CODEWITHSUNDEEP
c++inputcastingcharint
user2403642
user2403642

Reputation:

Determining input to be int or char in C++

I'm working on a small program for school to compute a factorial of an inputted number. I've got a good amount of experience in Java, but this is my first go at C++.

My problem: I need to be able to have a single input from the user, which is either an integer or the character 'q', which signifies the application needs to quit.

Here is my current attempt:

#include <stdio.h>
#include <iostream>
#include "Header.h"
using namespace std;

int x=0;
char y;


int main(int argc, char *argv[])
{
    printf("Hello, please enter a number to compute a factorial (or 'q' to quit): ");

    cin >> y;
    x= (y-'0');


if(y=='q')
    {   printf("Thanks for playing!\n");
        exit(1);
    }   

    long result= print_Factorial(x);
    cout << x << "!= " << result << "\n";

    return result;

}

This casting does not work, however. If I enter a double digit number, such as 12, it only converts to x the first digit of the two and computes that factorial. I'm sure this is simple, what am I missing?

Explicit answer or a lead to where I can learn more about this problem, anything is appreciated.

Upvotes: 0

Views: 4504

Answers (4)

Emilio Garavaglia
Emilio Garavaglia

Reputation: 20730

Your user can enter whatever line of text, you you have to read a "line of text" to validate.

#include <iostream>
#include <string>
#include <stdexcept>

int main()
{
    std::string text;
    std::getline(std::cin,text);

    if(text.size()==1 && text[0]=='q')
    { 
        std::cout << "quit command"; 
        return 0; 
    }

    try
    {
        int i = std::stoi(text); //may throw if text is not convertible
        /* whatever elaboration and output */
        return 0;
    }
    catch(const std::exception& e)
    {
        std::cout << "bad input: " << text << '\n';
        std::cout << "caused excpetion: " << e.what() << std::endl;
    }
    return 3; //means "excpetion thorown"
}

Upvotes: 0

Some programmer dude
Some programmer dude

Reputation: 409176

There are functions you can use to try and covert a string into a number, and which you can check if the conversion was a success or not. The std::strtol function is one of them:

std::string input;
std::cin >> input;

char* endptr = nullptr;
const char *input_ptr = input.c_str();
long value = std::strtol(input_ptr, &endptr, 10);

if (endptr == input_ptr)
{
    // Input was not a valid number
}
else if (*endptr != '\0')
{
    // Input starts with a valid number, but ends with some extra characters
    // (for example "123abc")
    // `value` is set to the numeric part of the string
}
else
{
    // Input was a valid number
}

If you don't mind exceptions, then you can use e.g. std::stoi instead:

std::string input;
std::cin >> input;

int value = 0;
try
{
    size_t endpos = 0;

    value = std::stoi(input, &endpos);

    if (endpos != input.length())
    {
        // Input starts with a valid number, but ends with some extra characters
        // (for example "123abc")
        // `value` is set to the numeric part of the string
    }
    else
    {
        // Input is a valid number
    }
}
catch (std::invalid_argument&)
{
    // Input not a valid number
}
catch (std::out_of_range&)
{
    // Input is a valid number, but to big to fit in an `int`
}

Upvotes: 3

user2249683
user2249683

Reputation: 

#include <iostream>
#include <sstream>

int main() {
    std::string in;
    std::cout << "Please enter a digit: ";
    while(std::cin >> in) {
        std::cout << "Input: " << in << std::endl;
        if(in.size() == 1) {
            if(in[0] == 'q' || in[0] == 'Q') {
                std::cout << "Quit" << std::endl;
                return 0;
            }
        }
        std::istringstream parse(in);
        int value;
        if(parse >> value) {
            if(parse.eof()) {
                std::cout << "Success" << std::endl;
                return 0;
            }
        }
        std::cout << "Please try again: ";
    }
    std::cerr << "This should not happen <control + d>" << std::endl;
    return 1;
}

Upvotes: 0

Kyranstar
Kyranstar

Reputation: 1720

The reason you're getting the first digit is because you're using cin >> y; where y is a char, which holds one character. So you're only getting one character.

What you probably would want to do is get the answer as a string, and once you check that the string is not == "q", then you could convert it to an int.

Upvotes: 1

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