How to make elements of each column zero before nth element ??

Question

Rabia Zulfiqar 2020-5-16

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编辑： Walter Roberson 2020-5-17

Hi I am trying to do something like this, I have a matrix where I have to find the min value of each column after that I have to convert elements occuring before minimum values to zero in each column.

For example:

5x4 matrix

[5 12 8 19

4 2 9 22

6 14 7 1

8 16 4 3

10 3 5 5]

The min value in each column is 4,2,4,1

so the new matrix should be

[0 0 0 0

4 2 0 0

6 14 0 1

8 16 4 3

10 3 5 5]

Can someone please help??? i really appreciate your help.Thanks

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Answer 1

Walter Roberson 2020-5-16

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在 MATLAB Online 中打开

A = [5      12      8      19
 4       2       9     22
 6      14      7       1
 8      16      4       3
 10     3       5        5];
A .* ~cumprod(A>min(A,[],1))

SIgh. I suspect that I just answered a homework problem. I will, however, leave it for you to figure out how it works.

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Rabia Zulfiqar 2020-5-16

Thankyou so much for this but I am not doing any MATLAB assignment I am trying to form an algorithm for my degree project and I got stuck at this:)

Walter Roberson 2020-5-16

编辑：Walter Roberson 2020-5-17

I will give you the same advice that I give other people:

Get it working first

That is, you have full permission to use loops or slower algorithms, or do things "brute force" ways, write them however you think about them, as long as you concentrate on getting it working. Don't waste days trying to come up with a clever way to do something when it is blocking your progress.

You can always go back later and clean up the code, make it prettier or more efficient or more MATLAB-y. And when you do, preserve a copy of the version you got working for comparison, so that you can test to prove that the new version gets the same results as the version you got working.

Software engineering tests have found that it is typical that about 80% of the execution time is spent in about 20% of the lines of code. It is not productive to spend days making a nice version of a small section of code that will only account for perhaps 1% of the execution time.

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Answer 2

Image Analyst 2020-5-16

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https://ww2.mathworks.cn/matlabcentral/answers/525826-how-to-make-elements-of-each-column-zero-before-nth-element#answer_432849

在 MATLAB Online 中打开

Here's one way that's easy to understand and implement:

m = [5      12      8      19
 4       2       9     22
 6      14      7       1
 8      16      4       3
 10     3       5        5]
columnMins = min(m, [], 1) % Get min value in each column.
for col = 1 : length(columnMins)
	row = find(m(:, col) == columnMins(col), 1);
	m(1:row-1, col) = 0;
end