MATLAB: Creating a tridiagonal matrix

matrix manipulationtridiagonals

I am currently trying to create a 500*500 matrix in matlab with diagonals a=-1, b=4, c=2. My teacher has said that the best way to go about it is using loops, but is there a coded in function to use?

Best Answer

"My teacher has said that the best way to go about it is using loops"
Why on earth would they say that? Either diag :
>> N = 10;
>> a = -1;
>> b = 4;
>> c = 2;
>> M = diag(a*ones(1,N)) + diag(b*ones(1,N-1),1) + diag(c*ones(1,N-1),-1)
M =
  -1   4   0   0   0   0   0   0   0   0
   2  -1   4   0   0   0   0   0   0   0
   0   2  -1   4   0   0   0   0   0   0
   0   0   2  -1   4   0   0   0   0   0
   0   0   0   2  -1   4   0   0   0   0
   0   0   0   0   2  -1   4   0   0   0
   0   0   0   0   0   2  -1   4   0   0
   0   0   0   0   0   0   2  -1   4   0
   0   0   0   0   0   0   0   2  -1   4
   0   0   0   0   0   0   0   0   2  -1
or indexing would likely be simpler:
>> M = zeros(N,N);
>> M(  1:1+N:N*N) = a;
>> M(N+1:1+N:N*N) = b;
>> M(  2:1+N:N*N-N) = c
M =
  -1   4   0   0   0   0   0   0   0   0
   2  -1   4   0   0   0   0   0   0   0
   0   2  -1   4   0   0   0   0   0   0
   0   0   2  -1   4   0   0   0   0   0
   0   0   0   2  -1   4   0   0   0   0
   0   0   0   0   2  -1   4   0   0   0
   0   0   0   0   0   2  -1   4   0   0
   0   0   0   0   0   0   2  -1   4   0
   0   0   0   0   0   0   0   2  -1   4
   0   0   0   0   0   0   0   0   2  -1
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