#include<
stdio.h>
main()
{
int i,sum=0;
for(i=1;i<=1000;i+=2)
sum += i;
for(i=2;i<=1000;i+=2)
sum -= i;
printf("s = %d.\n",sum);
}
另,因为1-2 + 3-4 +5-6 ..... +999-1000 = (1-2)+ (3-4) + ...... +(999-1000)
= -1 +(-1) + ...... + (-1) = -500。 所以也可以直接
#include<stdio.h>
main()
{
int i,sum=0;
for(i=1;i<=1000;i+=2)
sum += -1;
printf("s = %d.\n",sum);
}