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A flag variable is a boolean variable (having only two possible values - True or False) used in programming to signal the occurrence of an event or a specific condition. It is usually employed to control the flow of a program, especially when it comes to decision-making and looping structures.

The flag variable serves the purpose of providing control over the execution of certain parts of the code. It allows the program to make decisions based on its value, thus enabling or disabling specific code segments. For example, it can be used to check if a certain condition is met or to control the number of iterations in a loop.

Let's consider a Python example to demonstrate how this can be done: ```python found = False # Initialize a flag variable numbers = [5, 8, 3, 10, 12] search_number = 10 for number in numbers: if number == search_number: found = True # Flag variable set to True if the search_number is present in the list break if found: # Executes if flag variable is True print("The number {} is present in the list.".format(search_number)) else: print("The number {} is not present in the list.".format(search_number)) ``` In this example, we use the flag variable "found" to indicate if the `search_number` is present in the list of numbers. When the program finds the specified number, it sets the flag variable to True and then proceeds to the decision-making part where it prints the appropriate message based on the value of the flag variable.