# How can I fix my for loop to iterate diagonally?

17 views (last 30 days)
Karen Smith on 18 Dec 2021
Commented: Voss on 26 Dec 2021
Hello all, I am extremely new to matlab and would like to know if anyone can help me fix/clarify my code.
I am trying to built a matrix of zeros with the A1 and A2 variables, so it can be any size, and then use a for loop to read a variable across the left diagonal, with indexes (1,1), (2,2), (3,3). For now, I have put it in manually at (1,1), but I would like to be able to fix this, as it won't work. Here is my code currently. I tried a for loop to basically say if (1,1) , (2,2) then it is a value of 6, but cannot understand. Any help is appreciated!
A1=2;
A2=2;
band=1;
e=1;
eps=2;
epss=3;
t=4;
tp=5;
M = zeros((A1*A2)*2*band,(N1*N2)*2*band)
M(1,1)=6
% left diagonal (1,1),(2,2),(3,3).....
for ii=1:(A1*A2)*2*band
for jj=1:(A1*A2)*2*band
if ii==jj
M(ii,jj)=6;
end
end
end
Voss on 26 Dec 2021
@Karen Smith To do the other diagonal:
N = (A1*A2)*2*band;
for ii = 1:N
M(ii,N-ii+1) = 8; % set element (ii,N-ii+1) to 8 or whatever
end
Or, adapting @Stephen's second suggestion (use linear indexing):
N = (A1*A2)*2*band;
M(N:(N-1):numel(M)) = 8;
Both of those will set the "other" diagonal elements of an existing matrix M of size N-by-N.
If instead you want to create a new matrix with the "other" diagonal elements equal to some value and all other elements equal to zero, you can use @Stephen's first suggestion (use the diag() function) and flip() the result:
N = (A1*A2)*2*band;
M = flip(diag(8*ones(1,N)));

Voss on 18 Dec 2021
Assuming that N1 and N2 in the code should be A1 and A2 (so that M is a square matrix), the easiest way to set the values along the diagonal to be [1 2 3 ...] might be:
A1=2;
A2=2;
band=1;
n = (A1*A2)*2*band;
M = zeros(n);
for ii = 1:n
M(ii,ii) = ii;
end
display(M);
M = 8×8
1 0 0 0 0 0 0 0 0 2 0 0 0 0 0 0 0 0 3 0 0 0 0 0 0 0 0 4 0 0 0 0 0 0 0 0 5 0 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0 0 7 0 0 0 0 0 0 0 0 8
Or to set all the diagonal elements to be 6:
A1=2;
A2=2;
band=1;
n = (A1*A2)*2*band;
M = zeros(n);
for ii = 1:n
M(ii,ii) = 6;
end
display(M);
M = 8×8
6 0 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0 0 6
% An alternative:
M = 6*eye(n);
display(M);
M = 8×8
6 0 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0 0 6
In general, to create a square matrix of zeros and set the diagonal elements to some values vals, you can do this kind of thing:
vals = [90 91 92 93 94 95 96 97];
A1=2;
A2=2;
band=1;
n = (A1*A2)*2*band;
M = zeros(n);
for ii = 1:n
M(ii,ii) = vals(ii);
end
display(M);
M = 8×8
90 0 0 0 0 0 0 0 0 91 0 0 0 0 0 0 0 0 92 0 0 0 0 0 0 0 0 93 0 0 0 0 0 0 0 0 94 0 0 0 0 0 0 0 0 95 0 0 0 0 0 0 0 0 96 0 0 0 0 0 0 0 0 97
Stephen23 on 24 Dec 2021
Edited: Stephen23 on 24 Dec 2021
"In general, to create a square matrix of zeros and set the diagonal elements to some values vals, you can do this kind of thing:"
The general MATLAB approach for that task:
vals = [90,91,92,93,94,95,96,97];
M = diag(vals)
M = 8×8
90 0 0 0 0 0 0 0 0 91 0 0 0 0 0 0 0 0 92 0 0 0 0 0 0 0 0 93 0 0 0 0 0 0 0 0 94 0 0 0 0 0 0 0 0 95 0 0 0 0 0 0 0 0 96 0 0 0 0 0 0 0 0 97