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'为什么i++放在sum上面就变成5150

#include <stdio.h>

int main()

{

    int i,sum=0;

    i=1;

    while(   i<=100  )  //循环条件应该是什么呢？

    {

        i++;

        sum=sum+i;

  

                 //这里是不是应该改变循环变量的值

    }

    printf("100以内所有整数之和为：%d\n", sum);

    return 0;

}


weixin_慕侠2446508

2021-08-07

源自：C语言入门  4-6

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weixin_慕前端3492105
2021-08-07

你把i++放在sum上面之后，程序是这样运行的：

（第一次）：i++就是i+1=2   然后i=2代入sum=sum+i中，即sum=0+2

（第二次）：i++就是i+1=3   然后i=3代入sum=sum+i中，即sum=2+3

但是实际上程序是这样运行才对的

（第一次）：i=1先代入sum=sum+i中，即sum=0+1，然后i++即i=2

（第二次）：i=2先代入sum=sum+i中，即sum=1+2，然后i++即i=3.


这里是一个循环体，也就是说循环100次，每次都比正确的多加了数

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#1

weixin_慕前端3492105

同时这里要注意一点就是： i++; 这一句单独成一行的时候，表示i最后+1了，但是未+1之前的i是没有用到的，因为;号就表示这一行已经结束了，只有这一行得到的结果i+1才能用到下一行去。因此在如果把i++放在sum前面，这样sum=0+2了，因为此时的i已+1了，上一行的i++；未代入的i=1因为上一行已经结束，所以就不能用了。 如果要i=1能用，那就要在;号之前，也就是语句结束之前加一个i的表达式 如：printf("%d\n",i++),这个时候就是i先代入表达式，然后i再+1了
2021-08-07 回复 有任何疑惑可以回复我~
#2

weixin_慕侠2446508 提问者

那题目变成 ++i; sum=sum+i; 那第一次运行是什么样的 i++和++i在这里体现不出区别吗
2021-08-07 回复 有任何疑惑可以回复我~
#3

weixin_慕前端3492105 回复 weixin_慕侠2446508 提问者

体现不出来，++i和i++如果单独成一行的话，实际上得出的结果是一样的，那就是i=i+1，其他因为分号;的原因，表示语句已经结束，未+1之前的i不能作用于上一行或者下一行，所以可以说单独成一行的话，++i和i++是一样的
2021-08-08 回复 有任何疑惑可以回复我~

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