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.

// 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"; 
} 

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.

// 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; 
} 

Output:

PRIME

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