while loop condtion and termination in C programming - c

Question

I am trying to write a while loop which only runs as user input is 'F' or 'C'. However, my while loop seems doesn't work. Could you show me what is wrong with my scanf and while loop condition
#include <stdio.h>
#include <stdlib.h>
int main()
{
char response;
while (response == 'F' || response == 'C')
{
printf(" Please enter F or C\n");
scanf(" %c", &response);
}
return 0;
}

Answer 1

response is uninitialized. Your code hasn't even gotten to the scanf() yet. Consider changing it to a do-while loop.

Answer 2

You have to initialize response, try this
#include <stdio.h>
#include <stdlib.h>
int main()
{
char response;
response = 'F';
while (response == 'F' || response == 'C')
{
printf(" Please enter F or C\n");
scanf(" %c", &response);
}
return 0;
}
and you can of course do it this way
#include <stdio.h>
#include <stdlib.h>
int main()
{
char response;
/* since you don't check for `scanf()`'s return value,
* this prevents undefined behavior
*/
response = 'F';
do {
printf(" Please enter F or C\n");
scanf(" %c", &response);
} while (response == 'F' || response == 'C');
return 0;
}

Answer 3

Your loop runs as long as response is 'F' or 'C'. What you need is
while (response != 'F' && response != 'C')
Also, response is not initialized. Start with
char response = '\0';

Answer 4

There are two common ways to do a terminate-on-input loop in C:
do {
...get input...
do_stuff();
} while (...input does not indicate exit...);
or
while (1) {
...get input...
if (...input indicates exit...) break;
do_stuff();
}
I think the latter might work better for your case:
while (1) {
printf(" Please enter F or C\n");
scanf(" %c", &response);
if ('F' == response) {
do_f();
} else if ('C' == response) {
do_c();
} else {
break;
}
}