**In this article, you’ll learn about various techniques of C++ User-defined Function** **to solve a single problem.**

**C++ User-defined Function** can be classified into the following categories **to help comprehend parameters and returns in functions:**

**No-argument and no-return-value function****No-argument function that returns a value****No return value for a function with an argument.****Return value and argument function**

**Examples**

**void add() // No return without argument****void sub(int,int)// No return argument****int mul() // Return without argument****float div(int ,int) // Return with argument**

Function with No arguments passed and no return value

- Each
**function is independent**in this form of function. - They require data, data values, and
**the ability to calculate**and show the**same block.** - These features can be
**used to display information**or they can be**totally controlled by the user.**

Contents

## Example: Function with No arguments passed and no return value

**Consider a circumstance** where you need to determine the **prime number.** As noted above, this challenge is overcome by **creating user-defined functions** in** four different methods.**

// This program to check Prime number: # include <iostream> using namespace std; void prime(); int main(){ // No argument is passed to prime() prime(); return 0; } // Return type of function is void because value is not returned. void prime(){ int num, i, a = 0; cout << "Enter a positive integer enter to check: "; cin >> num; for(i = 2; i <= num/2; ++i) { if(num % i == 0) { a = 1; break; } } if (a == 1) { cout << num << " is not a prime number."; } else { cout << num << " is a prime number."; } }

**Output 1**

**Enter a positive integer enter to check: 2 , Output: 2 is a prime number.**

Output 2

**Enter a positive integer enter to check: 4, Output: 4 is not a prime number.**

**Working of above program**

**prime**is called with**no arguments from main**in the above application.**prime**accepts a positive**number from the user**and determines whether it is a**prime number.**- The
**function returns**no value since the return type of**prime**is void.

Function with No arguments passed but a return value

**The number of arguments**is passed from the calling function to the called**function****in this sort of function**, but the called function returns a value.- The called
**function variable**is declared and initialized in the same called function module. The called**function is self-contained.**

## Example: Function with No arguments passed but a return value

#include <iostream> using namespace std; int prime(); int main() { int number, a, flag = 0; // No argument is passed to prime() number = prime(); for (a = 2; a <= number/2; ++a) { if (number%a == 0) { flag = 1; break; } } if (flag == 1) { cout<<num<<" is not a prime number."; } else { cout<<num<<" is a prime number."; } return 0; } // Return type of function is int int prime() { int i; printf("Enter a positive integer to check: "); cin >> i; return i; }

**Output 1**

**Enter a positive integer enter to check: 2, Output: 2 is a prime number.**

**Output 2**

**Enter a positive integer enter to check: 4, Output: 4 is not a prime number.**

**Working of the above program**

- The
**prime function**is called with no arguments from the**main method**in the above application. - prime asks the user
**for a positive integer.**Because the function’s return type is an int, it returns the user’s**inputted number**to the calling main function. - The
**number is then tested**for primeness in the main and**printed on the screen if it is.**

Function with Arguments passed but no return value

- This type of
**function passes inputs**to the called function through the calling**function but does not return a value.** - Such functions are
**practically dependent on one another.**

## Example: Function with Arguments passed but no return value

#include <iostream> using namespace std; void prime(int n); int main() { int number; cout << "Enter a positive integer to check: "; cin >> number; // Argument num is passed to the function prime prime(number); return 0; } // There is no return value to calling function. Hence, return type of function is void. */ void prime(int i) { int n, flag = 0; for (n = 2; n <= i/2; ++n) { if (i%n == 0) { flag = 1; break; } } if (flag == 1) { cout << i << " is not a prime number."; } else { cout << i << " is a prime number."; } }

**Output 1**

**Enter a positive integer enter to check: 3, Output: 3 is a prime number.**

**Output 2**

**Enter a positive integer enter to check: 6, Output: 6 is not a prime number.**

**Working of the above program**

- The user is
**initially prompted**for a positive integer, which is then stored in the variable**num.** - The user is initially prompted for a
**positive integer,**which is then stored in**the variable num.** - The number is supplied to the
**prime**function, which checks whether**the number is prime**and**prints the result.** - The function returns no value since the
**return type of prime is void.**

Function with Arguments passed and a return value.

- This is the
**greatest kind since it completely isolates the function from its inputs and outputs,**leaving only the logic to be described inside the function body. - Both functions are
**mutually exclusive.**

## Example: Function with Arguments passed and a return value

#include <iostream> using namespace std; int prime(int n); int main() { int number, flag = 0; cout << "Enter positive integer to check: "; cin >> number; // Argument num is passed to check() function flag = prime(num); if(flag == 1) cout << number << " is not a prime number."; else cout<< number << " is a prime number."; return 0; } /* This function returns integer value. */ int prime(int n) { int i; for(i = 2; i <= n/2; ++i) { if(n % i == 0) return 1; } return 0; }

**Output 1**

**Enter a positive integer enter to check: 3, Output: 3 is a prime number.**

**Output 2**

**Enter a positive integer enter to check: 6, Output: 6 is not a prime number.**

**Working of the above program**

C++ User-defined Function, C++ User-defined Function,C++ User-defined Function, user defined functions in c, what is user defined functions in c++

- A
**positive integer**is requested from the user and stored in the variable**num**in the above software. - The
**number is sent to the prime function**, which determines**whether or not the number is prime.** - Prime returns 1 or 0 to the main calling function because its return type is an int.
**1 is returned if the number is a prime number.**Otherwise, 0 is returned. - The returning 1 or 0 is placed in the
**variable flag in the main method,**and the corresponding text is printed on the screen.

## Which method is better?

**All four programs above provide the same result and are technically sound.****There is no hard and fast rule for deciding which method to use.****The method you use is determined by the situation and how you wish to tackle the problem.**

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