C语言isalnum()函数：判断字符是否为英文字母或数字

头文件：#include <ctype.h>

isalnum() 用来判断一个字符是否为英文字母或数字，相当于 isalpha(c) || isdigit(c)，其原型为：
    int isalnum(int c);

【参数】c 为需要检测的字符。

【返回值】若参数c 为字母或数字，若 c 为 0 ~ 9  a ~ z  A ~ Z 则返回非 0，否则返回 0。

注意，isalnum()为宏定义，非真正函数。

【实例】找出str 字符串中为英文字母或数字的字符。

#include <ctype.h>
main(){
    char str[] = "123c@#FDsP[e?";
    int i;
    for (i = 0; str[i] != 0; i++)
        if(isalnum(str[i]))
            printf("%c is an alphanumeric character\n", str[i]);
}

输出结果：
1 is an apphabetic character
2 is an apphabetic character
3 is an apphabetic character
c is an apphabetic character
F is an apphabetic character
D is an apphabetic character
s is an apphabetic character
P is an apphabetic character
e is an apphabetic character