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cmathpowcalculation
Prince Billy Graham
Prince Billy Graham

Reputation: 3578

Pow function returning wrong result

When I use the pow() function, sometimes the results are off by one. For example, this code produces 124, but I know that 5³ should be 125.

#include<stdio.h>
#include<math.h>

int main(){
    int i = pow(5, 3);
    printf("%d", i);
}

Why is the result wrong?

Upvotes: -2

Views: 4801

Answers (4)

Bathsheba
Bathsheba

Reputation: 234635

pow(x, y) is likely implemented as exp(y * log(x)): modern CPUs can evaluate exp and log in a couple of flicks of the wrist.

Although adequate for many scientific applications, when truncating the result to an integer, the result can be off for even trivial arguments. That's what is happening here.

Your best bet is to roll your own version of pow for integer arguments; i.e. find one from a good library. As a starting point, see The most efficient way to implement an integer based power function pow(int, int)

Upvotes: 4

Sujikanth
Sujikanth

Reputation: 133

Use Float Data Type

#include <stdio.h>
#include <math.h>

int main()
{
    float x=2;
    float y=2;
    float p= pow(x,y);
    printf("%f",p);
    return 0;
}

Upvotes: 2

Spektre
Spektre

Reputation: 51835

Your problem is that you are mixing integer variables with floating point math. My bet is that the result of 5^3 is something like 124.999999 due to rounding problems and when cast into integer variable get floored to 124.

There are 3 ways to deal with this:

  1. more safely mix floating math and integers

    int x=5,y=3,z;
z=floor(pow(x,y)+0.5);
// or
z=round(pow(x,y));

    but using this will always present a possible risk of rounding errors affecting the result especially for higher exponents.

  2. compute on floating variables only

    so replace int with float or double. This is a bit safer than #1 but still in some cases is this not usable (depends on the task). and may need occasional floor,ceil,round along the way to get the wanted result correctly.

  3. Use integer math only

    This is the safest way (unless you cross the int limit). The pow can be computed on integer math relatively easily see:

Upvotes: 9

goodvibration
goodvibration

Reputation: 6206

You can use this function instead of pow:

long long int Pow(long long int base, unsigned int exp)
{
    if (exp > 0)
        return base * Pow(base, exp-1);
    return 1;
}

Upvotes: -4

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