Inverting Capitalization of letter in C - c

Question

ok so i was asked to invert capitalization in C using a function inver_caps. My function works and prints the new letter correctly, but im having trouble as to why in the main it will not print correctly?
Any ideas?
void invert_caps (char letter);
int main(void){
char lettermain;
printf("Enter a letter: ");
scanf(" %c", &lettermain);
invert_caps(lettermain);
printf("The invert of the letter is %c \n", lettermain);
system("PAUSE");
return 0;
}
void invert_caps (char letter){
printf("\nletter is %d\n",letter); /*this was used for debugging*/
if ((int)letter >=65 && (int)letter<=90){
letter = (int)letter+32;
}else{
letter = (int)letter - 32;
}
printf("\nnew letter is %d or %c\n",letter, letter); /*this was used for debugging*/
return letter;
}

Answer 1

You are doing pass by value (copy value), that doesn't reflects change made in calling at caller function. Do
(1) pass by pointer (pass address), Or
(2) simply return converted value from function.
Side note: Don't use ascii values in code, just use char constants to keep code readable (you don't have to remember ascii value).
For example I write (2) solution for you (I believe that will be easy for you presently, avoiding pointer at this stage).
to understand the code read comments:
char invert_caps (char letter){
// ^ added return type, its not void now
if ( letter >= 'A' && letter<= 'Z'){ // not using ASCII value but char Constants
letter = letter + ('a' - 'A'); // its more readable
// Note 32 = 'a' - 'A' that is 97 - 54
}
else {
if ( letter >= 'a' && letter<= 'z'){// Add this case, to be safe is good practice
letter = letter - ('a' - 'A');
}
else
letter = '\0'; // if letter is neither Upper or lower alphabetic case
} // then convened it into nul symbol (exception case)
return letter; // added a line
}
// I removed unnecessary typecasts and debug statements
In main() you need to call it like at same place:
lettermain = invert_caps(lettermain);
// ^ return value assigned to variable `lettermain`
return is a key word in C. The return statement terminates the execution of a function and returns control to the calling function. A return statement can also return a value to the calling function and from function invert_caps() we are returning converted value.

Answer 2

Here is a small routine using library functions to detect and change letter case.
#include <ctype.h>
int invert_case(int c)
{
return islower(c) ? toupper(c) : isupper(c) ? tolower(c) : c;
}
If you don't like the nested ?: ternary operators, here is the same logic using if.
int invert_case(int c)
{
if (islower(c)) {
return toupper(c);
}
if (isupper(c)) {
return tolower(c);
}
return c;
}
Note: this can be made even simpler, see Dave's comment below.

Answer 3

basic reason you are not getting the inversion in the main method is you are only passing a copy of lettermain to invert_caps() method.
Rather than that do the following,
void invert_caps (char letter);
int main(void){
char lettermain;
printf("\n=========Question 8=========\n");
printf("Enter a letter: ");
scanf(" %c", &lettermain);
lettermain=invert_caps(lettermain);
printf("The invert of the letter is %c \n", lettermain);
system("PAUSE");
return 0;
}
char invert_caps (char letter){
printf("\nletter is %d\n",letter);
if ((int)letter >=65 && (int)letter<=90){
letter = (int)letter+32;
}else{
letter = (int)letter - 32;
}
printf("\nnew letter is %d or %c\n",letter, letter); /*this was used for debugging*/
return letter;
}

Answer 4

Because when you pass the lettermain variable from main, it's copied.
You need to either return the new character, or pass the variable by reference.