#include?<stdio.h>
int?main()
{
????int?sum?=?0;??//定義計算結果變量sum
????int?i?=?1;????//定義循環數字變量i
????int?flag?=-1;?//定義符號狀態變量flag，，，這兒我習慣-1?
????while(i<100)
????{
???????
????????sum+=i*flag;
????????++i;
????????flag*=-1;
????}
????
????printf("sum=%d\n",sum);?//先輸出，后賦值。?
????i?=?1;??
????sum?=?0;
????do{
?????????//printf("i=%d\n",i*flag);
????????sum+=i*flag;
????????//printf("sum=%d\n",sum);
????????i++;
????????flag*=-1;
????}while(i<=100);
????printf("sum=%d\n",sum);
?i?=?1;?//先輸出，后賦值。?
?sum?=?0;?
????for(i=1;i<=100;i++)
????{
???????sum+=i*flag;
????????flag*=-1;
????}??
????printf("sum=%d\n",sum);?
????return?0;????
}

我改了一下你的代碼。

有什么再問吧

#include <stdio.h>
int main()
{
??? /* 小伙伴們：
?????????? 選擇你們認為最合理的循環結構完成功能吧 */
??? int sum = 0;? //定義計算結果變量sum
??? int i = 1;??? //定義循環數字變量i
??? int flag = 1; //定義符號狀態變量flag
???
??? //使用while循環
??? while(i<=100)
??? {
??????
??????? sum+=i*flag;
??????? i++;
??????? flag*=-1;
??? }
???
??? i = 1;? //重新初始化變量i
??? sum = 0;
??? //do-while循環
??? do{
???????? //printf("i=%d\n",i*flag);
??????? sum=sum+(i*flag);
??????? //printf("sum=%d\n",sum);
??????? i++;
??????? flag*=-1;
??? }while(i<=100);
???
??? i = 1;? //重新初始化變量i
??? sum = 0;
??? //使用for循環
??? for(i=1;i<=100;i++)
??? {
?????? sum+=i*flag;
??????? flag*=-1;
??? }?
????
??? printf("sum=%d\n",sum);
??
??? return 0;???
}