Create a matrix with ones on any offset diagonal [duplicate]

Answer 1

triu(B+1,1)-triu(B+1,2)

creates ones on the first upper off-diagonal, and

tril(B+1,-2)-tril(B+1,-3)

does the same for the first lower off-diagonal.

Answer 2

diag has this functionality built in:

diag(ones(4,1),1)
ans =

     0     1     0     0     0
     0     0     1     0     0
     0     0     0     1     0
     0     0     0     0     1
     0     0     0     0     0

diag(ones(4,1),-1)

ans =

     0     0     0     0     0
     1     0     0     0     0
     0     1     0     0     0
     0     0     1     0     0
     0     0     0     1     0

Where the syntax of diag(V,k) is: V is the vector to be put on the diagonal (be it ones, or any odd vector), and k is the label of the diagonal. 0 is the main diagonal, positive integers are increasingly further away upper diagonals and negative integers the same for the lower diagonals; i.e. k=1 gives the first upper diagonal, k=-4 gives the lower left corner in this example.

For completeness, if you just want the indices instead of a full matrix (since you suggested you wanted to insert a vector into a present matrix) you can use the following function:

function [idx] = diagidx(n,k)
% n size of square matrix
% k number of diagonal
if k==0 % identity
    idx = [(1:n).' (1:n).']; % [row col]
elseif k>0 % Upper diagonal
    idx = [(1:n-k).' (1+k:n).'];
elseif k<0 % lower diagonal
    idx = [(1+abs(k):n).' (1:n-abs(k)).'];
end
end

where each row of idx contains the indices for the matrix.