#include<stdio.h>
int main(void)
{
    char ch;
    int ch_num=0,dig_num=0,oth_num=0;
    printf("Enter a string:");            /*scanf("%c",ch);这句话不用*/
    ch=getchar();
    do
    {
        if((ch>='A')&&(ch<='Z')||(ch>='a')&&(ch<='z'))
            ch_num++;
        else
            if((ch>='0')&&(ch<='9'))
                dig_num++;
            else
                oth_num++;
    }while(ch=getchar()!='\n');
    printf("%4d%4d%4d\n",ch_num,dig_num,oth_num);
    return(0);
}

while(ch=getchar()!='\n');
改成
while((ch=getchar())!='\n');
再编译运行试试看

你说这个是一个~，还有一个就是要把ch=getchar()放进if里面，谢谢啦