C++ while loop insight request - c++

Question

basically I am having weird trouble with my while loop near the beginning of the program which checks for user validation on their choice of activity. When they choose the first activity and complete it, it works fine, but when they complete the second activity, it will go into runtime and keep requesting the user to input a valid choice, even though they haven't even gotten the chance to input a choice. Any tips?
#include <iostream>
using namespace std;
int main()
{
const int DIGITS_CHOICE = 1, IDENTIFIER_CHOICE = 2, DOUBLE_CHOICE = 3, EXIT_CHOICE = 4;
int choice;
int userNumber, storedNumber, factor = 10, digitCounter = 0, subtractor;
char ch;
do
{
cout << "\n\n\t\tPlease choose an option:\n\n"
<< "1. How many digits?\n"
<< "2. Is this a valid C++ Identifer?\n"
<< "3. Is this a double letter word?\n"
<< "4. Exit\n";
cout << endl << "Choice: ";
cin >> choice;
while (choice < DIGITS_CHOICE || choice > EXIT_CHOICE)
{
cout << endl << "Please enter a valid menu option: ";
cin >> choice;
}
if (choice != EXIT_CHOICE)
{
switch (choice)
{
case DIGITS_CHOICE:
cout << "Please enter an integer: ";
cin >> userNumber;
storedNumber = userNumber;
if (userNumber < 10)
{
digitCounter = 1;
}
else
{
while (userNumber != 0)
{
subtractor = userNumber % factor;
if (subtractor > 0)
{
userNumber = userNumber - subtractor;
factor *= 10;
digitCounter++;
}
else
{
userNumber = 0;
}
}
}
cout << storedNumber << " has " << digitCounter << " digit(s)." << endl;
factor = 10;
digitCounter = 0;
break;
case IDENTIFIER_CHOICE:
cout << "Please enter an identifier and press [Enter] immediately after. ";
cin >> ch;
if (ch >= 0 || ch <= 9 || ch <= 'a' || ch >= 'z' || ch <= 'A' || ch >= 'Z' || ch != '_')
{
if (ch >= 0 || ch <= 9)
{
cout << "Not a valid identifier." << endl;
cout << "Identifiers cannot start with a digit." << endl;
ch = '\n';
}
else
{
cout << "Not a valid identifier." << endl;
cout << "Inavlid character." << endl;
ch = '\n';
}
}
while (ch != '\n')
{
if (ch >= 'a' || ch <= 'z' || ch >= 'A' || ch <= 'Z')
{
cin.get(ch);
}
}
break;
case DOUBLE_CHOICE:
break;
}
}
} while (choice != EXIT_CHOICE);
return 0;
}
Also the program isn't complete yet. the third option has nothing and the 2nd option is almost complete. the first activity though is complete :)

Answer 1

Your check for valid characters is too broad, and doesn't really make sense:
if (ch >= 0 || ch <= 9 || ch <= 'a' || ch >= 'z' || ch <= 'A' || ch >= 'Z' || ch != '_')
Every possible value of ch is going to be greater than or equal to zero, so this expression is equivalent to (true || a || b || c || ... || z) and it's always going to resolve to true.
Instead, see if it's below 'A', between 'Z' and 'a' or beyond 'z' and if so, it's invalid.
Also, when checking if it's a digit, you need to check if it's ≥ '0' and ≤ '9' as characters. It's important that you compare it to the character representation of 0 and 9 because the value of the character '0' not actually 0 (it turns out it's actually 48) and likewise with '9':
if ( ch < 'A'
|| (ch > 'Z' && ch < 'a')
|| ch > 'z')
{
if (ch >= '0' && ch <= '9')
{
cout << "Not a valid identifier." << endl;
cout << "Identifiers cannot start with a digit." << endl;
ch = '\n';
}
else
{
cout << "Not a valid identifier." << endl;
cout << "Invalid character." << endl;
ch = '\n';
}
}
It's not really clear what the check after that is meant to do? Is it only meant to allow letters? That seems strange after saying "Identifiers cannot start with a digit." anyway:
if (ch >= 'a' || ch <= 'z' || ch >= 'A' || ch <= 'Z')
This has essentially the same issue where every character is going to be either above 'a' or below 'z' or both, so this will always resolve to true. Instead, use && to check for being within a range:
if ((ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z'))
Hopefully that addresses your question.

Answer 2

The logic is not great; i is valid, but your code says it's not. It at least goes back to the menu just fine. Your code is also behaving like it will respond immediately as letters are being typed. That is not the case. It won't print anything until the user presses Enter.
It seems the issue is your variable ch being a char. If I type anything that's longer than a single character, what will happen is that the single character gets evaluated, and the remaining characters I typed remain in the input stream. It looks like you are attempting to handle that, but it's not working. I am not going to spend time delving into the why, partly because it's complex, partly because I don't know the full intricacies of istream behavior.
What I will say is that if you want to handle a multi-character input, use cin.get() everywhere and not just sometimes. You can do processing of each character, but again, nothing will go to the screen until the user presses Enter.
But here's code that appears to work:
#include <cctype> // isalpha() and isalnum()
#include <string> // ch is now a std::string
// ...
case IDENTIFIER_CHOICE:
cout << "Please enter an identifier and press [Enter] immediately after. ";
std::cin.ignore(256, '\n'); // needed because of getline behavior
std::getline(std::cin, ch);
if (!(isalpha(ch[0]) || ch[0] == '_')) {
cout << "Not valid.\n";
break;
}
for (int i = 1; i < ch.size(); ++i) {
if (!isalnum(ch[i])) {
cout << "Not valid.\n";
break;
}
}
cout << "Valid.\n";
break;
// ...
With cin, when you press Enter, that keystroke is saved in the input stream. getline() doesn't behave the way we expect because while cin will typically ignore that keystroke, getline does not. So I just tell cin to ignore an arbitrary (but sufficient in this case) amount of characters in the stream up to and including the Enter keystroke (Mac and Linux, should still behave for Windows (I think)).
This is still far from bulletproof input validation (that's impossible), but I think it suffices for what you're working on.