Guassian Function, for loop only runs once???

a=[A,B];

for i=1:4;

a(2,i)=a(2,i)-(a(2,1)/a(1,1)).*a(1,i)

end

This is one of my first lines in my function but I do not know what this for loop only runs once A is a 3X3 matrix and B is a 3X1 which I combined for an augmented matrix

[Adam Danz]

The four-loop definitely runs 4 times.

As evidence, insert this line anywhere inside your for-loop and the iteration number will be displayed in the command window.

   fprintf('iteration #%d\n', i)

Only the 2nd row of 'a' will be affected by your code. Perhaps that's causing unexpected results?

[Abigail Osborne]

Thank you, it is running four times but after the first iteration it doesn't change values. I am working row by row, so I was waiting to ensure my second row worked before moving onto the third.

A=[1 2 3; 3 2 1; 1 2 2];

B=[3; 4; 5];

I have been using these numbers as practice values since I worked it out by hand previously, and the second row should change to 0 -4 -8 -5

But the code I had only changes the second row to 0 2 1 4

Answer 1

[Adam Danz]

You're probably indexing incorrectly and instead of using the variable 'i', you're accidentally using a hard-coded value (maybe 1, by accident?)

Here's why your values aren't changing.

On the first iteration, you are replacing the value a(2,1) with a zero (see below)

i = 1 
a(2,i) = a(2,i)-(a(2,1)/a(1,1)).*a(1,i)
%               |______________________|  this section is merely (Q/R) * R which just equals Q on the first iteration
% that reduces to 
a(2,i) = a(2,i) - a(2,i)  % which equals 0

On all following iterations, you're using that zero in the numerator of a fraction which will always equal 0 (see below)

i = 1 %, 3, 4 
a(2,i) = a(2,i)-(a(2,1)/a(1,1)).*a(1,i)
%               (  0   /a(1,1)).*a(1,i)  and that will always equal 0
% so   = a(2,i) - 0                        that will never change the value of a(2,i)

All of this results in only 1 change and that change happens on the first iteration in position a(2,1).

[Abigail Osborne]

Great! Thank you this was very helpful!