MATLAB: Find the cell where the sum equals a threshold

findsummationthreshold

Hello,
I have a 32 column by one row array. I want to sum across the values in each column until I match a threshold value. The result should provide the column # (which will be between 1:32)
I've tried a few things and havent been able to iron this down.
Here's the data:
A = [0 0 4520 51418 101386 90907 78735 65863 45766 26831 30586 14864 7905 4741 2840 2475 872 290 99 44 15 2 0 0 0 1 0 2 0 0 0 0];
Threshold value is = 0.5*sum(A).
So I want to find the cell where A = 265081.
The result should provide the cell number (or bin if you will) where this value occurs while summing from the first cell of A to the last.

Best Answer

Compare the cumulative sum against your threshold. The line below returns the first index of A that is greater than or equal to your threshold. 
A = [0	0	4520	51418	101386	90907	78735	65863	45766	26831	30586	14864	7905	4741	2840	2475	872	290	99	44	15	2	0	0	0	1	0	2	0	0	0	0];
threshold = 5000; 
idx = find(cumsum(A) >= threshold,1);
If you'd rather have the index of the last element that is less than or equal to your threshold, 
idx = find(cumsum(A) <= threshold,1,'last');
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