C语言运算时类型自动转换及精度损失问题

C语言运算时类型自动转换及精度损失问题

#include<stdio.h>
#include<math.h>
int main(){
   unsigned long long a=0;
   int i;
    for(i=0;i<64;i++){
        a+=pow(2,i);
    }
    unsigned long long b=a/25000;
    printf("%llu\n",a);
    printf("%llu",b);
    return 0;
}
/*
a: 0
b: 0
*/

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

int main() {
    unsigned long long a = 0;
    int i;
    for(i = 0; i < 64; i++) {
        double temp = pow(2, i);
        if(i == 63) {
            printf("temp: %lf\n", temp);
            printf("a: %llu\n", a);
            unsigned long long s = a + temp;
            printf("s: %llu\n", s);
        }
        a += temp;
    }
    unsigned long long b = a / 25000;
    printf("%llu\n", a);
    printf("%llu\n", b);
    return 0;
}

/*
temp: 9223372036854775808.000000
a: 9223372036854775808
s: 0
*/

9223372036854775808 + 9223372036854775808 = 18446744073709551616
18446744073709551616 已经超过了 unsigned long long 能表示的范围，加法溢出了

#include<stdio.h>
#include<math.h>
int main(){
   unsigned long long a=0;
   int i;
    for(i=0;i<63;i++){
     a+=(unsigned long long)pow(2,i);//没有进行强制类型转换，则该步的加法转成double类型进行相加，结果失真。
    }
    unsigned long long b=a/25000;
    printf("%llu\n",a);
    printf("%llu",b);
    return 0;
}