what is wrong with my 'While' function

i want to increase (n) until TT=s

    v=0.5;

    d=0.1;

    density=1000;

    w=0.446;

    t=0.2;

    rr=30.494*v^5-86.479*v^4+89.045*v^3-34.023*v^2+6.6418*v;

    va=v*(1-w);

    s=rr/(1-t);

    n=1;

    while n>1;

    n=n+1;

    j=va/(n*d);

    kt=0.392*(1-(j/0.95))^0.8;

    TT=kt*density*n^2*d^4;

    if TT=s;

        break

    end

    end

This is my program.... when i run it i received this error message '>> T=T Undefined function or variable 'T'.'

Answer 1

[Ilham Hardy]

I ran your code and found:

- Matlab is about precision, in terms that your while loops only break if the TT value is exactly the same as s. So the script will executes until n<12000 instead of stopping at the desired vessel speed. To circumvent the problem, check your script well! and use no exact value but a tolerance as shown by Azzi above.

- DO NOT name your variable i or j

- the variable n is usually expressed in rps instead of rpm.

See the code below

v=0.5;

d=0.1;

density=1000;

w=0.446;

t=0.2;

rr=30.494*v^5-86.479*v^4+89.045*v^3-34.023*v^2+6.6418*v;

va=v*(1-w);

s=rr/(1-t);

n=1;

while n<12000;

      n=n+1;

      j_=va/(n*d);

      kt=(0.392*(1-(j_/0.95)))^0.8;

      TT=kt*density*n^2*d^4;

      if TT>=(s-0.5) && TT<=(s+0.5) ; %example

          break
      end

end

Answer 2

[Azzi Abdelmalek]

   n=1;
   while n>1;

You will never be in the loop

Also, there is no variable T in your code

To test if TT is equal to s use

if TT==s

% or

if abs(TT-s)<=tolerence %or  if abs(TT-s)>=tolerence

Answer 3

[Matt J]

The error message is not being triggered by any of the lines you've shown. The command T=T appears nowhere in your posted code.

I suspect you meant to type this

 >>TT,

but accidentally typed this

 >>T=T,

Answer 4

[Image Analyst]

Lots and lots wrong with that code. First of all the line TT=s. It would normally be TT==s, except for the FAQ: http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F so let's get rid of that line totally - you don't even need it because we'll put the stopping condition on the while line, like I'll show you later.

Next, kt is complex number. Is that what you expected? So now TT is complex. That combined with my previous comments suggests something like

n=1;
TT = -inf;
while abs(TT) < s
  n=n+1;
  j=va/(n*d);
  kt=0.392*(1-(j/0.95))^0.8;
  TT = kt*density*n^2*d^4
end

but I think that's not what you want. Another thing to do with while loops is to put in a failsafe to prevent runaway infinite loops. Like a check on n, where you'll break if it's greater than a million, or whatever you think is the greatest number n will ever possibly get to.

while abs(TT) < s && n < 1000000

So fix all those things, post your new code, and then let's continue if you're still having problems.

[ameen]

this is my new code and i still have the same error message when i run it

v=0.5;

d=0.1;

density=1000;

w=0.446;

t=0.2;

rr=30.494*v^5-86.479*v^4+89.045*v^3-34.023*v^2+6.6418*v;

va=v*(1-w);

s=rr/(1-t);

n=1;

while n<12000;

n=n+1;

j=va/(n*d);

kt=0.392*(1-(j/0.95))^0.8;

TT=kt*density*n^2*d^4;

if TT==s;

    break

end

end

The error message is ' T=T

Undefined function or variable 'T'.'

although i don't have a variable named (T)

[Iain]

On what line is the error being thrown?

[Image Analyst]

You didn't read my answer, did you? First of all, there is no T=T line in your code. Secondly, you still have the TT==s line, which shows you didn't read the FAQ link I gave you. Third, kt and TT are still complex numbers. Please read my answer and try again.

[ameen]

i wrote the code in a new script

when i press run

i have this error in the command window

[Image Analyst]

No you don't, because I simply copied and pasted and it ran through 12000 iterations and quit, thanks to the one suggestion of mine that you did take which was to put in a failsafe (n<12000) - otherwise you would have got into an infinite loop and hung the computer. I ran the exact code you put there and there was no error whatsoever.

[ameen]

when i run it form the new script i have an error message in the command window

when i copied it and paste in the command window there is no error message but the i found that TT==s as i want and the value of n is 12000

what i want from this code is 'increasing (n) until TT=s and then giving me the value of (n) and (TT)

[Image Analyst]

But what does it mean when TT is a complex number a + i * b?

[ameen]

i don't want it complex number that's why i fixed by putting while 100<n<12000;

[Ilham Hardy]

IA,

I think the i and j is a real number. the parameters name might be confusing though.

Edit: He is missing the line

   j=va/(n*d);

Edit: No, he is not :)

[Image Analyst]

j = 1.385 the first time into the loop. So, if I plug that in for j to the command line, look what that does to kt:

K>> kt=0.392*(1-(1.385/0.95))^0.8
kt =
  -0.1698 + 0.1233i

kt is complex!

[Ilham Hardy]

Yes, you are correct for n = 2, but when n>2, parameter j becomes smaller than 1, which makes kt no longer complex.

Answer 5

[ameen]

Thank you all very much for your help and replys