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Problem finding max value of column of an Matrix

Asked by Sayanta on 13 May 2011
Dear Experts,
I have some loop problem in matlab, I can't find where is the exact problem in my loop , Could please help me in that :)
A = [ 0 1 2 3 4 25 6 7 8 9 ; 9 5 1 20 3 6 4 7 8 5]'
for i=1:size(A,1) for n=1:size(A,2) if A(i,n) == max(A(:,n)) A(i,n)=1 else A(i,n)=0 end end end
the Correct result I expect from the program like this :
0 0
0 0
0 0
0 1
0 0
1 0
0 0
0 0
0 0
0 0
But I getting the error result like
0 0
0 0
0 0
0 1
0 0
1 0
0 0
0 0
0 1
1 1
Many thanks
Good weekend
With Regards
Sayanta

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2 Answers

Answer by Laura Proctor on 13 May 2011
 Accepted Answer

A = [ 0 1 2 3 4 25 6 7 8 9 ; 9 5 1 20 3 6 4 7 8 5]'
c1 = A(:,1) == max(A(:,1));
c2 = A(:,2) == max(A(:,2));
A = [c1 c2];
You'll end up with a logical array as a result, but it will give the results you want. If it needs to be an array of type double, just cast it to double:
double(A)
If you would like to not create variables c1 and c2, then you can just replace c1 with A(:,1) and c2 with A(:,2) which will result in A as type double.
The problem with your initial code is that you're overwriting values in A in each loop.

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Answer by Andy
on 13 May 2011

The problem you're having is that you recalculate the max of each column on each iteration of the loop. You could move that calculation out of the loop, or not use a loop at all:
A = [ 0 1 2 3 4 25 6 7 8 9 ;
9 5 1 20 3 6 4 7 8 5]';
[dummy,mA] = max(A,[],1); % max of each column
B = zeros(size(A));
B(sub2ind(size(A),mA,[1 2]))=1; % enter 1s for each max

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