How Can I Effectively Verify Integer User Input in C ?

Verifying User Input as Integers

Problem Statement

In a C program, it is crucial to ensure that user input is in the expected format, especially when performing mathematical operations. This article addresses how to determine if the input provided by the user is an integer. If the input is not an integer, the program will prompt the user to re-enter a valid integer.

Solution

There are two main approaches to verify user input as an integer:

1. Using cin.fail():

• Read the input using cin >>.
• Check the state of the input stream using if (cin.fail()).
• If cin.fail() is true, it means the input is not an integer.

2. Using std::getline and String Manipulation:

• Use std::getline(std::cin, theInput) to capture the input as a string.
• Check the string for any character that is not a number using if (theInput.find_first_not_of("0123456789") != std::string::npos).
• If the condition is true, it means the input contains non-numeric characters.

Implementation

Below is a code snippet that implements the std::getline and string manipulation approach:

#include <iostream>
#include <string>

int main() {

    std::string theInput;
    int inputAsInt;

    std::getline(std::cin, theInput);

    while(std::cin.fail() || std::cin.eof() || theInput.find_first_not_of("0123456789") != std::string::npos) {

        std::cout << "Error" << std::endl;

        if( theInput.find_first_not_of("0123456789") == std::string::npos) {
            std::cin.clear();
            std::cin.ignore(256,'\n');
        }

        std::getline(std::cin, theInput);
    }

    std::string::size_type st;
    inputAsInt = std::stoi(theInput,&amp;st);
    std::cout << inputAsInt << std::endl;
    return 0;
}

This code continuously prompts the user to enter an integer until a valid integer is entered. It handles situations where the user enters non-numeric characters or provides empty input.

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