# Index exceeds the number of array elements (1)

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Yago Ferrara Román on 20 Oct 2021
Commented: Jan on 21 Oct 2021
I`m trying to do a function to solve a lower triangular matrix but whe I try to run the code, it says there's a problem.
function [x]=sistriinferior(U,b,n)
x(1) = b(1)/U(1,1);
for i=2:n
s=0;
for j=1:n-1
s= s + U(i,j)*x(j); %apparently the problem is in this line
end
x(i) = (b(i)-s)/U(i,i);
end
end

Jan on 20 Oct 2021
Edited: Jan on 20 Oct 2021
Replace
for j = 1:n-1
by
for j = 1:i-1
By the way, you can write
s = 0;
for j = 1:i - 1
s = s + U(i, j) * x(j);
end
as
s = U(i, 1:i - 1) * x(1:i - 1); % Dot product
Preallocate the output x before the loop to avoid the very expensive iterative growing of arrays:
x = zeros(n, 1);
##### 2 CommentsShowHide 1 older comment
Jan on 21 Oct 2021
Do you know Matlab Onramp? This is a useful tutorial.
The debugger helps also to examine what's going on: Set a breakpoint on the left side of the editor. Then you can step through the code line by line during it is processed and control the values of the used variables.
Mistakes like the exchanged index in your case are hard to find, because your brain knows, what you wanted to type. This impedes the reading psychologically. In such cases it is the best option to ask colleagues, who are not involved or foreigners of the forum, who see the problem in a second. :-)