Use of FLAG in programming

Flag variable is used as a signal in programming to let the program know that a certain condition has met. It usually acts as a boolean variable indicating a condition to be either true or false. 
Example 1: Check if an array has any even number.

Input : arr[] = {1, 3, 7, 5} 
Output : No All numbers are odd.

Input : arr[] = {1, 2, 7, 5} 
Output : Yes There is one even number in the array.

We initialize a flag variable as false, then traverse the array. As soon as we find an even element, we set flag as true and break the loop. Finally we return flag. 

CPP

// C++ program to check if given array is has
// any even number
#include <iostream>
using namespace std;

bool checkIfAnyEven(int arr[], int n)
{
    bool flag = false;
    for (int i=0; i<n; i++)
    {
        if (arr[i] % 2 == 0)
        {
            flag = true;
            break;
        }
    }
    return flag;
}

int main()
{
int arr[] = {1, 3, 2, 5, 6, 7};
int n = sizeof(arr)/sizeof(arr[0]);
if (checkIfAnyEven(arr, n))
    cout << "Yes";
else
    cout << "No";
}

Java

//Java program to check if given array is has
// any even number
public class GFG {

    boolean checkIfAnyEven(int arr[], int n)
    {
        boolean flag = false;
        for (int i = 0; i < n; i++) {
            if (arr[i] % 2 == 0) {
                flag = true;
                break;
            }
        }
        return flag;
    }
    public static void main(String args[])
    {
        GFG obj = new GFG();
        int arr[] = { 1, 3, 2, 5, 6, 7 };
        int n = arr.length;
        if (obj.checkIfAnyEven(arr, n)) {
            System.out.println("YES");
        }
        else {
            System.out.println("NO");
        }
    }
}

Python

# Python program to check if given array has
# any even number
my_list=[1,3,5,2,7,9]
def IsListEven(my_list):

    flag = False

    for i in range(len(my_list)):

        if my_list[i] % 2 == 0: # As pointed in comment, use != for isEven
            flag = True
            print("Yes given list has even numbers.")
            break
    print(flag)
IsListEven(my_list)

C#

using System;

public class GFG {

    public static bool checkIfAnyEven(int[] numberArray)
    {
        foreach(var n in numberArray)
        {
            if (n % 2 != 0)
                return true;
        }
        return false;
    }
    public static void Main()
    {
        int[] numberArray = { 2, 4, 7, 8, 6 };

        if (checkIfAnyEven(numberArray)) {
            Console.WriteLine("YES");
        }
        else {
            Console.WriteLine("NO");
        }
    }
}
// This code is contributed by Rahul Chauhan

Javascript

// Javascript program to check if given array is has
// any even number
let arr = [1,3,2,5,6,7];
let flag = false;
 { 1, 3, 2, 5, 6, 7 };
for (let elem of arr) {
    if (elem % 2 == 0) {
        flag = true;
        break; // get out of the loop
    }
}
if (flag)
console.log("YES")
else
console.log("NO")
//This code is Contributed By Rahul Chauhan

Output

Yes

Example 2 : Check if given number is prime or not.

Input : n = 5 
Output : Yes 

Input : n = 18
 Output : No

We initialize a flag variable as true. Then we traverse through all numbers from 2 to n-1. As soon as we find a number that divides n, we set flag as false. Finally we return flag. 

CPP

// C++ implementation to show the use of flag variable
#include <iostream>
using namespace std;

// Function to return true if n is prime
bool isPrime(int n)
{
    bool flag = true;

    // Corner case
    if (n <= 1)
        return false;

    // Check from 2 to n-1
    for (int i = 2; i < n; i++) {

        // Set flag to false and break out of the loop
        // if the condition is not satisfied
        if (n % i == 0) {
            flag = false;
            break;
        }
    }

    // flag variable here can tell whether the previous loop
    // broke without completion or it completed the execution
    // satisfying all the conditions
    return flag;
}

// Driver code
int main()
{
    if(isPrime(13))
        cout << "PRIME";
    else
        cout << "NOT A PRIME";
    return 0;
}

Java

/*package whatever //do not write package name here */
// Java  implementation to show the use of flag variable
import java.io.*;

class GFG {
    boolean  isPrime(int n)
{
    boolean flag = true;

    // Corner case
    if (n <= 1)
        return false;

    // Check from 2 to n-1
    for (int i = 2; i < n; i++) {

        // Set flag to false and break out of the loop
        // if the condition is not satisfied
        if (n % i == 0) {
            flag = false;
            break;
        }
    }

    // flag variable here can tell whether the previous loop
    // broke without completion or it completed the execution
    // satisfying all the conditions
    return flag;
}
    public static void main(String[] args) {
        GFG obj=new GFG();
        int n=13;
        if(obj.isPrime(n))
        {
            System.out.println("PRIME");
        }else
        {
            System.out.println("NOT A PRIME");
        }
            }
}

Python3

#  Python3 Program to check if a number is prime or not

num =13

# To take input from the user
#num = int(input("Enter Any Number: "))

# define a flag variable
flag = False

if num == 1:
    print(num, "NOT A PRIME")
elif num > 1:
    for i in range(2, num):
        if (num % i) == 0:
            flag = True
            break

    # check if flag is True
    if flag:
        print("NOT A PRIME")
    else:
        print("PRIME")

  #This code is contributed By Rahul Chauhan

C#

// C#  implementation to show the use of flag variable
using System;
public class PrimeNumberExample {
    public static void Main(string[] args)
    {
        int n, i, m = 0, flag = 0;
        n = 13;
        m = n / 2;

      // Check from 2 to n-1
        for (i = 2; i <= m; i++) {

         // Set flag to false and break out of the loop
        // if the condition is not satisfied
            if (n % i == 0) {
                Console.Write("NOT A PRIME");
                flag = 1;
                break;
            }
        }
        if (flag == 0)
            Console.Write("PRIME");
    }
}

Javascript

//JAVASCRIPT PROGRAM  to check if a number is prime or not
let number=13;
let flag = true;

// check if number is equal to 1
if (number === 1) {
    console.log("1 is neither prime nor composite number.");
}

// check if number is greater than 1
else if (number > 1) {

        for (let i = 2; i < number; i++) {
        if (number % i == 0) {
            flag = false;
            break;
        }
    }

    if (flag) {
        console.log(`PRIME`);
    } else {
        console.log(`NOT A PRIME NUMBER`);
    }
}

// check if number is less than 1
else {
    console.log("NOT A PRIME");
}
// This code is contributed By Rahul Chauhan

Output

PRIME