Generating Permutation Matrices in Octave / Matlab

I have been doing Gilbert Strang’s linear algebra assignments, some of which require you to write short scripts in MatLab, though I use GNU Octave (which is kind of like a free MatLab). I was trying out this problem:

permutationMatricesTo solve this quickly, it would have been nice to have a function that would give a list of permutation matrices for every n-sized square matrix, but there was none in Octave, so I wrote a function permMatrices which creates a list of permutation matrices for a square matrix of size n.

% function to generate permutation matrices given the size of the desired permutation matrices
function x = permMatrices(n)
x = zeros(n,n,factorial(n));
permutations = perms(1:n);
for i = 1:size(x,3)
x(:,:,i) = eye(n)(permutations(i,:),:);
end
endfunction
view raw permMatrices.m hosted with ❤ by GitHub

For example:

permMatrExample

The MatLab / Octave code to solve this problem is shown below:

% Solution for part (a)
p = permMatrices(3);
n = size(p,3); % number of permutation matrices
v = zeros(n,1); % vector of zeros with dimension equalling number of permutation matrices
% check for permutation matrices other than identity matrix with 3rd power equalling identity matrix
for i = 1:n
if p(:,:,i)^3 == eye(3)
v(i,1) = 1;
end
end
v(1,1) = 0; % exclude identity matrix
ans1 = p(:,:,v == 1)
% Solution for part (b)
P = permMatrices(4);
m = size(P,3); % number of permutation matrices
t = zeros(m,1); % vector of zeros with dimension equalling number of permutation matrices
% check for permutation matrices with 4th power equalling identity matrix
for i = 1:m
if P(:,:,i)^4 == eye(4)
t(i,1) = 1;
end
end
% print the permutation matrices
ans2 = P(:,:,t == 0)
view raw Section2_7_13.m hosted with ❤ by GitHub

Output:

op13a
Output for 13(a)
op13b
Output for 13(b)

 

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