# Can someone tell me what is wrong with this?

3 views (last 30 days)
Ewa Jablonska on 13 Nov 2019
Answered: Ewa Jablonska on 13 Nov 2019
x1=5;
tau=0.1;
k=1;
while x(k)-x(k+1)>0.01
k=k+1;
x(k+1)=x(k)-tau*(2*x(k)-2);
end
figure(1)
plot(x,'b*-.');
grid;

Ewa Jablonska on 13 Nov 2019
That's the task and I have it using for loop, but I can not figure out how to do it with while loop.
M on 13 Nov 2019
Why do you want it to work with a while loop ?
To see a comparison between for and while loops, you can have a look here:
Ewa Jablonska on 13 Nov 2019
Because our lecturer said we should figure it out with while loop. :/

Ewa Jablonska on 13 Nov 2019
Thanks for effort, i mixed up + with -. :/ Correct:
%Value of step coefficient tau=0.1
x(1)=5;
k=1;
tau=0.1;
while x(k)>1.01
k=k+1;
x(k+1)=x(k)-tau*(2*x(k)-2);
end
plot(x,'b*-');
grid;

Fangjun Jiang on 13 Nov 2019
The problem is, vector x is not defined. When you do x1=5, I think you meant x(1)=5. But even with that, when k=1, x(2) is not defined yet.
The solution is to add these lines at the begining.
N=100;x=zeros(N,1);x(1)=5;

#### 1 Comment

Ewa Jablonska on 13 Nov 2019
%Value of step coefficient tau=0.1
x(1)=5;
k=1;
tau=0.1;
while x(k)>0.1
k=k+1;
x(k+1)=x(k)+tau*(2*x(k)+2);
end
plot(x,'b*-')
error:
Index exceeds array bounds.
x(k+1)=x(k)+tau*(2*x(k)+2);

David Hill on 13 Nov 2019
function x = Minimum(tau,xx)
x(1)=xx;
x(2)=x(1)-tau*(2*x(1)-2);
k=2;
while abs(x(k)-x(k-1))>0.01
x(k+1)=x(k)-tau*(2*x(k)-2);
k=k+1;
end
end

Ewa Jablonska on 13 Nov 2019
and how to see the result?
David Hill on 13 Nov 2019
you can see x in the workspace,
type 'disp(x)' and return,
or just type 'x' and return.

Steven Lord on 13 Nov 2019
I'm guessing from the fact that you define x1 but don't use it that the x variable may not exist. You may have been thinking / hoping that MATLAB would use x1, x2, etc. when it saw x(1), x(2), etc. but it does not.
The fact that you're using x(k+1) (not x(k-1)) in your while condition also smells a bit strange. You may be walking off the end of the x array.
But if correcting these does not resolve the problem, please state more clearly the specific difficulty you're experiencing.
Do you receive an error message (text displayed in red)? If so, tell us on which line the error occurs and show us the full and exact text of the error, all the text displayed in red exactly as it's displayed in the Command Window.
Do you receive a warning (text displayed in orange)? If so, do the same as for an error but show us all the text displayed in orange.
Do you get a different answer than you expected? If so, show us the answer you received and the answer you expected and explain why you expected what you expected.
Does something else happen? If so, what?

#### 1 Comment

Ewa Jablonska on 13 Nov 2019
%Value of step coefficient tau=0.1
x(1)=5;
k=1;
tau=0.1;
while x(k)>0.1
k=k+1;
x(k+1)=x(k)-tau*(2*x(k)+2);
end
plot(x,'b*-')
error: Index exceeds array bounds.
x(k+1)=x(k)-tau*(2*x(k)+2);
No oragne just red.
Tasks given and must use while loop, because we did for "for loop":
The ponit is that function should end at specific value but it does not even with higher go to value.

R2019a

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by