#include<stdio.h>
#include<ctype.h>
int get_letter(char);
int main(void)
{
    char ch;
    printf("Enter a string by analysis:");
    scanf("%c",&ch); //输入C
    printf("Now %c No. %d\n",ch,get_letter(ch));
    return 0;
}
int get_letter(char ch)
{
    if(isalpha(ch))
        return tolower(ch)-'a'+1; //如果输入的是c为毛返回的是3？应该是c才对吧？我的理解是c-a=b;b+1=c;
    else
        return -1;
}

char:c-b+1;
ASCⅡ:63-61+1=3;