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. 

Yes

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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. 

PRIME