C++ manually Convert string to int?

Question

I was trying to manually convert a string to an int, and I have had difficulties. I first check to see if the string entered is an integer. Next, I want to convert that string to an integer without using any library methods.

When I ran the code inside my loop line by line, it did what I needed it to, but when I ran it inside a loop, it spat out incorrect binary results.

==> I am going to check if the value you enter is a number.

==> Enter a number: 234

==> Input result: 1

==> Press enter to continue...

==> 11001

// Darian Nwankwo, Random Programs, August 2, 2015

#include <iostream>
#include <string>
#include <cmath>

int main(int argc, const char * argv[]) {
    std::string number = "";
    bool isNumber = false;

    std::cout << "I am going to check if the value you enter is a number." << std::endl;
    std::cout << "Enter a number: ";
    std::cin >> number;
    std::cin.ignore();

    // Iterate through variable number to check if it is a number
    for ( int i = 0 ; i < number.length() ; i++ ) {
        if ( number[i] < 48 || number[i] > 57) {
            break;
        } else {
            isNumber = true;
        }
    }

    std::cout << "Input result: " << isNumber << std::endl;

    int newNumber = 0;             
    // Iterates over the number string variable and converts value to an integer
    if (isNumber) {
        for ( int i = 0 ; i < number.length() ; i++ ) {
            // newNumber += std::pow( 10.0, number.length() )
            newNumber = std::pow(10.0, ( number.length() - ( i + 1 ) ) * ( number[i] - '0' ));
        }
    } else {
        std::cout << "Can't convert." << std::endl;
    }

    char cont;
    std::cout << "Press enter to continue..." << std::endl;
    std::cin.get(cont);
    std::cout << newNumber;

    return 0;
}

Answer 1

I modified your code:

    // Darian Nwankwo, Random Programs, August 2, 2015

#include <iostream>
#include <string>
#include <cmath>

int main(int argc, const char * argv[]) {
    std::string number = "";
    bool isNumber = true;     // <-----should be default true

    std::cout << "I am going to check if the value you enter is a number." << std::endl;
    std::cout << "Enter a number: ";
    std::cin >> number;
    std::cin.ignore();
    // Iterate through variable number to check if it is a number
    for ( int i = 0 ; i < number.length() ; i++ ) {
        if ( number[i] < 48 || number[i] > 57) {
            isNumber = false;       // <--- if not number, turn isNumber to be false
            break;
        }
    }

    std::cout << "Input result: " << isNumber << std::endl;

    int newNumber = 0;             
    // Iterates over the number string variable and converts value to an integer
    if (isNumber) {
        for ( int i = 0 ; i < number.length() ; i++ ) {
            // newNumber += std::pow( 10.0, number.length() )
            newNumber = newNumber + std::pow(10.0, ( number.length() - ( i + 1 ) )) * ( number[i] - '0' );   // <--- pow() should not include (number[i] - '0') and newNumber should be cumulative
        }
    } else {
        std::cout << "Can't convert." << std::endl;
    }

    char cont;
    std::cout << "Press enter to continue..." << std::endl;
    std::cin.get(cont);
    std::cout << newNumber;

    return 0;
}

Hope it can help.

Answer 2

Change your for loop to:

    for ( int i = number.length() -1 ; i >= 0 ; i-- ) {
        int power = number.length() - i -1;
        newNumber += (std::pow( 10.0,  power) * (number[i] - '0'));

and make newNumber to double.

Hope it helps !!