problem with 'sum' command

Question

itsik 2011-5-22

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https://ww2.mathworks.cn/matlabcentral/answers/7976-problem-with-sum-command

hello if i have this code: x=[10,1000] a=sum(x) i get after the sum that a=[1,1000] so i need to find the inverse command for 'sum' to create the original [10,1000] matrix how does i do that? maybe with 'fft'? thanks!

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Answer 1

the cyclist 2011-5-22

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https://ww2.mathworks.cn/matlabcentral/answers/7976-problem-with-sum-command#answer_10963

You mean that the size of x is 10x10000, right? Not that x is the vector [10,10000], which is what you wrote.

There is no unique inverse to the sum() command. Think about it. If I tell you that 2+2 equals 4, there is no way to for you to undo that, and obtain 2 and 2 from 4. It could have been 1 and 3, or 0 and 4, or -1.1 and 5.1, etc.

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John D'Errico 2011-5-22

I recall hearing the next release of matlab is supposed to have the unsum function in it, as part of the clairvoyance toolbox. They will naturally move the function magic.m into the clairvoyance toolbox at that time.

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