Diagonals in Matrices Matlab

i have a matrix A=zeros(400,400).

m=size(A,1)

A(1:m+1:end) = 1 % main diagonal

A(21:m+1:end) = -.25 % lower diagonal

A(8001:m+1:end) = -.25 %upper diag

A(2:m+1:end) = -.25 %close lower diag

A(401:m+1:end) = -.25 %close upper diag

The first three diagonals are continous and dont skip. However, i need the close lower and upper diagonals to be -.25 for every 2 instances. As in, the 3rd column of both close diagonals will be 0 and then the next two are -.25 followed by another 0 for the entire 400x400 matrix. NOTE: the close upper diagonal starts in column 2 while the lower starts in the first diagonal. EX. i need A(3,4)=0 ...A(6,7)=0... etc i am aware of the pattern i just dont know how solve in matlab

Answer 1

[Jan]

I'm not sure if I can follow your explanations. A short example might be useful.

The term "be -.25 for every 2 instances" might mean:

A(2:(2*m+2):end)   = -0.25;  % close lower diag
A(401:(2*m+2):end) = -0.25;  % close upper diag

What does this mean: "As in, the 3rd column of both close diagonals will be 0 and then the next two are -.25 followed by another 0"? With some guessing:

A(2:(3*m+3):end)     = -0.25;
A(2+m+1:(3*m+3):end) = -0.25;

[Buster]

This actually worked but it did every other instance when i need every three. would i just change the +2 to +3?