How can I fix my for loop troubles in Matlab?

[danbone87]

Hi, I have this for loop where i want it to iterate for a variable ap = ap-1 -f(x)

and i want ap-1 to replace ap in each iteration.

It's pretty simple but i just can't get it to work out. any help would be appreciated and here's my code btw.

clc
a=1
fa=exp(a)*((-a^2)+(2*a)-2)+2.723684615938410;
fprimea=exp(a)*(a^2);
ap=a+(fa/fprimea);
tol=1*(10^-6);
f=abs(ap-a);
n=15;

for k=1:10
ap=a-(fa/fprimea);
f=abs(ap-a)
a=ap
end

 

[chroot]

From the look of it, you need to learn how to write functions. Look them up in the help.

Right now you're just calculating the fa and fprimea junk once.

- Warren

 

[danbone87]

I've been working with those and i keep getting this dang error "index must be a positive integer or logical."

not sure what that means... any ideas?

 

[chroot]

It probably means you're trying to use 0 to index a vector or matrix. MATLAB indices begin at 1.

- Warren

 

[danbone87]

sorry, but I am not sure what that means at all :[

 

[chroot]

Well, I can't say for sure unless you show me the code and indicate what line is causing the error.

- Warren

 

[danbone87]

function [f ap]=NR(fa,fprimea,a,tol,f,iterations)

a=1
fa=exp(a)*((-a^2)+(2*a)-2)+2.723684615938410;
fprimea=exp(a)*(a^2);
ap=a+(fa/fprimea);
tol=1*(10^-6);
f=abs(ap-a);
n=15;
iterations= input('how many interations? ')

for k=1:iterations
ap=a-(fa(a)/fprimea(a)); <----- error is in this line
f=abs(ap-a)
a=ap
end

end


error: ? Attempted to access fa(0.998012); index must be a positive integer or logical.

Error in ==> matlabsss at 14
ap=a-(fa(a)/fprimea(a));

 

[chroot]

In this line:

ap=a-(fa(a)/fprimea(a));

Are you just trying to multiply fa by a and fprimea by a? If so, you need to use * to represent multiplication. MATLAB is interpreting your use of parentheses as an array lookup, e.g. fa(a) is the ath element of fa.

- Warren

 

[danbone87]

okay, i have a function going and all that and it's still doing the same thing. it's not modifying the a to make it equal to the ap of the last iteration...

 

[chroot]

a only exists inside your function. It disappears as soon as the function is done running. You're passing ap out.

- Warren

 

[danbone87]

I got it!

thanks so much for the help