2015
 Accepted Answer

for k=1:length(B)
x = A(:,1)==B(k,1);
B(k,2) = A(x,2);
end
B =
1 5
1 5
1 5
1 5
2 6
2 6
3 6
3 6
4 7

  2 Comments

Andrei Bobrov's solution is much neater.

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Answer by Andrei Bobrov
on 5 May 2015
Edited by Andrei Bobrov
on 5 May 2015

[lo,ii] = ismember(B(:,1),A(:,1));
out = B;
out(lo,2) = A(ii(lo),2);

  3 Comments

Very nice solution, although it took me a while to realize that l0 is a variable. it looks similar to the number 10, especially when it is being used as an index.
If the nomenclature is clearer then this solution is much easier to read:
>> [ism,idx] = ismember(B(:,1),A(:,1));
>> out = B;
>> out(ism,2) = A(idx(ism),2)
out =
1 5
1 5
1 5
1 5
2 6
2 6
3 6
3 6
4 7
Hi Stephen! I corrected..

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Answer by Nobel Mondal on 5 May 2015

Assuming, the 'a' matrix has unique values in the first column
for bRow= 1:size(b, 1)
aRow = find(a(:,1)==b(bRow,1), 1);
if ~isempty(aRow)
b(thisrow,2) = a(aRow,2);
end
end

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