MathGroup Archive 2012

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: Bug in NIntegrate[]?


NIntegrate first tries to evaluate its argument symbolically and evaluates to 1 since your definition defines f as one for symbolic input.

You could change your default to

f[x_?NumericQ] = 1;

to preclude evaluation for symbolic input; however, it would be better to define f using Piecewise.

f[x_] := Piecewise[{{1, 0 <= x <= 1}}]


Bob Hanlon


On Jun 15, 2012, at 3:41 AM, GS <vokaputs at gmail.com> wrote:

> I define the function f[x] as follows:
>
> f[x_] := 0 /; x < 0 || x > 1;
> f[x_] := 1
>
> It is zero outside of the interval [0,1]. This can be verified by plotting
> Plot[f[x], {x, -1, 2}]
>
> Now I integrate it from -1 to 2:
> In[270]:= NIntegrate[f[x], {x, -1, 2}]
> Out[270]= 3.
>
> The result should be 1, but it is 3. Clearly Mathematica ignores the fact that f[x] is zero outside of [0,1].
>
> This caused a lot of headache for me recently when I encountered such behavior in one of my research code.
> GS
>



  • Prev by Date: Re: Bug in NIntegrate[]?
  • Next by Date: Re: Bug in NIntegrate[]?
  • Previous by thread: Re: Bug in NIntegrate[]?
  • Next by thread: Re: Bug in NIntegrate[]?