Power function returns 1 less result

问题:

Whenever I input a number in this program the program return a value which is 1 less than the actual result ... What is the problem here??

#include<stdio.h>
#include<math.h>
 int main(void)
 {
     int a,b,c,n;

     scanf("%d",&n);

     c=pow((5),(n));

    printf("%d",c);

 }

回答1:

pow() returns a double, the implicit conversion from double to int is "rounding towards zero".

So it depends on the behavior of the pow() function.

If it's perfect then no problem, the conversion is exact.

If not:

1) the result is slightly larger, then the conversion will round it down to the expected value.

2) if the result is slightly smaller, then the conversion will round down which is what you see.

solution:

Change the conversion to "round to nearest integer" by using rounding functions

c=lround(pow((5),(n)));

In this case, as long as pow() has an error of less than +-0.5 you will get the expected result.

回答2:

pow() takes double arguments and returns a double.

If you store the return value into an int and print that, you may not get the desired result.

回答3:

If you need accurate results for big numbers, you should use a arbitrary precision math library like GMP. It's easy:

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

int main(void) {
    int n;
    mpz_t c;

    scanf("%d",&n);
    mpz_ui_pow_ui(c, 5, n);
    gmp_printf("%Zd\n", c);

    return 0;
}