adding next values in an array
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Edited: Nikolas Spiliopoulos on 17 Feb 2017
hi all, I have a question
I have a vector like this [1;2;3;-5;-6;2;3], when it's negative i would like to add in the next value and get something like that [1;2;3;-5;-11;-9;-6]. Which means when it's negative it starts adding the next value!
the thing is that i have an array with 48x258 dimensions, so the example above would be for each column!
thanks a lot!
Guillaume on 17 Feb 2017
Edited: Guillaume on 17 Feb 2017
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
More Answers (1)
Alexandra Harkai on 17 Feb 2017
Considering the last element on a column will not be added to any other elements, you can loop through the whole thing:
function [a] = addNegatives(a)
for col = 1:size(a,2) % for each column
for k = 1:size(a, 1)-1 % don't do it for the last element
if a(k, col) < 0
a(k+1, col) = a(k, col) + a(k+1, col);
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