user3639557 - 11 months ago 56

C++ Question

I am puzzled, when evaluating the following function it produces number till F(0.8, 172, 1), but when I increase 172 to 173, the result becomes infinite. I suspect there is a numerical precision problem?

`double F(double d, int c, int t) {`

// base cases

if ((c==1 && t==1) || (c==0 && t==0))

return 1.;

if (c==0 || t==0)

return 0.;

if (t>c)

return 0.;

return F(d,c-1,t-1) + (c-1 - t*d)*F(d,c-1,t);

}

Answer Source

I do not know what your function is supposed to do, but given the arguments: `F(0.8, 172, 1)`

the return value is `4.41861e+306`

which is just short of the maximum value a `double`

can represent:

```
// 1.79769e+308
std::cout << std::numeric_limits<double>::max() << std::endl;
```

When `172`

is replaced with `173`

, the return value exceeds the maximum value a `double`

can represent and becomes positive infinity. This is made clear by changing the return type of `F`

to be `long double`

which results in the value `7.56466e+308`