Lesson 07 - References & Arrays

Example Program

// If you are not sure what some lines of code do, try looking back at
// previous example programs, notes, or ask a question.

#include <iostream>

using namespace std;

// The function prototype of a function with a reference parameter. To specify the
// reference parameter, simply type and ampersand after the data type.
// Note that it does not return anything, as the data will be passed back through
// the reference parameter.
void addNumber(int &addTo, int value);

// This function prototype includes an array, which automatically acts as a reference
// parameter -- changes to the array within the function will also effect MAIN.
// Note that you do not need to specify the array size.
void setArray(int arrayParam[], int arraySize, int setValue);

int main() {
	
	int result = 5;

	cout << "Number before function: " << result << endl;

	// Passes the result value as a reference parameter, so that its value will
	// be changed within MAIN.
	addNumber(result, 10);

	cout << "Number after function: " << result << endl;

	// This declares an array of integers with a size of ten.
	int array[10];

	cout << endl;

	// This will output a garbage value, as the values in the array have not been
	// initialized. Note the use of array[0] to denote the first element.
	cout << "Garbage first value in array: " << array[0] << endl;

	// For loops are particularly useful in looping through arrays. This loop sets
	// the first value in the array to 1, the second to 2, and so on.
	for(int i = 0; i < 10; i++) {
		array[i] = i + 1;
	}

	// Output the fourth and sixth values in the array, which will now be 4. Note the 
	// use of array[3] and array[5] to denote the fourth and sixth elements.
	cout << "After the loop, the fourth value in the array is: " << array[3] << endl;
	cout << "After the loop, the sixth value in the array is: " << array[5] << endl;
	
	cout << endl;
	
	// This calls the setArray function, which will set the first 10 values in the
	// array "array" to 25. This will effect the "array" variable in MAIN.
	setArray(array, 10, 25);

	// Again, loop through the array, outputting each value. This shows that all the
	// elements in the array have indeed been set to 25.
	for(int i = 0; i < 10; i++) {
		cout << "Array element " << i << ": " << array[i] << endl;
	}

	cout << endl;

	// Declares a c-string that can hold 50 characters, or 49 characters if you don't
	// count the null character.
	char testString[50];

	// Input the string "in the aggregate," so that the word from the console will be
	// automatically copied to the array, with a null terminator at the end.
	cout << "Enter string: ";
	cin >> testString;

	// Loop though the array until you get to the null character, and output each
	// character in the string on a new line
	for(int i = 0; testString[i]; i++)
 		cout << testString[i] << endl;

	cout << endl;
	system("pause");
	
	return 0;
}

// This is the implementation of the function with a reference parameter. As with the
// prototype, the reference parameter is specified with an ampersand. The function
// simply plus-equals the value into the result, but the important part is that this
// code will change the value of the "result" variable in MAIN
void addNumber(int &addTo, int value) {
	addTo += value;
}

// This function loops through the elements in "arrayParam," setting each one to the
// value specified by the parameter "setValue." Note that this will change the values
// in the "array" variable in MAIN.
void setArray(int arrayParam[], int arraySize, int setValue) {
	for(int i = 0; i < arraySize; i++) {
		arrayParam[i] = setValue;
	}
}

Reference Parameters

Before we get to arrays, there’s a small but important concept that you should be familiar with: reference parameters. In the past, I’ve emphasized how the formal parameters of your functions are simply copies of the actual parameters passed into the function. However, if you define a parameter as a reference parameter, how it is changed within the function will actually change the value that was passed into it. For example, if you passed an integer variable by reference from your main function, after the function, the variable—in main—could hold a different value.

The syntax to declare a reference parameter is exactly the same as a normal parameter, except that you add an ampersand before the parameter name. Also remember to also include it in the function prototype.

void refFunction(int &refer);

main() {
	int variable = 5;

	cout << “Variable before function: “ << variable << endl; 		// will be 5

	refFunction(variable);

	refFunction << “Variable after function: “ << variable << endl; 	// will be 10
}

void refFunction(int &refer) {
	refer += 5;
}

This short example will first output the value 5, as you would expect. However, after the function is called, the value will actually be changed within main, and the program will output the value 10.

Concept of Arrays

Now, on to another important concept of programming—arrays! Arrays allow you to contain more than one piece of data within a variable. They’re quite simple: an array is essentially a collection of a type of variable. For example, you could have an array of 10 integers, meaning you can store 10 integer values in that one array. We'll learn more about how they are stored in lesson 10.

Declaring Arrays

Arrays are declared in the same way as variables, except with the addition of square brackets. As with normal variables, you first specify the type of your variable (int, double, char, etc). Then, add your identifier, and finally the array size in square brackets at the end of the identifier.

Additionally, when you declare an array of a fundamental type (and only when you declare it), you can set all values to a certain initial value by adding “= {value}” to your declaration.

int integerArray[50];

Declares an array of 50 integers

char characterArray[10] = {‘a’};

Declares an array of 10 characters, and initializes all of them to ‘a.'

Using Arrays

Accessing data within an array is also quite simple. To access an element in the array, simply write the array’s identifier followed by brackets with the position of the element in the array. The positions of the elements, referred to as indices, always begin at 0. This means that the first element in the array is at index 0, the second at 1, and so on. The final element is stored at the index of your array's total size minus one.

While it may be tempting to simply set one array identifier equal to another to copy the data, this is not allowed (again, see lesson 10). Hence, if you want to cope one array to another, you must loop through it and copy each element individually.

Working with arrays is a big source of errors for beginning programmers, as the process is susceptible to many “off-by-one” errors. However, that's not all: if you try to access or assign a value that is past the end of an array or before it begins, you may or may not generate an error, but it will always be a bug. You won’t always generate an error because if you go off the end of an array, you will start assigning values to whatever memory was after the end of your array. If that memory was used by the operating system, your program will throw an error known as a segmentation fault, or seg fault for short. People tend to have a love/hate relationship with seg faults, as if you have one, it’s good to know that you have a bug, and it's relatively easy to find where the error is. However, they’re also bad—they might mean that you have other seg faults waiting to happen, but haven’t—remember that going into memory past or before your array might not cause your program to crash. To top it all off, the memory arrangement will be different on different computers, so your program may behave differently. If there’s anything to take from all this, just remember to be very diligent in looking for errors when using array.

If you are using linux, the utility valgrind is extremely useful in memory profiling and finding segmentation faults (also memory leaks, see lesson 11).

integerArray[0] = 5;

Sets the first element in the array to 5.

characterArray[5] = ‘l’;

Sets the sixth element in the array to ‘l.’

char testValue = characterArray[10];

This example will try to access the 11th element in the array, when it only contains 10 values. Hence, it will try to read the memory after your array, and might throw a segmentation fault.

Arrays and Loops

The ability to assign values to individual elements in an array is useful, but most the array processing you’ll be doing will be using loops. There isn’t anything new to learn here, but realize that you can apply loops in many ways: for example, loop from 0 to your array size, and assign a value to each element in the array using your counter variable. This is one of the ways that the "for" loop is most useful. Finally, remember to be careful of off-by-one errors—for example, if the first example looped until "i" was less than or equal to 10, in the last iteration it would try to access array[10], the 11th element—possibly causing an error.

for(int i = 0; i < 50; i++)
	integerArray[i] = 0;

This example simply loops through each element in a 50 integer array and sets it to 0.

for(int i = 0; i < 10; i++)
	characterArray[i] += i;

This example will loop through each element of the array, adding adding an increasing amount to each one (i.e. add zero to the first element, so it will remain ‘a’, add one to the second element, so it will be ‘b’, etc.).

Array Parameters

When you pass an array to a function, it will automatically act like a reference parameter, even if you don’t use the ampersand. Because of this, remember that when you pass arrays to functions, the function will change the values of the array within main.

To pass an array to a function, you must first make it clear that your function will take an array: in the prototype, simply type a pair of empty square brackets. You don’t actually have to specify the size of the array when you pass it to a function, it just has to know that the data will be an array. You do the same in the function implementation, and finally, when calling the function with actual parameters, simply type the array identifier without any brackets at all.

void clearArray(int array[],int size);

main() {
	int integerArray[10];

	clearArray(integerArray,10);
}

void clearArray(int array[], int size) {
	for(int I = 0; I < size; I++)
		array[i] = 0;
	}
}

C-Strings

The final subtopic in this section covers C-Style strings, or just c-strings for short. C-strings are technically just arrays of characters and have specific ways of working with them. The basic principle is quite simple—the array holds the characters of the string, and is "null-terminated." This means that a null character (‘\0’) must always signify the end of the string. Because of this property, the size of the array must always be at least one more than the number of actual characters in your string.

You can use C-Strings how you would expect a string type to work: for example, you can cin to a c-string and your program will automatically copy the input word to the array, adding a null character to the end. When you use a c-string like that, it is called using it in the aggregate.

Finally, you can easily do character-by-character manipulation of c-strings, as you know that you have reached the end of the string when you find a null character. We won’t be going very in-depth with c-strings here (see lesson 11), but you should learn how to declare and use them.

char cstring[50];

The first example declares a c-string that can hold 50 characters. Because at least one element must contain the null character, it can hold 49 "real" characters.

cin >> cstring;

This will input one word from the console and store it in the array named "cstring," automatically adding a null character after the word.

for(int i = 0; cstring[i]; i++)
	cout << cstring[i] << endl;

The final example will loop through the array "cstring" until it comes to a null character (you can simply use the character as the conditional statement, as it will evaluate to true if it is anything but null) and output each character on a new line.

Programming Exercises

Write a function (using reference parameters) that calculates the roots of a quadratic function (feel free to use cmath for square roots and powers).
Write a function that puts values into an array of integers, another function that sorts the values from lowest to highest, and finally a function to output the array. There are many methods of sorting, but they’re all pretty simple. If you’re stuck, look at lesson 09.
Write a program to input a c-style string and output it backwards, character by character, encoded with a code-wheel type offset (each character is offset), or anything else you can think of. See lesson 11 for more.

Example Program

Download Run this Code

// If you are not sure what some lines of code do, try looking back at
// previous example programs, notes, or ask a question.

#include <iostream>

using namespace std;

// The function prototype of a function with a reference parameter. To specify the
// reference parameter, simply type and ampersand after the data type.
// Note that it does not return anything, as the data will be passed back through
// the reference parameter.
void addNumber(int &addTo, int value);

// This function prototype includes an array, which automatically acts as a reference
// parameter -- changes to the array within the function will also effect MAIN.
// Note that you do not need to specify the array size.
void setArray(int arrayParam[], int arraySize, int setValue);

int main() {
	
	int result = 5;

	cout << "Number before function: " << result << endl;

	// Passes the result value as a reference parameter, so that its value will
	// be changed within MAIN.
	addNumber(result, 10);

	cout << "Number after function: " << result << endl;

	// This declares an array of integers with a size of ten.
	int array[10];

	cout << endl;

	// This will output a garbage value, as the values in the array have not been
	// initialized. Note the use of array[0] to denote the first element.
	cout << "Garbage first value in array: " << array[0] << endl;

	// For loops are particularly useful in looping through arrays. This loop sets
	// the first value in the array to 1, the second to 2, and so on.
	for(int i = 0; i < 10; i++) {
		array[i] = i + 1;
	}

	// Output the fourth and sixth values in the array, which will now be 4. Note the 
	// use of array[3] and array[5] to denote the fourth and sixth elements.
	cout << "After the loop, the fourth value in the array is: " << array[3] << endl;
	cout << "After the loop, the sixth value in the array is: " << array[5] << endl;
	
	cout << endl;
	
	// This calls the setArray function, which will set the first 10 values in the
	// array "array" to 25. This will effect the "array" variable in MAIN.
	setArray(array, 10, 25);

	// Again, loop through the array, outputting each value. This shows that all the
	// elements in the array have indeed been set to 25.
	for(int i = 0; i < 10; i++) {
		cout << "Array element " << i << ": " << array[i] << endl;
	}

	cout << endl;

	// Declares a c-string that can hold 50 characters, or 49 characters if you don't
	// count the null character.
	char testString[50];

	// Input the string "in the aggregate," so that the word from the console will be
	// automatically copied to the array, with a null terminator at the end.
	cout << "Enter string: ";
	cin >> testString;

	// Loop though the array until you get to the null character, and output each
	// character in the string on a new line
	for(int i = 0; testString[i]; i++)
 		cout << testString[i] << endl;

	cout << endl;
	system("pause");
	
	return 0;
}

// This is the implementation of the function with a reference parameter. As with the
// prototype, the reference parameter is specified with an ampersand. The function
// simply plus-equals the value into the result, but the important part is that this
// code will change the value of the "result" variable in MAIN
void addNumber(int &addTo, int value) {
	addTo += value;
}

// This function loops through the elements in "arrayParam," setting each one to the
// value specified by the parameter "setValue." Note that this will change the values
// in the "array" variable in MAIN.
void setArray(int arrayParam[], int arraySize, int setValue) {
	for(int i = 0; i < arraySize; i++) {
		arrayParam[i] = setValue;
	}
}