Subprograms

In C++, a subprogram (also known as a function) is a block of code that performs a specific task and can be called whenever needed in a program. Functions help structure and organize code, making it easier to understand, maintain, and reuse.

Example:

Given two natural numbers n and m. Determine the sum of the reversed digits of n and m. Try it!

#include < iostream >
using namespace std;

int reverse(int x)
{
    int r = 0;
    do
    {
        r = 10 * r + x % 10;
        x /= 10;
    }
    while(x != 0);
    return r;
}

int main(){
    int n , m;
    cin >> n >> m;
    cout << reverse(n) + reverse(m);
    return 0;
}

Function Header

The function has a header: int reverse(int x), from which we deduce that:

• the function is named reverse;
• the function has one parameter, x, of type int. The parameter is important because it specifies which number's digits are to be reversed:
• a function can have multiple parameters;
• parameters appearing in the function header are called formal parameters;
• function parameters are also called arguments;
• the function has a specific type, here int, which specifies the type of the result;

Function Body

The function has a block of statements, the function body, delimited by braces {}, which specifies the operations to obtain the result. Furthermore:

• the function body can have its own variables (here r);
• these variables are called local variables;
• in the function body, the parameter behaves like a local variable;
• we cannot have a local variable with the same identifier as the formal parameter;
• in the function body, the values of formal parameters are not known. The function must determine the correct result regardless of their value;
• the result determined within the function is returned to the calling program by the return statement. The result is typically computed in the variable r. With the statement return r;, the current value of the variable r is returned to the calling program;

Void Functions

Functions with a return type of void in C++ are functions that do not return any value. These functions are used to perform actions or operations that do not require an explicit result. Usually, void functions are used to manipulate data, display information on the screen, or perform other auxiliary actions within a program.

Characteristics of void Functions:

• Have no return value: Instead of returning a specific value, void functions terminate their execution and return to the point where they were called.
• Can have parameters: Like other functions, void functions can accept one or more parameters to perform their task.
• Structure similar to other functions: The main difference lies in the return type, which is void.

Example: Try it!

#include < iostream >
using namespace std;

// Function that displays a greeting message
void greet()
{
    cout << "Hello! Welcome!" << endl;
}

int main()
{
    greet(); // Calling the greet function
    return 0;
}

In this example, the greet function displays a message on the screen and does not return any value.

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