Use of FLAG in programming (original) (raw)
Last Updated : 11 Jul, 2025
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 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
`
To learn more about flags and how to effectively use them in your programs, our **C programming course offers detailed lessons on using flags, control flow, and improving your program logic.
**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 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"); } } }
Python
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
``