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Matlab loop help

Posted on 2008-10-22
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Hey

i dont really understand whats happening in this example code i have. i dont understand matlabs implicit algebra. If you look at the attatched code can you please tell me how to write the bottom part of it with just "for loops" instead of the built in matlab algebra so i can better understand whats happening.

how do i write the:

  x(i) = ( b(i) - A(i,i+1:n)*x(i+1:n) ) / A(i,i);

line of the loop:

for i = n-1:-1:1
  x(i) = ( b(i) - A(i,i+1:n)*x(i+1:n) ) / A(i,i);
end  


as a series of nested loops. when i it like this i think i'll be able to understand whats happening more easily and then hopefully i'll be able to play with the function!

thanks
function x = UpperTriangularSolve(A,b,n)
 
% function x = UpperTriangularSolve(A,b,n)
%
% Solve the system  A x = b  where A is assumed to be upper triangular,
% i.e. A(i,j) = 0 for j < i, and the diagonal is assumed to be nonzero,
% i.e. A(i,i) \= 0.
%
% ARGUMENTS:  A   upper triangular n x n matrix
%             b   right hand side column n-vector
%             n   integer matrix dimension
%
% RETURNS:    x   column n-vector solution
 
  x = zeros(n,1);
  x(n) = b(n)/A(n,n);
 
for i = n-1:-1:1
  x(i) = ( b(i) - A(i,i+1:n)*x(i+1:n) ) / A(i,i);
end

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Question by:Ashwee
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Ashwee earned 0 total points
ID: 22776012
erm i kind of worked it out :P

for i = n-1:-1:1
  f = 0;
  x(i) = b(i);
  
  for m = i+1:n
    m;
    f = f + A(i,m)*x(m);
  end
 
  x(i) = x(i) - f;
  x(i) = x(i) / A(i,i);
end

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