in a matrix m, how to select the rows after m(m(:,1)==1,2)

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Loriann Chevalier 2022-7-12

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评论： dpb 2022-7-12

采纳的回答： dpb

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Hi everyone,

I have a matrix, say 13x2 like this :

m = [0 0;
0;
0;
0;
0;
0;
0;
0;
0;
0;
0;
0;
0]

Now I want to set that at the row where column 1 has a value of 1, the value of the same row in column 2 is 0.25, and also the 3 next rows (still column 2), i.e., I ultimately want this :

m = [0 0;
0;
0;
0;
0;
0.25;
0.25;
0.25;
0.25;
0;
0;
0;
0]

For the row with m(row,1) = 1, I use this :

m(m(:,1)==1,2) = 0.25;

but then I cannot find the right notation for the 3 next rows. E.g. I would imagine this below, but it doesn't work :

m((m(:,1)==1):(m(:,1)==1)+3,2) = 0.25;

Thanks!

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Answer 1

dpb 2022-7-12

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编辑：dpb 2022-7-12

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Don't try to play MATLAB golf; use a temporary variable to help...

ix=find(m);             % ~=0 is implied; only the one element is nonzero
m(ix:ix+3,2)=0.25;      % set the desired column 2 values

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Loriann Chevalier 2022-7-12

That's great. Thank you very much

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Answer 2

Sanyam 2022-7-12

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Hey @Loriann Chevalier

Answer of @dpb is cool, but if you don't remember the syntax to perform vectorized operations in MATLAB, then writing the code from basics help a lot. Below is the snippet of code attached, accomplishing your task with just if-else and for-loop

Explanation:

iterate over all the rows
check if the value in first column is 1
if not then do nothing
if one then change the value of 2nd column to 0.25 -> change the value of 2nd column of next 2 rows, given the rows lie inside your matrix

Hope it's useful. Thanks!!

for i = 1:size(m, 1)
    if m(i, 1) == 1
        m(i, 2) = 0.25;
        if i+1 <= size(m, 1)
            m(i+1, 2) = 0.25;
        end
        if i+2 <= size(m, 1)
            m(i+2, 2) = 0.25;
        end
    end
end