Handling Input to Avoid Program Crashes

• Download CelciusToFerenheit.txt - 2.72 KB

Introduction

This article is about validating input in order to avoid crashing when invalid input is given by the user.

Background

Everyone has had, at one point of time, experienced a crash due to incorrect input. This is an example of one way it could be handled in C++.

Using the Code

You can create a project using the attached CPP code. Compile the application and try to enter string where number is required. The program will complain instead of crashing. Just play around with it.

		/*
Developed by	: Dhruvkumar Rangunwala
Email			: instrukahero@yahoo.com
*/
#include <iostream>
#include <string>
#include <cctype>
using namespace std;
bool CheckIfNumber(string , int, int&);
const int asciiStartForDigit(48);
int main()
{
	double celcius, fahrenheit;
	string choice, numberEntered;
	int numberEnteredVerified, multiplesOf10 = 1, choiceInNumber;
	
	do
	{
		system("cls");
		cout << "This program converts C -> F and F -> C temperature unit" << endl;
		cout << "Enter 1 for C -> F" << endl;
		cout << "Enter 2 for F -> C" << endl;
		cout << "Enter x or X to exit" << endl;
		cin >> choice;
		if ((choice.at(0) == 'x' || choice.at(0) == 'X') && choice.length() == 1)
		{
			break;
		}
		numberEnteredVerified = 0;
		if(CheckIfNumber(choice, multiplesOf10, numberEnteredVerified))
		{
			cout << "Choice entered is :" << numberEnteredVerified << endl;
			switch(numberEnteredVerified)
			{
			case 1: 
				cout << "Enter temperature to convert from C to F:";
				break;
			case 2:
				cout << "Enter temperature to convert from F to C:";
				break;
			default:
				cout << "You did not enter choice as 1 or 2\n";
				system("pause");
				continue;
			}
			cin >> numberEntered;
		}
		else
		{
			cout << "\nYou did not enter a number for choice\n\n\n";
			system("pause");
			continue;
		}
		choiceInNumber = numberEnteredVerified;
		numberEnteredVerified = 0;
		if(CheckIfNumber(numberEntered, multiplesOf10, numberEnteredVerified))
		{
			cout << "Number entered is :" << numberEnteredVerified << endl;
			switch(choiceInNumber)
			{
			case 1:
				celcius = numberEnteredVerified;
				fahrenheit = (9.0/5) * celcius + 32;
				cout << "Celcius " << celcius << 
				" when converted to Fahrenheit is " << fahrenheit << endl;
				break;
			case 2:
				fahrenheit = numberEnteredVerified;
				celcius = (fahrenheit - 32) * (5.0/9);
				cout << "Fahrenheit " << fahrenheit <<
				" when converted to Celcius is " << celcius << endl;
				break;
			}
			system("pause");
		}
		else
		{
			cout << "\nYou did not enter a number for conversion\n\n\n";
			system("pause");
			continue;
		}
	}while (true);
	
	cout << "\n\n\nExiting the program\n\n\n";
}
bool CheckIfNumber(string numberEntered, int multiplesOf10, int& numberEnteredVerified)
{
	char numberEnteredToChar[10];
	size_t i;
	strcpy(numberEnteredToChar, numberEntered.c_str());
	for(i = 0; i < strlen(numberEnteredToChar); i++)
	{
		if(!(isdigit(numberEnteredToChar[i])))
		{
			return false;
		}
		else
		{
			numberEnteredVerified = (numberEnteredVerified * multiplesOf10) + 
			    ((int) numberEnteredToChar[i] - asciiStartForDigit);
			multiplesOf10 *= 10;
		}
	}
	return true;
}