How to quickly fill in a matrix

Hi there,

Does anybody have a "fix" on how to make this function go faster? Essentially, I am trying to fill up the matrix Hpsi using the vector phi in a special way.

Thanks in advance, Andy.

T= 1000;
phi=[1:10];
Hpsi = speye(T);
 phi=phi';
 pl=length(phi);
 for i=1:pl
 Hpsi = Hpsi + sparse((i+1):T,1:(T-i),phi(i)*ones(1,T-i),T,T);
 end

Answer 1

[Christine Tobler]

Use spdiags:

spdiags(repmat(phi([end:-1:1 1])', T), -10:0, T, T)

[akkp]

I find that your code is slower than what I proposed:

clear all;
clc;

T=1000;
phi=1:10;
pl=length(phi);

disp('my code:');
tic;

Hpsi = speye(T);
for i=1:pl
    Hpsi = Hpsi + sparse((i+1):T,1:(T-i),phi(i)*ones(1,T-i),T,T);
end

toc;

disp('your code:');

tic;
Hpsi2 = spdiags(repmat(phi([end:-1:1 1])', T), -10:0, T, T);
toc

my code:
Elapsed time is 0.026569 seconds.
your code:
Elapsed time is 0.042326 seconds.

[Christine Tobler]

Sorry, I forgot an input:

spdiags(repmat(phi([end:-1:1 1])', T, 1), -10:0, T, T)