Function basics

By it's definition, a function is a group of code that is accessed at some point in the execution of a program which carries out some specific task(s). They are accessed when they are called. Below is how a function would look:

type name ( type parameter1, type parameter2, ...) {

     //code here

}

where type is the return type of the function, name is the appropriate name given to the function and each parameter (or argument) is of an appropriate type.

By definition, a function can return a value of a certain type to where the function was called. Just like variables, a function can be of type int, double, float, bool etc... However, there is a special type of function type called void.

By definition, a void function DOES NOT RETURN A VALUE. Mostly, void functions are used when printing something to the screen such as a welcome message or even the triangle of stars.

Declaring functions

In a similar fashion to variables, functions need to be declared in order to be used. There are a few ways of doing this.

Method 1: Declare first, Define later

#include <iostream>
using namespace std;

void triple(int a, int b);

int main(){

    //some code

    return 0;
}

void triple(int a, int b){

     a = a * 3;
     b = b * 3;

}

As you will observe, the trip(...) function was declared between the main() function and the heading of the program. It was then defined in full AFTER the main function. These are called header lines for a function as they occur in the head of the program. This is the prefered was of declaring functions but there is another (method 2).

Method 2: Define in full

#include <iostream>
using namespace std;

void triple(int a, int b){

     a = a * 3;
     b = b * 3;

}

int main(){

    //some code

    return 0;
}

In this method, the function is declared fully BEFORE the main() function. THIS IS DANGEROUS primarily because of other functions. As an example, say that you have another function call inside the triple(...) function and you did not declare it ABOVE the triple(...) function, you are in trouble. C++ will not know where the other function is in memory!!! Try to use method number 1 when declaring functions.

Calling functions

In order for a function to be used in a program, it must be called. To do this, the name of the function and any parameters you are passing to it must be typed out EXACTLY as how you declared it. The below example shows how to call as well as declare a function:

//  for loop to print out index
#include <iostream>
using namespace std;

void printMessage(){
	cout << "Hello and welcome!" << endl;
}

int main(){
	printMessage();
	return 0;
}

Passing by value

When dealing with function parameters, there are two ways of passing variables or other values. One ways, and the most common, is called passing by value. This will give a COPY of the value to the function and the ORIGINAL value WILL NOT CHANGE IN THE PROGRAM. The below example shows a simple adding function that tries to change a value but does not.

//  for loop to print out index
#include <iostream>
using namespace std;

int add( int a, int b ){
	int sum = a+b;
	a = 7;
        return sum;
}

int main(){
	int n1 = 5, n2 = 6;
	cout << add(n1,n2) << endl;
	cout << n1 << endl;
	return 0;
}

Passing by reference

This method of passing values is very different from the first method. Here, values CAN CHANGE if any of them are being passed by reference.

In order to denote a pass by reference, an ampersand ( & ) must be placed after the type of the parameter.

//  for loop to print out index
#include <iostream>
using namespace std;

void dbl(int& a, int& b){
	a*= 2;
	b*= 2;
}

void trip(int a, int b){
	a*=3;
	b*=3;
}

int main(){

int x = 4, y = 6;

cout << "AT START:" << endl;
cout << "x= " << x << " y= " << y << endl;

//call the dbl function:
dbl(x,y);

cout << "AFTER DBL CALL:" << endl;
cout << "x= " << x << " y= " << y << endl;

//call the trip function:
trip(x,y);

cout << "AFTER TRIP CALL:" << endl;
cout << "x= " << x << " y= " << y << endl;

return 0;
}

AT START:
x=4 y=6

AFTER DBL CALL:
x=8 y=12

AFTER TRIP CALL:
x=8 y=12

Member Functions

With C++ as many other languages, there are whats called member functions. They are "pre-made" by the language that you are using. In general, they are part of the class it comes from. All member functions are accessed with the dot operator ( . )

With the cout class, there are member functions that control output formatting. Let's see the cout stream and some of its member functions.

width( numCharacters )

sets the spacing of the output in terms of the number of characters (the argument). The width is valid only on the next cout statement. Here is an example:

#include <iostream>
using namespace std;

int main(){

cout << "Hi!";
cout.width(11);
cout << "Bye!";

return 0;
}

The width will set 11 spaces for the "Hi!" string. Then, the "Bye!" string will print with no restriction in terms of size. The output is:

Hi!       Bye!

fill( character )

sets the character that will be used in output that has a width larger than the expression needs. Using the example above, let's add another line of code to the program that uses fill:

#include <iostream>
using namespace std;

int main(){

cout << "Hi!";
cout.width(11);
cout.fill('-');
cout << "Bye!";

return 0;
}

The fill() function will accept a character as its parameter and will use it to fill the unused spaces from the string. Here, there are 7 dashes that are used. This is the output:

Hi!-------Bye!

With the cin class, there are member functions that deal with input. Let's see two of them.

get( charArray, capacity )

The cin.get() function will take a line of input (tutorial 10), and be able to include whitespaces. Until now, the cin stream stops the input at a white space. Now, with this function and the next, we can control that. Here is an example.

#include <iostream>
using namespace std;

int main(){

char line[100];

cout << "Please enter a string no more than 100 characters:\n";
cin.get(line, 100);

return 0;
}

This example will allow a user to enter a string that will include whitespaces. There are 2 arguments that need to be passed to the function. The first is the name of the character array and the second is the space in that array.

getline( charArray, capacity, endingCharacter )

The cin.getline() function will take a line of input (tutorial 10), and be able to include whitespaces. The only difference between the get() and getline() functions is that getline() will have a default terminating character of the newline character '\n'.

#include <iostream>
using namespace std;

int main(){

char line[100];

cout << "Please enter a string no more than 100 characters:\n";
cin.getline(line, 100, '-');

return 0;
}

If you need another character other than the newline, you may specify that as a third parameter, as seen above.