MATLAB: Hi, guys. How would one extract the lower triangle of a matrix without using the tril function and witout a for loop

bsxfunlower trianglematrix manipulationrot90triangletril

I tried
B = zeros(size(A)); n=size(A,2);
for(i=1:n) B(end+1:end+n-i+1)=A(i:n,i); end
but I seem to be getting some sort of error.
Thanks

Best Answer

You could use bsxfun and rot90 to generate a matrix of indices:
>> N = 5;
>> K = 0; % select the diagonal
>> A = 1:N;
>> B = rot90(bsxfun(@plus,A,A(:)))-1;
Now lets try it on matrix of random values and extract only the lower diagonal elements:
>> X = rand(N);
>> X(B<=N+K)
ans =
    0.81472
    0.90579
    0.12699
    0.91338
    0.63236
     0.2785
    0.54688
    0.95751
    0.96489
    0.95717
    0.48538
    0.80028
    0.79221
    0.95949
    0.67874
And you can see that it extracts all of the lower-triangle values, matching those given by tril:
>> tril(X)
ans =
    0.81472            0            0            0            0
    0.90579       0.2785            0            0            0
    0.12699      0.54688      0.95717            0            0
    0.91338      0.95751      0.48538      0.79221            0
    0.63236      0.96489      0.80028      0.95949      0.67874
Note that this method also gives direct control over which diagonal you start with using the value K, just like the second argument of tril.
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