C++ do/while loop with two different conditions - c++

Question

I want the do while loop to check to see if the entered input is R OR P. I think it is checking for both and when I get to that part when I run, it pauses for a minute and then I get "CPU Limit Exceeded (core dumped). On another related note, am I in danger of breaking something?
/************************************************/
/* Name: servcode */
/* Description: Get service type */
/* Parameters: N/A */
/* Return Value: servcode */
/************************************************/
char servcode()
{
char servcode = 'a'; // Define variable for service code
char serviceyn = 'n'; // Define variable for user verify
int i = 1; // Define variable for sentinel loop
do {
cout << "\n" << "\n" << "Please enter your service code, [R]egular or [P]remium: " << "\n";
cin >> servcode;
while ((servcode != 'R', 'P') && (i < 3));
{
cout << "\n" << "Error - invalid service code, please try again.";
cout << "\n" << "Please enter your service code: ";
cin >> servcode;
i++;
if (i == 3)
{
cout << "\n" << "Too many invalid attempts, program terminating." << "\n"
<< "Have a nice day. " << "\n" << "\n";
exit (0);
} //end if
} //end while
cout << "\n" << "You entered: " << servcode << "\n"
<< "Is that correct? [y,n]";
cin >> serviceyn;
} while (serviceyn != 'y'); // end do/while loop
return servcode;
}

Answer 1

The correct syntax is:
while (servcode != 'R' && servcode != 'P' && i < 3)
Note the expanded comparison and the removal of the semicolon at the end:
(servcode != 'R', 'P') is valid C++ but doesn't do what you're expecting it to do;
the semicolon makes the statement into a loop with an empty body, so it continues executing forever since the loop condition never changes.

Answer 2

Change:
while ((servcode != 'R', 'P') && (i < 3));
to:
while ((servcode != 'R') && (servcode != 'P') && (i < 3))
Note the removal of an unwanted semicolon.

Answer 3

You need to do something like this:
while(something != 'a' && something != 'b')
You're using the comma operator, which discards the results of every expression except the last, so this:
while(something != 'a', 'b')
Will compare something with a, ignore the result, and use 'b' as the condition of the loop. 'b' is a non-zero value, so it's always true and the loop goes on forever (or until memory runs out, or something else stops it).

Answer 4

(servcode != 'R', 'P')
Should be:
(servcode != 'R') && (servcode != 'P')
Kudos to PaulR for correction. My brain is not in gear.

Answer 5

Let's break down this statement and see what it's doing. I've put in some extra punctuation that does not change the meaning of the statement.
while (((servcode != 'R'), ('P')) && (i < 3)) { };
The comma operator separates two different expressions and returns the value of the second one. The first part probably does what you expect, but the second part is just a literal character 'P' which is always true!
The semicolon at the end of the statement marks the body of what will be executed as the while loop. It's a do-nothing which does not change the value of servcode or i, so obviously once you enter the loop you will never leave it. This is a common mistake.