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C++ Arrays & Structures - Notes | Study C++ Programming for Beginners - Class 8

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C++ Structures
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C++ Arrays

Arrays are used to store multiple values in a single variable, instead of declaring separate variables for each value.
To declare an array, define the variable type, specify the name of the array followed by square brackets and specify the number of elements it should store:

string cars[4];

We have now declared a variable that holds an array of four strings. To insert values to it, we can use an array literal - place the values in a comma-separated list, inside curly braces:

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

To create an array of three integers, you could write:

int myNum[3] = {10, 20, 30};

Access the Elements of an Array

You access an array element by referring to the index number inside square brackets [].
This statement accesses the value of the first element in cars:

Example

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

cout << cars[0];

// Outputs Volvo

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

Change an Array Element

To change the value of a specific element, refer to the index number:

cars[0] = "Opel";

Example

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

cars[0] = "Opel";

cout << cars[0];

// Now outputs Opel instead of Volvo

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", "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";

...

Get the 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++ Structures

Structures (also called structs) are a way to group several related variables into one place. Each variable in the structure is known as a member of the structure.
Unlike an array, a structure can contain many different data types (int, string, bool, etc.).

Create a Structure

To create a structure, use the struct keyword and declare each of its members inside curly braces.
After the declaration, specify the name of the structure variable (myStructure in the example below):

struct {             // Structure declaration

  int myNum;         // Member (int variable)

  string myString;   // Member (string variable)

} myStructure;       // Structure variable

Access Structure Members

To access members of a structure, use the dot syntax (.):
Example

Assign data to members of a structure and print it:

// Create a structure variable called myStructure

struct {

  int myNum;

  string myString;

} myStructure;


// Assign values to members of myStructure

myStructure.myNum = 1;

myStructure.myString = "Hello World!";


// Print members of myStructure

cout << myStructure.myNum << "\n";

cout << myStructure.myString << "\n";

One Structure in Multiple Variables

You can use a comma (,) to use one structure in many variables:

struct {

  int myNum;

  string myString;

} myStruct1, myStruct2, myStruct3; // Multiple structure variables separated with commas

This example shows how to use a structure in two different variables:
Example

Use one structure to represent two cars:

struct {

  string brand;

  string model;

  int year;

} myCar1, myCar2; // We can add variables by separating them with a comma here


// Put data into the first structure

myCar1.brand = "BMW";

myCar1.model = "X5";

myCar1.year = 1999;


// Put data into the second structure

myCar2.brand = "Ford";

myCar2.model = "Mustang";

myCar2.year = 1969;


// Print the structure members

cout << myCar1.brand << " " << myCar1.model << " " << myCar1.year << "\n";

cout << myCar2.brand << " " << myCar2.model << " " << myCar2.year << "\n";

Named Structures

By giving a name to the structure, you can treat it as a data type. This means that you can create variables with this structure anywhere in the program at any time.
To create a named structure, put the name of the structure right after the struct keyword:

struct myDataType { // This structure is named "myDataType"

  int myNum;

  string myString;

};

To declare a variable that uses the structure, use the name of the structure as the data type of the variable:

myDataType myVar;

Example

Use one structure to represent two cars:

// Declare a structure named "car"

struct car {

  string brand;

  string model;

  int year;

};


int main() {

  // Create a car structure and store it in myCar1;

  car myCar1;

  myCar1.brand = "BMW";

  myCar1.model = "X5";

  myCar1.year = 1999;


  // Create another car structure and store it in myCar2;

  car myCar2;

  myCar2.brand = "Ford";

  myCar2.model = "Mustang";

  myCar2.year = 1969;

 

  // Print the structure members

  cout << myCar1.brand << " " << myCar1.model << " " << myCar1.year << "\n";

  cout << myCar2.brand << " " << myCar2.model << " " << myCar2.year << "\n";

 

  return 0;

}

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