# computing the cumulative matrix with some conditions

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Zahra Sheikhbahaee on 6 Mar 2021
Answered: Matt J on 7 Mar 2021
Is there a very concise way to code up computing the cumulative matrix of the following matrix on each column given the following condition that if the element of the column is 1 it adds 0. If it would be 2 it adds 2, if it is 3 it adds to the previous value 4 and if the element of the matrix is 4 it add to the previous value -5
AR =
1 1 1 1 1 1
1 1 1 1 1 1
1 1 2 1 2 1
1 1 2 1 2 2
1 1 1 1 1 1
1 1 1 1 1 1
2 2 2 2 2 2
2 2 2 2 2 2
1 1 1 1 1 1
1 1 1 1 1 1
1 2 2 2 2 2
2 2 2 2 2 2
1 1 1 1 1 1
1 1 1 1 1 1
2 2 2 2 2 2
2 2 1 2 2 2
1 1 1 1 1 1
1 1 1 2 1 1
2 2 1 2 2 2
2 2 1 2 2 2
1 1 1 1 1 1
1 1 1 1 1 1
2 1 2 2 2 1

Image Analyst on 6 Mar 2021
Why can't you just make up a look up table like [0, 2, 4, -5] and so on. Then just do a simple for loop for the various conditions such as
1. adding the number from the look up table to the current value, and replacing the current value with it, or
2. adding the number from the look up table to the "previous" element's value
for case #2, I'm not sure whether the result replaces the current element or the previous element.
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Image Analyst on 7 Mar 2021
That's doing a pure assignment to the number you specify. I thought you wanted to add some numbers together.
Explain exactly, with a sample array, what "it adds to the previous value" means to you.
Also explain what "it adds 2" means to you.

Matt J on 7 Mar 2021
Is this what you want?
T=[0, 2, 4, -5] ;
result=cumsum(T(AR))