CODEWITHSUNDEEP
c++validation
byBanachTarskiIamcorrect
byBanachTarskiIamcorrect

Reputation: 15

C++ integer validation

Currently having issues creating an integer validation loop in my code.

Trying to validate for an integer, however this code fails to work if you input, for example the string '22a' it sets trial_no as 22. Is there any way to check that every charachter input is indeed a string, such that '22a' or '2a2' would be considered erroneous and the loop would continue until a valid integer was input?

int trial_no;

bool valid = false;

while(!valid) 
{
    valid = true; //assume trial_no will be an integer
    cout << "Enter the number of die throws to simulate" << endl;
    cin >> trial_no;

    if(cin.fail()) // exit loop condition is dependent on trail_no being a valid integer
    {
        cin.clear(); //corrects stream
        cin.ignore(numeric_limits<streamsize>::max(), '\n'); //skips left over stream data (numeric_limit
        // is in case user enters more than one letter))
        cout << "Please enter an integer value" << endl;
        valid = false; //cin not an integer so loop goes round to try again
    }
}

Upvotes: 0

Views: 1237

Answers (1)

Ron
Ron

Reputation: 15501

Arguably the best way is to read the entire line as a string and utilize the std::stoi function:

#include <iostream>
#include <string>
int main() {
    std::cout << "Enter an integer: ";
    std::string tempstr;
    bool valid = false;
    std::getline(std::cin, tempstr);
    try {
        int result = std::stoi(tempstr);
        std::cout << "The result is: " << result;
        valid = true;
    }
    catch (std::invalid_argument) {
        std::cout << "Could not convert to integer.";
        valid = false;
    }
}

As pointed out in the comments, this function can also throw a std::out_of_range exception. This assumes your compiler is C++11 (+) capable. If not, go down the std::stringstream route:

std::string tempstr;
std::getline(std::cin, tempstr);
std::stringstream ss(tempstr);
int result;
bool valid = false;
if (ss >> result) {
    valid = true;
}
else {
    valid = false;
}

Upvotes: 0

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