#include <stdio.h>

#include <string>

using namespace std;

bool isNumberString(const string& s) {
    int len = s.length();
    for (int i = 0; i < len; i++) {
        if (s[i] < '0' || s[i] > '9')
            return false;
    }
    return true;
}

int main() {
    string ccNumber;
    
    cout << "This program uses the Luhn Algorigthm to validate a CC number." << endl;
    cout << "You can enter 'exit' anytime to quit." << endl;
    
    while (true) {
        
        cout << "Please enter a CC number to validate: ";
        cin >> ccNumber;
        
        if (ccNumber == "exit")
            break;
        
        else if (!isNumberString(ccNumber)) {
            cout << "Bad input! ";
            continue;
        }
            
        int len = ccNumber.length();
        int doubleEvenSum = 0;
        
        // Step 1 is to double every second digit, starting from the right. If it
        // results in a two digit number, add both the digits to obtain a single
        // digit number. Finally, sum all the answers to obtain 'doubleEvenSum'.   
        
        for (int i = len - 2; i >= 0; i = i - 2) {
            int dbl = ((ccNumber[i] - 48) * 2);
            if (dbl > 9) {
                dbl = (dbl / 10) + (dbl % 10);
            }
            doubleEvenSum += dbl;
        }
        
        // Step 2 is to add every odd placed digit from the right to the value
        // 'doubleEvenSum'.
        
        for (int i = len - 1; i >= 0; i = i - 2) {
            doubleEvenSum += (ccNumber[i] - 48);
        }
        
        // Step 3 is to check if the final 'doubleEvenSum' is a multiple of 10.
        // If yes, it is a valid CC number. Otherwise, not.
        
        cout << (doubleEvenSum % 10 == 0 ? "Valid!" : "Invalid!") << endl;
        
        continue;        
    }

    return 0;
}

C++ Online Compiler

Write, Run & Share C++ code online using OneCompiler's C++ online compiler for free. It's one of the robust, feature-rich online compilers for C++ language, running on the latest version 17. Getting started with the OneCompiler's C++ compiler is simple and pretty fast. The editor shows sample boilerplate code when you choose language as C++ and start coding!

Read inputs from stdin

OneCompiler's C++ online compiler supports stdin and users can give inputs to programs using the STDIN textbox under the I/O tab. Following is a sample program which takes name as input and print your name with hello.

#include <iostream>
#include <string>
using namespace std;

int main() 
{
    string name;
    cout << "Enter name:";
    getline (cin, name);
    cout << "Hello " << name;
    return 0;
}

About C++

C++ is a widely used middle-level programming language.

  • Supports different platforms like Windows, various Linux flavours, MacOS etc
  • C++ supports OOPS concepts like Inheritance, Polymorphism, Encapsulation and Abstraction.
  • Case-sensitive
  • C++ is a compiler based language
  • C++ supports structured programming language
  • C++ provides alot of inbuilt functions and also supports dynamic memory allocation.
  • Like C, C++ also allows you to play with memory using Pointers.

Syntax help

Loops

1. If-Else:

When ever you want to perform a set of operations based on a condition If-Else is used.

if(conditional-expression) {
   //code
}
else {
   //code
}

You can also use if-else for nested Ifs and If-Else-If ladder when multiple conditions are to be performed on a single variable.

2. Switch:

Switch is an alternative to If-Else-If ladder.

switch(conditional-expression){    
case value1:    
 // code    
 break;  // optional  
case value2:    
 // code    
 break;  // optional  
......    
    
default:     
 code to be executed when all the above cases are not matched;    
}

3. For:

For loop is used to iterate a set of statements based on a condition.

for(Initialization; Condition; Increment/decrement){  
  //code  
}

4. While:

While is also used to iterate a set of statements based on a condition. Usually while is preferred when number of iterations are not known in advance.

while (condition) {  
// code 
}

5. Do-While:

Do-while is also used to iterate a set of statements based on a condition. It is mostly used when you need to execute the statements atleast once.

do {  
 // code 
} while (condition);

Functions

Function is a sub-routine which contains set of statements. Usually functions are written when multiple calls are required to same set of statements which increases re-usuability and modularity. Function gets run only when it is called.

How to declare a Function:

return_type function_name(parameters);

How to call a Function:

function_name (parameters)

How to define a Function:

return_type function_name(parameters) {  
 // code
}