Index in position 2 exceeds array bounds. Index must not exceed 5

hello evreyone I need some help :)

here is my code. the error is in this line, line 16%maksimum=max(abs(U(i:end,i))); I used this matrix to test the code [1 2 3 4 1;2 4 6 8 2;3 6 9 12 3;4 8 12 16 4;8 6 4 2 8;4 3 2 1 4], the code works for some matrices but not for all. I want it to work like matlab's lu() function :) but it gave me some errors like "Index in position 2 exceeds array bounds. Index must not exceed 5."

% This code decomposes matrix A into upper and lower triangular matrices.
clear;
clc;
disp('Matlab code for LU decomposition of a matrix using ''gaussian elimination''and ''partial pivoting''')
disp('Click Enter to continue.')
pause
clc;
A = input("enter a matrix: ");
tic
[n,m]=size(A);
U=A;%upper triangular matrix
L=eye(n);%lower triangular matrix
P=eye(n);%permutation matrix
for i = 1:n
    if true %partial pivoting 
        maksimum=max(abs(U(i:end,i)));
        for k=1:n
           if maksimum == abs(U(k,i))
               U([i k],:) = U([k i],:);
               P([i k],:) = P([k i],:);
           end
        end
    end
    for j = i+1:n
        L(j,i)=U(j,i)/U(i,i);
        U(j,:) = U(j,:)-U(j,i)*U(i,:)/U(i,i);
    end
end
disp('permutation matris(P):')
disp(P)
disp("A'in ust ucgen matris(U):")
disp(U)
disp("A'in alt ucgen matris(L):")
disp(L)
toc

Answer 1

[Adam Danz]

Let's look at the line that's reported to be causing the problem and the error message.

maksimum=max(abs(U(i:end,i)));

Index in position 2 exceeds array bounds. Index must not exceed 5

1. "Index in position 2" means the 2nd index value in u(i:end, i) which is the last i. That i is causing a problem.
2. "Index in position 2 exceeds array bounds" means that the last i is larger than the array size. Position 2 indexes the columns of an array. So at some point i becomes larger than the number of columns in U.
3. "Index must not exceed 5" : Matlab tells us that U has 5 columns so the error happens when i==6.

Now that we understand the error message, how do we fix it? This is where your expertise comes in as the owner of the code. Here are some possible problems:

• Maybe U isn't the size it should be. Maybe it should contain more than 5 columns.
• Maybe U is the right size but the indexing is the problem. Maybe i shouldn't ever be >5, or, greater than the number of columns in U.
• The problem might be caused at this line where n is the number of rows, not columns [n,m]=size(A);
• But this part also looks fishy: U(i:end,i); is that really what you want to do (I don't know), to index rows i:end of column i?

[Mohammed Izedin Mohammed]

you are right I solved it by changing the index n to min(n,m) in the first for loop :) thnaks a lot

% This code decomposes matrix A into upper and lower triangular matrices.
clear;
clc;
disp('Matlab code for LU decomposition of a matrix using ''gaussian elimination''and ''partial pivoting''')
disp('Click Enter to continue.')
pause
clc;
A = input("enter a matrix: ");
tic
[n,m]=size(A);
U=A;%upper triangular matrix
L=eye(n);%lower triangular matrix
P=eye(n);%permutation matrix
for i = 1:min(n,m)
    if true %partial pivoting 
        maksimum=max(abs(U(i:end,i)));
        for k=1:n
           if maksimum == abs(U(k,i))
               U([i k],:) = U([k i],:);
               P([i k],:) = P([k i],:);
           end
        end
    end
    for j = i+1:n
        L(j,i)=U(j,i)/U(i,i);
        U(j,:) = U(j,:)-U(j,i)*U(i,:)/U(i,i);
    end
end
disp('permutation matris(P):')
disp(P)
disp("A'in ust ucgen matris(U):")
disp(U)
disp("A'in alt ucgen matris(L):")
disp(L)
toc