Decimals give different integrals than fractions; why?

[joshmccraney]

Just like the title says. Is this due to roundoff?

 

[Orodruin]

You really cannot expect us to be able to help you unless you give a specific example of code.

 

[joshmccraney]

The code is kind of long. Are you sure you want me to give it? There's not a computational difference that you're aware of for exact numbers vs decimals?

 

[jedishrfu]

Can't you illustrate it with a simpler example?

Its well known in computerdom that some numbers can't be represented properly in floating pt format so if Mathematica does a numerical computation with them them then computational error will creep into the calculation.

As an example, if you had some expression like $3.0*sin(x)*(1.0/3.0)$ and Mathematica symbolically reduces it to sin(x) then that result might be different from $(3.0*sin(x)) / 3.0$ where Mathematica didn't see the ONES identity and did the numerical computations of 3.0 * sin(x) then dividing by 3.0.

For larger numbers the result difference might be more pronounced as floating pt numbers kep to a certain limited digit precision.

 

[joshmccraney]

Can't you illustrate it with a simpler example?

Its well known in computerdom that some numbers can't be represented properly in floating pt format so if Mathematica does a numerical computation with them them then computational error will creep into the calculation.

As an example, if you had some expression like $3.0*sin(x)*(1.0/3.0)$ and Mathematica symbolically reduces it to sin(x) then that result might be different from $(3.0*sin(x)) / 3.0$ where Mathematica didn't see the ONES identity and did the numerical computations of 3.0 * sin(x) then dividing by 3.0.

For larger numbers the result difference might be more pronounced as floating pt numbers kep to a certain limited digit precision.

I'm not sure how to illustrate it with a simpler example, which shows how little I know about the issue. I can say when I plot a function (takes a lot of code to construct) one plot has max of 50 and the other has max 6000. Do you want me to post the code or pictures so you can see?

 

[Dale]

Mathematica does treat approximate and exact numbers differently. Usually approximate numbers make Mathematica evaluate it numerically using NIntegrate. In contrast an integrand with exact numbers will be solved analytically and then the limits of integration will be substituted at the end.

 

[jim mcnamara]

If you are going to rely on programming/software then you should really consider reading this article, it has been cited a huge number of times because of its importance:
What Every Computer Scientist Should Know About Floating-Point Arithmetic
https://www.itu.dk/~sestoft/bachelor/IEEE754_article.pdf

I think that Dale is trying to tell you something about your understanding of what Mathematica does behind the scenes. Consider it seriously.