C 程序检查字符是否为小写字母并转换为大写字母，反之亦然

Question

C Program to check if character is a lower case letter and convert to upper case letter and vice versa

why this works?

(it seems to be working if i use if statement twice but not if i use if-else statement)

#include <stdio.h>
#include <stdlib.h>

int main()
{
   char ch , k ;
   printf("enter an alphabet : ");
   scanf("%c",&ch);

   if (ch>=97 && ch<=122)
   {
     printf(" \n small letter");
     k=ch-32;
     printf(" \n after conversion %c",k);
   }

   if (ch>=65 && ch<=90)
   {
     printf(" \n capital letter");
     k=ch+32;
     printf(" \n after conversion %c",k);
   }

  return 0;
}

but this displays a random value.

#include <stdio.h>
#include <stdlib.h>

int main()
{
char ch , k ;
printf("enter an alphabet : ");
scanf("%c",&ch);

if (ch>=97 && ch<=122)
{
  printf(" \n small letter");
  k=ch-32;
  printf(" \n after conversion %c",k);
}

else (ch>=65 && ch<=90);
{
  printf(" \n capital letter");
  k=ch+32;
  printf(" \n after conversion %c",k);
}

return 0;
}

Answer 1

if (ch>=97 && ch<=122) it will only work with ASCII codes. It is not portable. C has special functions for this task:

#include <ctype.h>

int swapCase(int ch)
{
    return isupper((unsigned char)ch) ? tolower((unsigned char)ch) : toupper((unsigned char)ch);
    /*
    //or
    if(isupper((unsigned char)ch))
        ch = tolower((unsigned char)ch);
    else
        ch = toupper((unsigned char)ch);
    return ch;
    */
}

and some helper function and the main program to demonstrate how it works:

char *strSwapCase(char *str)
{
    char *wrk = str;
    if(str)
    {
        while(*str) 
        {
            *str = swapCase(*str);
            str++;
        }
    }
    return wrk;
}


int main (void){
    char x[] = "asDrgdfDFGHFDSdfgf3546&&&434fdgffyhfghfdfsdgsfdDFDGdfgdf";

    printf("before: %s\n", x);
    printf("after : %s", strSwapCase(x));
}

https://godbolt.org/z/E4oe99sGd

Answer 2

Your second code, as written, does not compile. If you meant to use "else if", it should be written:

else if (ch>=65 && ch<=90)

Correcting this line causes your code to both compile and behave as expected. As it stands, I'm not sure how your code is running and giving you a "random value", but you may be using some unusual compiler that's discarding the statement after your else ?