Add the rows of a Matrix until a certain value.

Hi All, I have this code which will add values in a vector until a certain value and then give the corresponding location. However I would like to modify this so that it would work with a matrix. Here is the current code.

A=[3 4 10 3 9 10 12 4 11 9 11 10]';
sum1=0;
k=1;
while sum1<15
    sum1=sum1+A(k);
    k=k+1;
end
sum1
k = k-1;

Now the output is sum1=17 and k = 3. If A instead was A=[3 4 10 3 9 10; 12 4 11 9 11 10], then ideally the output would be sum1 = 17 16 and k = 3 2

Any help would be appreciated.

Answer 1

[Guillaume]

First, a much better way to implement your vector algorithm is to use cumsum amd find:

A = [3 4 10 3 9 10 12 4 11 9 11 10]';
sumA = cumsum(A);
k = find(sumA >= 15, 1)
sum1 = sumA(k)

You can use cumsum on a matrix to sum each row. The difficulty is that find does not work on rows, so you have to use a loop or split the rows into a cell array and use cellfun on that:

A = [3 4 10 3 9 10; 12 4 11 9 11 10];
threshold = 15;
rowsums = num2cell(cumsum(A, 2), 2);
thresholdcol = cellfun(@(row) find(row >= threshold, 1), rowsums)
thresholdsum = cellfun(@(row, k) row(k), rowsums, num2cell(thresholdcol))

[Walter Roberson]

Note: this code will have problems if a row does not meet the threshold.

Answer 2

[Walter Roberson]

cA = cumsum(A, 2);
k = reshape(sum(cA < 15,2) + 1, 1, []);
sum1 = cA(sub2ind(size(cA), 1:size(cA,1), k));

Guaranteed to break if one of the rows does not reach 15.

[Walter Roberson]

More compactly:

cA = cumsum(A, 2);
k = sum(cA < 15, 2).' + 1;
nA = size(A,1);
sum1 = cA((0:nA-1)*nA + k);