Unit 7 Arrays and Strings

Loop Through an Array

You can loop through the array elements with the for loop.

The following example outputs all elements in the cars array:

Example

string cars[4] = {"Volvo", "BMW", "Ford", "Mazda"};

for (int i = 0; i < 4; i++) {

cout << cars[i] << "\n";

}»

The following example outputs the index of each element together with its value:

Example

string cars[4] = {"Volvo", "BMW", "Ford", "Mazda"};

for (int i = 0; i < 4; i++) {

cout << i << ": " << cars[i] << "\n";

}

Omit Array Size

You don't have to specify the size of the array. But if you don't, it will only be as big as the elements that are inserted into it:

string cars[] = {"Volvo", "BMW", "Ford"}; // size of array is always 3

This is completely fine. However, the problem arise if you want extra space for future elements. Then you have to overwrite the existing values:

string cars[] = {"Volvo", "BMW", "Ford"};

string cars[] = {"Volvo", "BMW", "Ford", "Mazda", "Tesla"};

If you specify the size however, the array will reserve the extra space:

string cars[5] = {"Volvo", "BMW", "Ford"}; // size of array is 5, even though it's only three elements inside it

Now you can add a fourth and fifth element without overwriting the others:

Example

cars[3] = "Mazda";

cars[4] = "Tesla";

Omit Elements on Declaration

It is also possible to declare an array without specifying the elements on declaration, and add them later:

Example

string cars[5];

cars[0] = "Volvo";

cars[1] = "BMW";

...

Size of an Array

To get the size of an array, you can use the sizeof() operator:

Example

int myNumbers[5] = {10, 20, 30, 40, 50};

cout << sizeof(myNumbers);

Result: 20

Why did the result show 20 instead of 5, when the array contains 5 elements?

It is because the sizeof() operator returns the size of a type in bytes.

You learned from the Data Types chapter that an int type is usually 4 bytes, so from the example above, 4 x 5 (4 bytes x 5 elements) = 20 bytes.

To find out how many elements an array has, you have to divide the size of the array by the size of the data type it contains:

Example

int myNumbers[5] = {10, 20, 30, 40, 50};

int getArrayLength = sizeof(myNumbers) / sizeof(int);

cout << getArrayLength;

Result: 5

Multi-Dimensional Arrays

A multi-dimensional array is an array of arrays.

To declare a multi-dimensional array, define the variable type, specify the name of the array followed by square brackets which specify how many elements the main array has, followed by another set of square brackets which indicates how many elements the sub-arrays have:

string letters[2][4];

As with ordinary arrays, you can insert values with an array literal - a comma-separated list inside curly braces. In a multi-dimensional array, each element in an array literal is another array literal.

string letters[2][4] = {

{ "A", "B", "C", "D" },

{ "E", "F", "G", "H" }

};

Each set of square brackets in an array declaration adds another dimension to an array. An array like the one above is said to have two dimensions.

Arrays can have any number of dimensions. The more dimensions an array has, the more complex the code becomes. The following array has three dimensions:

string letters[2][2][2] = {

{

{ "A", "B" },

{ "C", "D" }

},

{

{ "E", "F" },

{ "G", "H" }

}

};

Access the Elements of a Multi-Dimensional Array

To access an element of a multi-dimensional array, specify an index number in each of the array's dimensions.

This statement accesses the value of the element in the first row (0) and third column (2) of the letters array.

Example

string letters[2][4] = {

{ "A", "B", "C", "D" },

{ "E", "F", "G", "H" }

};

cout << letters[0][2]; // Outputs "C"

Remember that: Array indexes start with 0: [0] is the first element. [1] is the second element, etc.

Change Elements in a Multi-Dimensional Array

To change the value of an element, refer to the index number of the element in each of the dimensions:

Example

string letters[2][4] = {

{ "A", "B", "C", "D" },

{ "E", "F", "G", "H" }

};

letters[0][0] = "Z";

cout << letters[0][0]; // Now outputs "Z" instead of "A"

Loop Through a Multi-Dimensional Array

To loop through a multi-dimensional array, you need one loop for each of the array's dimensions.

The following example outputs all elements in the letters array:

Example

string letters[2][4] = {

{ "A", "B", "C", "D" },

{ "E", "F", "G", "H" }

};

for(int i = 0; i < 2; i++) {

for(int j = 0; j < 4; j++) {

cout << letters[i][j] << "\n";

}

}

This example shows how to loop through a three-dimensional array:

Example

string letters[2][2][2] = {

{

{ "A", "B" },

{ "C", "D" }

},

{

{ "E", "F" },

{ "G", "H" }

}

};

for(int i = 0; i < 2; i++) {

for(int j = 0; j < 2; j++) {

for(int k = 0; k < 2; k++) {

cout << letters[i][j][k] << "\n";

}

}

}

Why Multi-Dimensional Arrays?

Multi-dimensional arrays are great at representing grids. This example shows a practical use for them. In the following example we use a multi-dimensional array to represent a small game of Battleship:

Example

// We put "1" to indicate there is a ship.

bool ships[4][4] = {

{ 0, 1, 1, 0 },

{ 0, 0, 0, 0 },

{ 0, 0, 1, 0 },

{ 0, 0, 1, 0 }

};

// Keep track of how many hits the player has and how many turns they have played in these variables

int hits = 0;

int numberOfTurns = 0;

// Allow the player to keep going until they have hit all four ships

while (hits < 4) {

int row, column;

cout << "Selecting coordinates\n";

// Ask the player for a row

cout << "Choose a row number between 0 and 3: ";

cin >> row;

// Ask the player for a column

cout << "Choose a column number between 0 and 3: ";

cin >> column;

// Check if a ship exists in those coordinates

if (ships[row][column]) {

// If the player hit a ship, remove it by setting the value to zero.

ships[row][column] = 0;

// Increase the hit counter

hits++;

// Tell the player that they have hit a ship and how many ships are left

cout << "Hit! " << (4-hits) << " left.\n\n";

} else {

// Tell the player that they missed

cout << "Miss\n\n";

}

// Count how many turns the player has taken

numberOfTurns++;

}

cout << "Victory!\n";

cout << "You won in " << numberOfTurns << " turns";

C++ Strings

Strings are used for storing text.

A string variable contains a collection of characters surrounded by double quotes:

Example

Create a variable of type string and assign it a value:

string greeting = "Hello";

To use strings, you must include an additional header file in the source code, the <string> library:

Example

// Include the string library

#include <string>

// Create a string variable

string greeting = "Hello";

String Concatenation

The + operator can be used between strings to add them together to make a new string. This is called concatenation:

Example

string firstName = "John ";

string lastName = "Doe";

string fullName = firstName + lastName;

cout << fullName;

In the example above, we added a space after firstName to create a space between John and Doe on output. However, you could also add a space with quotes (" " or ' '):

Example

string firstName = "John";

string lastName = "Doe";

string fullName = firstName + " " + lastName;

cout << fullName;

Append

A string in C++ is actually an object, which contain functions that can perform certain operations on strings. For example, you can also concatenate strings with the append() function:

Example

string firstName = "John ";

string lastName = "Doe";

string fullName = firstName.append(lastName);

cout << fullName;

Adding Numbers and Strings

WARNING!

C++ uses the + operator for both addition and concatenation.

Numbers are added. Strings are concatenated.

If you add two numbers, the result will be a number:

Example

int x = 10;

int y = 20;

int z = x + y; // z will be 30 (an integer)

If you add two strings, the result will be a string concatenation:

Example

string x = "10";

string y = "20";

string z = x + y; // z will be 1020 (a string)

If you try to add a number to a string, an error occurs:

Example

string x = "10";

int y = 20;

string z = x + y;

String Length

To get the length of a string, use the length() function:

Example

string txt = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

cout << "The length of the txt string is: " << txt.length();

Tip: You might see some C++ programs that use the size() function to get the length of a string. This is just an alias of length(). It is completely up to you if you want to use length() or size():

Example

string txt = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

cout << "The length of the txt string is: " << txt.size();

Access Strings

You can access the characters in a string by referring to its index number inside square brackets [].

This example prints the first character in myString:

Example

string myString = "Hello";

cout << myString[0];

// Outputs H

Note: String indexes start with 0: [0] is the first character. [1] is the second character, etc.

This example prints the second character in myString:

Example

string myString = "Hello";

cout << myString[1];

// Outputs e

Change String Characters

To change the value of a specific character in a string, refer to the index number, and use single quotes:

Example

string myString = "Hello";

myString[0] = 'J';

cout << myString;

// Outputs Jello instead of Hello

User Input Strings

It is possible to use the extraction operator >> on cin to display a string entered by a user:

Example

string firstName;

cout << "Type your first name: ";

cin >> firstName; // get user input from the keyboard

cout << "Your name is: " << firstName;

// Type your first name: John

// Your name is: John

However, cin considers a space (whitespace, tabs, etc) as a terminating character, which means that it can only display a single word (even if you type many words):

Example

string fullName;

cout << "Type your full name: ";

cin >> fullName;

cout << "Your name is: " << fullName;

// Type your full name: John Doe

// Your name is: John

From the example above, you would expect the program to print "John Doe", but it only prints "John".

That's why, when working with strings, we often use the getline() function to read a line of text. It takes cin as the first parameter, and the string variable as second:

Example

string fullName;

cout << "Type your full name: ";

getline (cin, fullName);

cout << "Your name is: " << fullName;

// Type your full name: John Doe

// Your name is: John Doe

Omitting Namespace

You might see some C++ programs that runs without the standard namespace library. The using namespace std line can be omitted and replaced with the std keyword, followed by the :: operator for string (and cout) objects:

Example

#include <iostream>

#include <string>

int main() {

std::string greeting = "Hello";

std::cout << greeting;

return 0;

}