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Lorenzo

How to get a very small value rather than 0?

Asked by Lorenzo
on 19 Nov 2012

Dear all; I'm trying to work out a script which should give a ver small value as a result. The problem is that I keep getting 0 instead…

Any help would be really appreciated.

Here's the script:

rho=7860;
B=-1.00000000011701;
BL=23.562;
Bj=46.9362549800797;
k=1;
int1(k)=(exp(-2*BL(k))*(2*exp(2*BL(k))*sin(2*BL(k))+(-4*exp(3*BL(k))-4*exp(BL(k)))*sin(BL(k))+(4*exp(BL(k))-4*exp(3*BL(k)))*cos(BL(k))+exp(4*BL(k))+8*BL(k)*exp(2*BL(k))-1))/(8*Bj(k));
int2(k)=-(exp(-2*BL(k))*(2*exp(2*BL(k))*sin(2*BL(k))+(4*exp(3*BL(k))+4*exp(BL(k)))*sin(BL(k))+(4*exp(BL(k))-4*exp(3*BL(k)))*cos(BL(k))-exp(4*BL(k))+1))/(8*Bj(k));
int3(k)=-(exp(-BL(k))*((exp(2*BL(k))-1)*sin(BL(k))-exp(BL(k))*((exp(BL(k))+exp(-BL(k)))/2)^2+exp(BL(k))*cos(BL(k))^2))/(2*Bj(k));
m_cap1=rho.*(int1+B.^2.*int2+2.*B.*int3);

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2 Answers

Matt J
Answer by Matt J
on 19 Nov 2012
Edited by Matt J
on 19 Nov 2012

You haven't said what variable in your code is the problematic one.

Regardless, computers are inevitably discrete things. If you get too close to 0, then the number will have to get rounded to zero in order to be represented in floating point.

  2 Comments

Matt Fig
on 21 Nov 2012

Lorenzo Lore comments:

Sorry, the variable is m_cap1

Matt J
on 21 Nov 2012

If Matt Fig's suggestion doesn't work for you (because you don't have the Symbolic Toolbox), then there is this and

http://www.mathworks.com/matlabcentral/fileexchange/22725-variable-precision-integer-arithmetic

and others like it on the FEX.

However, it would be wise to re-evaluate your approach. It looks very dubious to be working with numbers as huge as

    >> exp(4*BL)
    ans =
       8.5386e+40

and still requiring precision<1.


Answer by Matt Fig
on 19 Nov 2012

Use the symbolic toolbox if you need more precision than double.

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