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How can I swap two elements in each row of a matrix without loops?

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Jack Rolf-Gokes
Jack Rolf-Gokes on 8 Apr 2021 at 18:16
Answered: Fangjun Jiang on 8 Apr 2021 at 19:52
If I have, say, a 5x4 matrix, and for each row I want to randomly select two elements and swap them, is there any way of doing this without using loops?
For example (A is the input, B is an example output):
A =
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16
17 18 19 20
B =
1 3 2 4
7 6 5 8
9 10 12 11
13 14 16 15
17 20 19 18

Accepted Answer

Fangjun Jiang
Fangjun Jiang on 8 Apr 2021 at 19:52
This is lengthy but does the job.
%%
flag=logical([0 0 1 1]); % pick two elements in each row
p=perms(flag); % all the possible permutation, total is 24
index1=randi(24,5,1); % pick 5 random numbers from 1 to 24
index=p(index1,:); % this is the logical index to pick the numbers in each row
A=reshape(1:20,5,4)% original matrix
B=A'; % need to transpose
data=B(index');
data=reshape(data,2,[]) %pick out the data, each column contains two numbers from each row in the matrix A
data=data([2,1],:) % swap
B(index')=data; % fill back the data
B=B' % transpose, this is the result

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