Diagonal matrix, wishing to solve Ax=b

I am not getting any errors, but I am certainly doing something wrong.

I wish to get output that is the result of x for x=b/A

where A is an nxn matrix

b a vector

I am trying to create function called solveDiag(A,b)

instead of getting desired output i get the original matrix A

so my output is:

"ans =

2

the answer is

ans= [3 0 0

0 2 0

0 0 7]"

This what I have:

function [A,b]=solveDiag(A,b)
%A=nxn diagonal matrix
%b=column vector
%n is number of unknowns in matrix
A=[3 0 0 ; 0 2 0 ; 0 0 7];
b=[4 8 21]
for i = 1, 2,...,n do
        x(1,:)=b(i)/A(i,i);
    fprintf('the answer is',x(1,:))
end
end

[Walter Roberson]

MATLAB does not use that syntax for for loops

[Walter Roberson]

You are not changing A so of course you get the same value back.

[CS]

thank you, yes i'm aware now.. I changed the syntax on loops. & yes, i don't want to get A back at all. How can i fix to get x back haha

[Walter Roberson]

function [A,b]=solveDiag(A,b)

says that the function is to receive two positional arguments, the first named A and the second named b. It also says that it is to output up to two outputs, the first of which is to be whatever value A has when the function returns, and the second is to be whatever value b has when the function returns.

If you want to return a different variable's value then that variable name would go on the left side instead of [A,b]

[CS]

TY. i have a better understanding by taking your advice & doing some additional research.

The function now returns appropriate value for x, though called "c" considering like so:

(also, im am still curious why it gives me "ans=2" output all the time)

[Walter Roberson]

You still need to fix your for loop syntax.

[CS]

okay, yes.. So, i tried changing that to "for i=1:n" and it kept giving me an error for that..

so i put it back this way & at least got the correct x value back somehow haha.. hmm, but if i do not change my syntax for i, i will never be able to expand this code (i suppose)

could you help me a little with that? if i change what i have exactly to the correct "for i=1:n" i get error.

[Walter Roberson]

Your comments imply that n is being passed in to the function but you do not list it as an input variable in the function header?

[CS]

Aha! That makes perfect sense, working on it now..

[Walter Roberson]

The syntax

for i = 1, 2,...,n do
        x(1,:)=b(i)/A(i,i);

is the same as

for i = 1
    display(2), ...,n do
    x(1,:)=b(i)/A(i,i);

where ...,n do is a line continuation indicator. After the line continuation indicator, everything else on the line would be ignored as a comment. Putting the lines together removing the line continuation indicator, your code is equivalent to

for i = 1
    display(2), x(1,:)=b(i)/A(i,i);

And that is why you are getting the ans = 2 .

Remember, in MATLAB, comma outside of a parameter list or [] indicates a statement separator and that the output of the statement just before it is to be displayed.

for i = 1, 2, 3, 4, 5

is

for i = 1
    display(2)
    display(3)
    display(4)
    display(5)

and would be completely different than

for i = [1, 2, 3, 4, 5]

because the commas inside the [] list building operation do not indicate statement separator.

for i = 1:5

would be almost the same as for i = [1, 2, 3, 4, 5] but the version with the : operator has some optimization compared to the version with the [] list.

[CS]

Thank you all very much, we have managed to create a function that delivers desired output!

Ignore the first line in command, i first used () to declare c instead of [].

Answer 1

[Les Beckham]

I thought that I had commented on this question earlier with something that should work for you. Here is the code that I think should work for you.

A = diag([3 2 7])
b = [4 8 21];
b/A

[CS]

yes, i apologize i believe i accidently posted the same question twice and have since deleted the other.

While I appreciate your advice, and it does very well work for me. I have used this code to see what my answer should be, but I have to create a function to perfom for any values A,b.

[Rik]

You haven't deleted the question, you have simply edited it to remove any usefulness to any future reader. Don't do that. There is a reason that you can only delete your own question if it hasn't received any answers yet. A stranger on the internet spent time reading, understanding, and answering your question. Removing it is a poor showing of gratitude.

[Abel Medina]

lol