Index exceeds the number of array elements (1)

Question

Yago Ferrara Román 2021-10-20

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https://ww2.mathworks.cn/matlabcentral/answers/1568248-index-exceeds-the-number-of-array-elements-1

评论： Jan 2021-10-21

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

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Answer 1

Jan 2021-10-20

1
链接

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https://ww2.mathworks.cn/matlabcentral/answers/1568248-index-exceeds-the-number-of-array-elements-1#answer_813023

编辑：Jan 2021-10-20

在 MATLAB Online 中打开

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);

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Yago Ferrara Román 2021-10-20

Thank you so much, I'm starting to use this program at college and I have zero experience

Jan 2021-10-21

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. :-)

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