Swapping two columns not working

Question

I want to select two random columns of a matrix and swap them. I use:

S(:,[round_i round_j]) = S(:,[round_j round_i]);

But my code seems to produce the same matrix as before. Below are a code snippet and output of the command window.

function swapped_schedule=swapRounds(S)
global weeks;

round_i=randi(weeks)
round_j=randi(weeks)

while round_j~=round_i
    round_j=randi(weeks);
end

S(:,[round_i round_j]) = S(:,[round_j round_i]);

swapped_schedule=S;

end

Schedule is the matrix I pass to the function swapRounds(). The output is shown as follows:

schedule =

     4    -4    -6     5    -2     6     2     3    -5    -3
     5    -6    -4     4     1     3    -1    -5    -3     6
    -6     5    -5     6     4    -2    -4    -1     2     1
    -1     1     2    -2    -3     5     3    -6     6    -5
    -2    -3     3    -1    -6    -4     6     2     1     4
     3     2     1    -3     5    -1    -5     4    -4    -2


round_i =

     4


round_j =

     6


ans =

     4    -4    -6     5    -2     6     2     3    -5    -3
     5    -6    -4     4     1     3    -1    -5    -3     6
    -6     5    -5     6     4    -2    -4    -1     2     1
    -1     1     2    -2    -3     5     3    -6     6    -5
    -2    -3     3    -1    -6    -4     6     2     1     4
     3     2     1    -3     5    -1    -5     4    -4    -2

How do I get this code to swap my two columns?

Just figured it out. I put a negation in the while loop. My Bad. — jannela vignendra, Mar 09 '18 at 11:42

Adriaan · Answer 1 · 2018-03-09T11:41:49.100

1

schedule = rand(6,10);
round_i = 4;
round_j = 6;

[rows,cols] = size(schedule); % get number of cols
colIDX = 1:cols; % create an index array
colIDX(round_i) = round_j;
colIDX(round_j) = round_i; % flip indices

schedule2 = schedule(:,colIDX); % rename to schedule if wanted

Basically you should index your columns with an array, not just two numbers. See this highly informative post on indexing in MATLAB

edited Mar 09 '18 at 11:41

answered Mar 09 '18 at 11:35

Adriaan

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Thanks for the help. But I seem to have put a negation in my while loop which seems to be logically wrong. Just fixed that, now it's working. Thank you anyways. – jannela vignendra Mar 09 '18 at 11:46

Swapping two columns not working

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