# include <stdio.h>
# include <math.h>
int main(void)
{
    int val,i;
    do
    {
    scanf("%d", &val);

    printf("%d = 1", val);
    for (i=2; i<=(int)sqrt(val); ++i)
        if (val%i == 0)
        {
            printf(" * %d", i);
            val = val/i;
            i = 1;
        }
        printf("\n");
    } while (1);



    return 0;
}
做了点小改动i<=(int)sqrt(val) 可以减少运算量

#include<stdio.h>
int main()
{
    int i=2,sum,d;
    scanf("%d",&sum);
    printf("%d=",sum);
    while(i<=sum){                   /*原来的代码这里面没有等号,如果是121,那么11<11不成立就会直接跳出去*/
        if(sum%i!=0)
            i=i+1;
        if(sum%i==0){
            sum=sum/i;
            if(sum==1)break;
            printf("%d*",i);
        }

    }
    if(sum==1)
        d=sum*i;
    printf("%d",d);
    return 0;
}其实i-=1可以不要,当时没注意.