C++ How to protect against invalid input? - c++

Question

In my program, I want users to choose between two choices. Simply, 1 or 2. I have written the program to where it will protect against an invalid numeric value such as 3 or 28, but I cannot protect against alphabetical input.
Code is as follows:
int whileInt = 0;
int choiceOne_One = 0;
while(whileInt == 0)
{
cin >> choiceOne_One;
if(choiceOne_One != 1 && choiceOne_One != 2)
{
cout << "Woah! That wasn't an option! Try Again.\n";
}
else if(choiceOne_One == 1)
{
whileInt++;
cout << "\nYou have died. Be more careful.\n";
}
else if(choiceOne_One == 2)
{
whileInt++;
cout << "\nYou have survived the alien invasion due to your cunning sense of "
"safety.\nCongratulations.\n";
}
}
My brain still works in Java, please help me figure this out. It will definitely be appreciated.

Answer 1

Try this :
#include <iostream>
using namespace std;
int main()
{
char input;
cin>>input;
switch(input)
{
case '0':
case '1':
case '2':
case '3':
case '4':
case '5':
case '6':
case '7':
case '8':
case '9':
cout<<"valid input"<<endl;
break;
default :
cout<<"Invalid input"<<endl;
}
return 0;
}
You can define a case for all your valid inputs, leave the rest to default.
Integers from 0-9 have their ASCII values from 48-57 respectively.
This solution wont be helpful if you have
. input>9 or input<0
. input such as 1abc

Answer 2

Might be a bit late but my favorite way to do and this is what i use :
#include <iostream>
#include <string>
#include <limits>
//T is for our variable and lambdaFunc will be our lambda function or
//function pointer
template<typename T, typename lambdaFunc>
auto secureEntry(T& variable, LambdaFunc lFunc, std::string errorMessage) //can ommit error message
{
while(!(std::cin(variable)) && !lFunc(variable)){
std::cout << errorMessage << std::endl;
std::cin.clear();
std::ignore((std::numeric_limits<std::streamsize>::max)(), '\n');
}
}
int main(int argc, char* argv[]){
int mynumber = {0};
std::string errorMessage = "Please use a number bigger than 0 and lower than 5"
secureEntry(myNumber, [](int& mynumber) -> bool{
return mynumber > 0 && mynumber < 5;
}, errorMessage)
}

Answer 3

you can do this also
int whileInt=0;
int choiceOne_One = 0;
while(whileInt == 0)
{
cin >> choiceOne_One;
if (choiceOne_One == 1 || choiceOne_One == 2){
if (choiceOne_One == 1){
whileInt++;
cout << "\nYou have died. Be more careful.\n";
}
else {
whileInt++;
cout << "\nYou have survived the alien invasion due to your cunning sense of "
"safety.\nCongratulations.\n";
}
}
else {
cout << "Woah! That wasn't an option! Try Again.\n";
}
}

Answer 4

Try reading a std::string with std::getline, then simply compare the string with "1" or "2". Otherwise, you can just read the whole line with std::getline, then use std::stoi (C++11) to transform the read string into an integer. The result of std::stoi tells you whether the transformation was successful or not (i.e., whether the string represented an integer). If successful, then you can simply compare the integer with 1 or 2. If not successful, then an std::invalid_argument or std::out_of_range exception is thrown.
First case (with std::string comparison):
#include <iostream>
int main()
{
std::string input;
std::getline(std::cin, input);
if (input != "1" && input != "2")
std::cout << "Invalid input!";
}
Second case (using std::stoi):
#include <iostream>
int main()
{
std::string input;
std::getline(std::cin, input);
int n = std::stoi(input); // throws exception if cannot convert to int
if (n != 1 && n != 2)
std::cout << "Invalid input!";
}

Answer 5

If you change the program so that cin puts a User's answer to a std::string, then you could do tests on the string value.
If the string's length() was more than 1, it cannot be '1' or '2'
There are additional tests you can do, like std::isalpha(int ch)
#include <stdio.h>
#include <string>
#include <iostream>
int main(int argc, char * argv[]) {
std::string value;
std::cin >> value;
std::cout << "Value is " << value << "\n";
std::cout << "length of value is " << value.length() << "\n";
char ch;
ch = value.at(0);
if(std::isalpha(ch)) std::cout << "You did not enter a number" << "\n";
return 0;
}
robert#debian:/tmp$ g++ -Wall test.cpp
robert#debian:/tmp$ ./a.out
123
Value is 123
length of value is 3
robert#debian:/tmp$ ./a.out
abc
Value is abc
length of value is 3
You did not enter a number