why the first loop doesnt change the string..

[transgalactic2]

i written the first loop so it will transform every upper case of the string into lower case

why its not working?

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

typedef struct node node;
struct node{
	int value;
	struct node * next;
};
 int countletters(char *str);
void main()
{
	int g;
	char  str[18]="aabx bXcb bBxaDAa";
	g=countletters(str);
}


int countletters(char *str)
{
  char ch='a';
  char *st;
  int cnt=0;
  for(st=str;*st!='\0';st++)
  {
	  if ((*str>='A')&&(*str<='Z'))
	  {
        *str=*str+32;  
	  }
  }
   for(;ch<='z';ch++)
   {

      for(st=str;*st!='\0';st++)
	  {
           if (*st==ch)
		   {
              cnt++;
			  break;
		   }
	  }
   }
   return cnt;
}

[Sharke]

Simple error:

#include <ctype.h>

for (st = str; *st != '\0'; st++)
{
    if (isupper(*st))
        *st = tolower(*st);
}

In your loop, you changed the value of *str, when the loop increments *st. So only the first character would be changed to lower case. Also I changed it to use isupper() and tolower() from <ctype.h> in the condition.