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Can someone tell me what is wrong with this?

Asked by Ewa Jablonska on 13 Nov 2019
Latest activity Answered by 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;

  6 Comments

Ewa Jablonska 2019 年 11 月 13 日
That's the task and I have it using for loop, but I can not figure out how to do it with while loop.
12.png
M
2019 年 11 月 13 日
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 2019 年 11 月 13 日
Because our lecturer said we should figure it out with while loop. :/

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4 Answers

回答者: Ewa Jablonska 2019 年 11 月 13 日
 採用された回答

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;

  0 Comments

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回答者: Fangjun Jiang
2019 年 11 月 13 日

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 2019 年 11 月 13 日
%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.
Error in zad12p (line 7)
x(k+1)=x(k)+tau*(2*x(k)+2);

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回答者: David Hill
2019 年 11 月 13 日

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

  2 Comments

Ewa Jablonska 2019 年 11 月 13 日
and how to see the result?
David Hill
2019 年 11 月 13 日
you can see x in the workspace,
type 'disp(x)' and return,
or just type 'x' and return.

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回答者: Steven Lord
2019 年 11 月 13 日

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 2019 年 11 月 13 日
%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.
Error in zad12p (line 7)
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":
12.png
The ponit is that function should end at specific value but it does not even with higher go to value.

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