Arrays

 

Chapter: 20

Arrays

Arrays

An array is a collection of variables of the same type.
When you need to store a list of values, such as numbers, you can store them in an array, instead of declaring separate variables for each number.

To declare an array, you need to define the type of the elements with square brackets.
For example, to declare an array of integers:

int[ ] arr;

JAVA

The name of the array is arr. The type of elements it will hold is int.

Now, you need to define the array's capacity, or the number of elements it will hold. To accomplish this, use the keyword new

int[ ] arr = new int[5];

JAVA

The code above declares an array of 5 integers.
In an array, the elements are ordered and each has a specific and constant position, which is called an index.

To reference elements in an array, type the name of the array followed by the index position within a pair of square brackets.
Example:

arr[2] = 42;

JAVA

This assigns a value of 42 to the element with 2 as its index.

Note that elements in the array are identified with zero-based index numbers, meaning that the first element's index is 0 rather than one. So, the maximum index of the array int[5] is 4.

Arrays

In Java, an array starts counting from...

1

2

0

Initializing Arrays

Java provides a shortcut for instantiating arrays of primitive types and strings.
If you already know what values to insert into the array, you can use an array literal.
Example of an array literal:

String[ ] myNames = { "A", "B", "C", "D"};
System.out.println(myNames[2]);

JAVA

Place the values in a comma-separated list, enclosed in curly braces.
The code above automatically initializes an array containing 4 elements, and stores the provided values.

Sometimes you might see the square brackets placed after the array name, which also works, but the preferred way is to place the brackets after the array's data type.

Initializing Arrays

What is the output of this code? int tmp[ ] = {5, 8, 9, 3}; System.out.println(tmp[2]);

Chapter: 21

Summing Elements in Arrays

Array Length

You can access the length of an array (the number of elements it stores) via its length property.
Example:

int[ ] intArr = new int[5];
System.out.println(intArr.length);

JAVA

Don't forget that in arrays, indexes start from 0. So, in the example above, the last index is 4.

Array Length

Fill in the blanks to retrieve the number of elements in an array called "test".

test.

Arrays

Now that we know how to set and get array elements, we can calculate the sum of all elements in an array by using loops.
The for loop is the most used loop when working with arrays, as we can use the length of the array to determine how many times to run the loop.

int [ ] myArr = {6, 42, 3, 7};
int sum=0;
for(int x=0; x<myArr.length; x++) {
sum += myArr[x];
}
System.out.println(sum);

// 58

JAVA

In the code above, we declared a variable sum to store the result and assigned it 0.
Then we used a for loop to iterate through the array, and added each element's value to the variable.

The condition of the for loop is x<myArr.length, as the last element's index is myArr.length-1.

Arrays

Fill in the blanks to calculate the sum of myArray's elements using the for loop, and print it to the screen. myArray is an array of doubles, containing 4 elements.

double sum = 0.0;

for (int x = 0; x < 4; x

) {

sum += myArray[x];

}

System.out.

Chapter: 22

Enhanced for Loop

Enhanced for Loop

The enhanced for loop (sometimes called a "for each" loop) is used to traverse elements in arrays.
The advantages are that it eliminates the possibility of bugs and makes the code easier to read.
Example:

int[ ] primes = {2, 3, 5, 7};

for (int t: primes) {
System.out.println(t);
}

/*
2
3
5
7
*/

JAVA

The enhanced for loop declares a variable of a type compatible with the elements of the array being accessed. The variable will be available within the for block, and its value will be the same as the current array element.
So, on each iteration of the loop, the variable t will be equal to the corresponding element in the array.

Notice the colon after the variable in the syntax.

Enhanced for Loop

Fill in the blanks to use the enhanced for loop for an array called "arr":

int total = 0;

for(int x

) {

total += x;

}

Chapter: 23

Multi-dimensional Arrays

Multidimensional Arrays

Multidimensional arrays are array that contain other arrays. The two-dimensional array is the most basic multidimensional array.
To create multidimensional arrays, place each array within its own set of square brackets.
Example of a two-dimensional array:

int[ ][ ] sample = { {1, 2, 3}, {4, 5, 6} };

JAVA

This declares an array with two arrays as its elements.
To access an element in the two-dimensional array, provide two indexes, one for the array, and another for the element inside that array.
The following example accesses the first element in the second array of sample.

int x = sample[1][0];
System.out.println(x);

JAVA

The array's two indexes are called row index and column index.

Multidimensional Arrays

What is the output of this code? String array[ ][ ] = { {"Hey", "John", "Bye"}, {"John", "Johnson", "Hello"} }; System.out.println(array[1][1]);

John

Hello

Johnson

Hey

Multidimensional Arrays

You can get and set a multidimensional array's elements using the same pair of square brackets.
Example:

int[ ][ ] myArr = { {1, 2, 3}, {4}, {5, 6, 7} };
myArr[0][2] = 42;
int x = myArr[1][0]; // 4

JAVA

The above two-dimensional array contains three arrays. The first array has three elements, the second has a single element and the last of these has three elements.

In Java, you're not limited to just two-dimensional arrays. Arrays can be nested within arrays to as many levels as your program needs. All you need to declare an array with more than two dimensions, is to add as many sets of empty brackets as you need. However, these are harder to maintain.
Remember, that all array members must be of the same type.

Multidimensional Arrays

What is the output of this code? int array[ ][ ] = {{3, 5, 8}, {7, 54, 1, 12, 4}}; System.out.println(array[0][2]);

Chapter: 24

Module 3 Quiz

What is the output of this code? int arr[ ] = new int[3]; for (int i = 0; i < 3; i++) { arr[i] = i; } int res = arr[0] + arr[2]; System.out.println(res);

What is the output of this code? int result = 0; for (int i = 0; i < 5; i++) { if (i == 3) { result += 10; } else { result += i; } } System.out.println(result);

Code Project

Reverse a String

Reverse a String

Write a program to take a string as input and output its reverse.
The given code takes a string as input and converts it into a char array, which contains letters of the string as its elements.

Sample Input:
hello there

Sample Output:
ereht olleh

Note: Comment down your answer after solving the question. 

You can loop through the char array, starting from the end, using arr.length to get the size of the array.