These 4 programs below check whether the integer entered by the user is a prime number or not.
The output of all these programs below is the same, and we have created a user-defined function in each example. However, the approach we have taken in each example is different.
Example 1: No Argument Passed and No Return Value
The checkPrimeNumber() function takes input from the user, checks whether it is a prime number or not, and displays it on the screen.
The empty parentheses in checkPrimeNumber(); inside the main() function indicates that no argument is passed to the function.
The return type of the function is void. Hence, no value is returned from the function.
Example 2: No Arguments Passed But Returns a Value
The empty parentheses in the n = getInteger(); statement indicates that no argument is passed to the function. And, the value returned from the function is assigned to n.
Here, the getInteger() function takes input from the user and returns it. The code to check whether a number is prime or not is inside the main() function.
Example 3: Argument Passed But No Return Value
The integer value entered by the user is passed to the checkPrimeAndDisplay() function.
Here, the checkPrimeAndDisplay() function checks whether the argument passed is a prime number or not and displays the appropriate message.
Example 4: Argument Passed and Returns a Value
The input from the user is passed to the checkPrimeNumber() function.
The checkPrimeNumber() function checks whether the passed argument is prime or not.
If the passed argument is a prime number, the function returns 0. If the passed argument is a non-prime number, the function returns 1. The return value is assigned to the flag variable.
Depending on whether flag is 0 or 1, an appropriate message is printed from the main() function.
Which approach is better?
Well, it depends on the problem you are trying to solve. In this case, passing an argument and returning a value from the function (example 4) is better.
A function should perform a specific task. The checkPrimeNumber() function doesn't take input from the user nor it displays the appropriate message. It only checks whether a number is prime or not.