How to use if-then with matrix indices

Hi, I got a matrix n by n of a symbolic value a. And I want to change the value of a based on the matrix indices.

syms a
n=50;
z=ones(n)*a;
for i=1:1:n
for j=1:1:n
if j=1 or j=i then a=4
else a=8

Could you please help? thanks

Answer 1

[Guillaume]

You certainly don't need a loop to generate your a.

a = ones(n) * 8;                   %default value everywhere
[rows, cols] = ndgrid(1:n);        %matrices of row and column indices
a(rows == 1 | rows == cols) = 4;   %substitute when condition is met

Note that because your i and j variables are meaningless, it's not clear whether you meant it to be column 1 or row 1 that get sets to 4. I assume it was rows but it's obvious how to change it to columns. Using variable names with meaning makes the code much clearer. Moreover i and j are actually functions in matlab, so are not recommended as variable names anyway.

Answer 2

[Andrei Bobrov]

n = 50;
z = ones(n)*8;
z(:,1) = 4;
z(eye(n)>0) = 4;

[Shan Chu]

sorry, actually a=4 when j=1 and j=i

[Andrei Bobrov]

corrected

Answer 3

[Torsten]

n = 50;
z = ones(n)*8;
z(:,1)=4;
z(eye(n)==1)=4;

Best wishes

Torsten.