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

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

Answer 1

[Fangjun Jiang]

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