Method other than for loop?

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Derek 2011-6-17

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My code is quite long and outputs two values called beam1 and beam2. The values of these variables are dependant on another variable labeled length.

What I want my code to do is output a length value that allows beam1-beam2<0.0001. As of now I am using a for loop to cycle through a range of different length values to output the ones that fit my criteria. However this method takes an extremely long time to run.

Do you have any suggestions that I could use instead of a for loop that might help the program take less time to run?

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Sean de Wolski 2011-6-17

Don't name your variable 'length' as that is a very useful MATLAB function that you don't want to have overwritten.

Sean de Wolski 2011-6-17

Other than that we're probably going to have to see the code (at least the relevant portions) to help.

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Walter Roberson 2011-6-17

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You could possibly use something like fmincon to minimize on (beam1-beam2).^2 with a tolerance of 0.0001^2 . Ummm -- can your code come out with beam2 > beam1 ?

Does your computation have a lot of local minima? If so then a more powerful minimizer might be needed.

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Derek 2011-6-21

Yes that is correct. I need to find a "length" value which causes |Psf(1)-Psb(1)|<0.0001.

Currently I am looping through different length values,

for length=1.7:0.01:1.9

and have an if statement telling the program to output the proper length that satisfys the condition as well as outputting Psb(1) and Psf(1).

My loop is outputting length=1.86 (which i already know is correct), I just need a faster way to do this.

Also, I keep getting the error,

"??? Attempted to access Pb1(1880); index must be a positive integer or logical.

Error in ==> cutandpaste2 at 143

Pb1(N)=5.6e-3;"

when I run the loop. It is not always at Pb1(1880) and changes depending on where I begin the loop. I have tried doing a long format (no help). I still get the optimal length with this error but no output for Psb and Psf.

finally for Walter,

g1=nSf*h*(Sesf*n2(N-x+1)-Sasf*n1(N-x+1)-alphaSf/(nSf*Nt))*Psf(N-x+1)*Nt;

gg1=nSb*h*(Sesb*n2(N-x+1)-Sasb*n1(N-x+1)-alphaSb/(nSb*Nt))*Psb(N-x+1)*Nt;

Walter Roberson 2011-6-21

In Pb1(N)=5.6e-3; your N has been calculated as an expression involving non-integers. The calculated value is very close to 1880 (or whatever) but is not *exactly* an integer. You should avoid such calculations. Meanwhile, you can work-around using round()

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