# Diagonals in Matrices Matlab

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Robert Herrell on 13 Sep 2021
Commented: Robert Herrell on 13 Sep 2021
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

Jan on 13 Sep 2021
Edited: Jan on 13 Sep 2021
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;
Robert Herrell on 13 Sep 2021
This actually worked but it did every other instance when i need every three. would i just change the +2 to +3? 