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# I need to produce a matrix that has one's along the diagonal from the upper right to the lower left

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DJ V on 12 Nov 2016
Commented: Walter Roberson on 13 Nov 2016
I need to produce a matrix along a square matrix of size "n" that has one's along the diagonal from the upper right to the lower left. I can't use "for" or an "if" statement. Any idea?
What doesn't work is:
function Y = reverse_diag(n)
zeroes = zero(n);
x = zero;
x(1:1:n,n:-1:1) =1;
Y = x;
end
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### Accepted Answer

DJ V on 12 Nov 2016
Thanks, but use of diag is explicit;y forbidden.
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Walter Roberson on 13 Nov 2016
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### More Answers (2)

Walter Roberson on 12 Nov 2016
n:n-1:something
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Walter Roberson on 13 Nov 2016
Another hint:
Z = zeros(3,3);
Z(sub2ind([3 3], [1 2 3], [3 2 1])) = 1

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Ahmet Cecen on 12 Nov 2016
x(1:1:n,n:-1:1) =1;
You cannot index points inside a matrix like that. MATLAB indexing reads like this:
Matrix(a,b) => For every a corresponding to every b.
So what you are doing is, say for the first row, fill every column (n:-1:1), then for the second row and so forth.
What you need to do is either use a for loop manually, or use the function:
diag()
followed by the function:
fliplr()
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