MATLAB: Summing up entries in the second column only when an entry in the third column is positive

MATLABmatrix manipulation

Suppose I have a matrix

N =[ 1 2 0; 4 5 1; 7 8 0];

I like to choose the row only with the positive third column and then sum up the elements in the second column. (In this case the answer is 5.) Please advise.

Best Answer

index  = (N(:, 3) > 0);
result = sum(N(index, 2))

Related Solutions

MATLAB: Adding next values in an array

 M = [1 2 3 -5 -6 2 3]'  %demo data
 M + cumsum([zeros(1, size(M, 2)); M(1:end-1, :)] .* (cumsum(M<0, 1)>1), 1)

Will do what you want on any sized matrix.

Explanation of above code:

M<0 finds sign of M
cumsum(M<0, 1) will be 1 or more at the first negative value in each column
cumsum(M<0, 1)>1 will be one after the first negative value all the way down
[zeros(1, size(M, 2)); M(1:end-1, :)] is M shifted down by one row
4.*5 filters the shifted M so that all positive values before the first negative values are 0 AND the first negative value is 0
cumsum of 6 + M is the desired result

MATLAB: How to find the index of first and last nonzero elements in each column

You could probably use

A = [0    0     0      0
	0     0     0      0
	1     0     4      8
	2     0     5      9
	3     1     6      7
	0     2     7      0
	0     0     0      0]
[rows, columns] = size(A)
B = zeros(2, columns)
for col = 1 : size(A, 2)
	B(1, col) = find(A(:, col), 1, 'first');
	B(2, col) = find(A(:, col), 1, 'last');
end

This gives

B =

3 5 3 3

5 6 6 5

which makes sense to me but I'm puzzled as to how you get

B=[3 12 17 24

5 13 21 26]

for your example. Can you explain?

Best Answer

Related Solutions

MATLAB: Adding next values in an array

MATLAB: How to find the index of first and last nonzero elements in each column

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