How to search for a previous element that is equal in prior loop iteration
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I have a Matrix called A as below
[1 1 3;
1 2 2;
1 3 3;
2 1 1;
2 2 3;
2 3 2;
2 4 4]
I want to produce a column vector that counts the number of times a value occurs in the 2nd column, I have tried the following function :
function count_stars(A)
total = 0;
for i = 1:size(A,1)
total = total + (A(i,2)==A(:,2));
end
total
This counts the number of matching values in the entire column e.g.
2
2
2
2
2
2
1
What I am wanting is to match to values between 1:i only rather than the entire column so that the output looks like this
1
1
1
2
2
2
1
Any insights you are able to share are greatly appreciated!
2 Comments
Image Analyst
on 28 Dec 2015
It's too late at night for me to work on this, but I'm thinking it might involve accumarray(), cumsum() or histcounts(). A question though, will the numbers always be integers? Or might they be floating point numbers with fractional parts?
Accepted Answer
harjeet singh
on 28 Dec 2015
hello tim try this code
a=[1 1 3;
1 2 2;
1 3 3;
2 1 1;
2 2 3;
2 3 2;
2 4 4];
for i=1:size(a,1)
[r,c]=find(a(1:i,2)==a(i,2));
b(i,1)=length(r);
end
b
More Answers (1)
Guillaume
on 29 Dec 2015
For small arrays, it does not matter but this is likely to be more efficient than harjeet's answer for large arrays:
a = [1 1 3;
1 2 2;
1 3 3;
2 1 1;
2 2 3;
2 3 2;
2 4 4];
[values] = unique(a(:, 3));
counts = zeros(size(a, 1), 1);
for val = values';
occurences = cumsum(a(:, 3) == val);
counts(a(:, 3) == val) = occurences(a(:, 3) == val);
end
Rather than looping over all the rows of a, it only loops over the unique values of a (4 iterations instead of 7 in this example).
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