方法一：常规计算

#include <stdio.h>
int main(){
	char c;
	int letters=0,space=0,digit=0,other=0;
	printf("请输入一行字符：\n");
	while((c=getchar())!='\n'){
		if(c>='a'&&c<='z'||c>='A'&&c<='Z')
			letters++;
		else if(c==' ')
			space++;
		else if(c>='0'&&c<='9')
			digit++;
		else
			other++;
	}
	printf("字母数：%d\n空格数：%d\n数字数：%d\n其他字符数：%d",letters,space,digit,other);
	return 0;
}

方法二：普通函数调用

#include <stdio.h>
#include <string.h> 
void count(char c[]){
	int letters=0,space=0,digit=0,other=0;
	int i;
	for (i = 0; c[i]; i++)
	{
		if(c[i]>='a'&&c[i]<='z'||c[i]>='A'&&c[i]<='Z')
			letters++;
		else if(c[i]==' ')
			space++;
		else if(c[i]>='0'&&c[i]<='9')
			digit++;
		else
            	other++;
	}
	printf("字母数：%d\n空格数：%d\n数字数：%d\n其他字符数：%d",letters,space,digit,other);
}
int main(){
	char c[32];
	printf("请输入一行字符：\n");
	gets(c);
	count(c);
	return 0;
}

方法三：指针函数调用

#include <stdio.h>
void count(char arr[]){
    int letters=0,space=0,digit=0,other=0;
    char *p=arr;
    char c;
    while(*p!='\n') {
      c =*p ;
        if(c>='a'&&c<='z'||c>='A'&&c<='Z')
        letters++;
        else if(c==' ')
        space++;
        else if(c>='0'&&c<='9')
        digit++;
        else
        other++;
        p++;
    }
    printf("字母数：%d\n空格数：%d\n数字数：%d\n其他字符数：%d",letters,space,digit,other);
}
int main(){
    char c[32];
    int i =0;
	printf("请输入一行字符：\n");
    while((c[i]=getchar())!='\n')
		i++;
    count(c);
    return 0;
}