为什么会跳过scanf?解决办法

为什么会跳过scanf?
#include<stdio.h>

void main()
{
int x;
while(1)
{

printf("Please input a integer:\n");
scanf("%d",&x);
if(x>=1000 || x<=0)
printf("The number you input is invalid.\n");
else
if(x>=100)
printf("The number has 3 digits.\n");
else if(x>=10)
printf("The number has 2 digits.\n");
else
printf("The number has 1 digit.\n");

}
}
源代码如上,当输入整数时反应正常, 但当输入小数(如3.4),就会陷入快速死循环, 好像跳过了scanf语句. 但把代码改成如下时, 反应正常. 不知是为什么, 我刚学编程,请大虾指导一下.
#include<stdio.h>

void main()
{
float x;
while(1)
{

printf("Please input a integer:\n");
scanf("%f",&x);
if(x>=1000 || x<=0)
printf("The number you input is invalid.\n");
else
if(x>=100)
printf("The number has 3 digits.\n");
else if(x>=10)
printf("The number has 2 digits.\n");
else
printf("The number has 1 digit.\n");

}
}


[解决办法]

C/C++ code

#include <stdio.h>int main(){    int x;    while (1)    {        printf("Please input a integer:\n");        scanf("%d", &x);        if (x>=1000 || x <=0)            printf("The number you input is invalid.\n");        else if (x >= 100)            printf("The number has 3 digits.\n");        else if (x >= 10)            printf("The number has 2 digits.\n");        else        printf("The number has 1 digit.\n");        fflush(stdin);    //清空一下缓冲区就好了    }    return 0;}
[解决办法]
最好不要使用fflush(stdin); 
首先看以下程序：
C/C++ code
#include <stdio.h>int main( void ){    int i;    for ( ; ; )     {        fputs("Please input an integer: ", stdout);        scanf("%d", &i);        printf("%d\n", i);    }    return 0;}