How can I fix my for loop to iterate diagonally?

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

[Awais Saeed]

If I understood it right, you want to have some number at the diagonal of your matrix. Your code is already working. Define your N1 and N2 though. To optimze your code, remove M(1,1)=6 and add a break; after M(ii,jj)=6;

[Voss]

The best way to loop across the diagonal of a square matrix is with a single for loop, like this:

for ii = 1:(A1*A2)*2*band
    M(ii,ii) = 6; % set element (ii,ii) to 6 or whatever
end

See my answer for more examples as applied to your existing code.

[Karen Smith]

Wow thank you so much! This is just what i needed. Do you know how to do the other diagonal with a value of let's say 8?

[Stephen23]

Why use a loop? MATLAB already has a function for this:

M = diag(6*ones(1,7))

M = 7×7

6 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0 6

Or alternatively, some simple indexing works too:

M(1:8:numel(M)) = 7

M = 7×7

7 0 0 0 0 0 0 0 7 0 0 0 0 0 0 0 7 0 0 0 0 0 0 0 7 0 0 0 0 0 0 0 7 0 0 0 0 0 0 0 7 0 0 0 0 0 0 0 7

[Voss]

@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)));

Answer 1

[Voss]

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]

"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