01 #include<stdio.h>
02 #include<string.h>
03 
04 /*
05  65-90 => ASCII 'A' to 'Z'
06  97-122 => ASCII 'a' to 'z'
07 
08  32 => Difference between lower case and upper case letter.
09 
10  'a' >> 97
11  'A' >> 65
12  ----------
13     32
14 
15 */
16 
17 
18 
19 void interchange(char[]);
20 
21 int main()
22 {
23     char str[50];
24     printf("Enter any String : ");
25     gets(str);
26 
27     printf("nBefore Interchange : %s",str);
28     interchange(str);
29     printf("nAfter Interchange : %s",str);
30 
31     return 0;
32 }
33 
34 void interchange(char str[])
35 {
36     int str_len=strlen(str);
37     int temp,i;
38     for(i=0; i<str_len; i++)
39     {
40         temp=(int)str[i];
41         if(temp>=65&&temp<=90)
42         {
43             str[i]=(char)temp+32;
44         }
45         else if(temp>=97&&temp<=122)
46         {
47             str[i]=(char)temp-32;
48         }
49     }
50 }

 OUTPUT :
 
 Enter any String : ThIs iS a dEMo !!!
 
 Before Interchange : ThIs iS a dEMo !!!
 After Interchange : tHiS Is A DemO !!!