Whike loop does not work and there is not error message

Question

MichaelO 2018-8-7

0
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https://ww2.mathworks.cn/matlabcentral/answers/413949-whike-loop-does-not-work-and-there-is-not-error-message

评论： MichaelO 2018-8-7

I am trying to do a loop until the last value and the value before the last one are equals. I run the code without the while (manually) and does work alright but when I run with while doesn't work. In this case the convergence is in 11 but if I change the data could be in 100 or maybe more.

If you can help me I would really appreciate. Thanks in advance.

P = [0.5,0.3,0.2;0.3,0.3,0.4;0.1,0.5,0.4];
pi=2;
py=P(pi,:);
TM=[]
kk=1
pky_new=py*P^kk
pky=[0 0 0]
while (pky_new(1,1)-pky(1,1))<0.000001
    kk=kk+1;
    pky=pky_new;
    pky_new=py*P^kk;
    TM=[TM;pky_new]
end
disp(['converge at y + ' num2str(kk)])

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jonas 2018-8-7

编辑：jonas 2018-8-7

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The condition is never met.

(pky_new(1,1)-pky(1,1))

ans =

0.2800

kk =

1

Perhaps it should say larger than (>)?

MichaelO 2018-8-7

Oh!!!! You are right. Thank you very much. I loose about 4 hours and I didn't realize.

Thanks again!

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Answer 1

Rik 2018-8-7

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You switched the condition: it is false on the first iteration and true when your loop should exit, which is the reverse from what it should be.

Also, in general you want a difference, so you should use abs. I don't understand the true goal of your code, so I don't know if that is what you should do.

P = [0.5,0.3,0.2;0.3,0.3,0.4;0.1,0.5,0.4];
pi=2;
py=P(pi,:);
TM=[];
kk=1;
pky_new=py*P^kk;
pky=[0 0 0];
while (pky_new(1,1)-pky(1,1))>0.000001% or maybe while abs(pky_new(1,1)-pky(1,1))>0.000001
    kk=kk+1;
    pky=pky_new;
    pky_new=py*P^kk;
    TM=[TM;pky_new];
end
disp(['converge at y + ' num2str(kk)])