语句中为什么不能这样写，num==0。求分析下这个过程

Question

#include
#include
#include

int whilenum(int num)
{
int i = 0;
while (num)
{
num /= 10;
i++;
}
return i;
}

int dowhilenum(int num)
{
int i = 0;
do
{
num /= 10;
i++;
}
while (num);
return i;
}

int fornum(int num)
{
int i = 0;
for (; num; num /= 10)
{

i++;
}
return i;
}

int gotonum(int num)
{
int i = 0;
AAA:if (num)
{
num /= 10;
i++;
goto AAA;
}
return i;
}

void main()
{
int num;
scanf_s("%d", &num);
printf("\nwhilenum=%d", whilenum(num));
printf("\ndowhilenum=%d", dowhilenum(num));
printf("\nfornum=%d", fornum(num));
printf("\ngotonum=%d\n", gotonum(num));
system("pause");
}

以上是准确语句，为什么不能改成这样？
int whilenum(int num)
{
int i = 0;
while (num==0)
{
num /= 10;
i++;
}
return i;
}

当改为num==0时，执行起来， printf("\nwhilenum=%d", whilenum(num));
不管我输入num是多少，输出的值就会变零

Answer 1

你这是
while (num==0){ //只有当num == 0的时候才执行

     num /= 10; 
     i++;

}