Java

ลำดับตอนที่ #7 : Java II - Arrays

Using Arrays

Arrays

          An array is a set of variables. It can manage multiple pieces of data, like a list of names, instead of declaring them separately. Each value in an array is called an element.

Array Data Types

          When declaring a variable for an array, you have to put [] after the type of each element. For example, an array of int values would be type int[] , and an array of String values would be type String[] .

Creating Arrays

          You can make arrays by writing values in { } and separating them with commas , . Check the example below. Be careful not to confuse [] and {}.

The Index Number

          Each element of an array is given a number like 0, 1, 2, ... . These are called index numbers. Keep in mind that index numbers start from 0. You can access individual elements by writing array_name[index] .

Updating the Elements of Arrays

          Let's also try updating an element of an array! Each element in an array is a value, so you can update it by assigning a different value to it.

When the Element Doesn't Exist

          You can't assign a value to an element that doesn't exist. It will result in an error, like in the example below!

Arrays and Loops

          When dealing with arrays, we often want to apply the same code to each element. You can easily do this with the loops we learned earlier. So, let's learn how to use loops with arrays!

Arrays and For Loops

          When you want to print all the elements of an array, it's not efficient to repeat the same code like in the left image. By using a for loop, like the right image, you can easily get the value of each element and print it.

Length of an Array

          You can use length to count the number of elements in an array, like the example on the left. By using length, you can replace i < 3 with i < names.length, like shown on the right. This way you don't need to count the number of elements yourself.

Enhanced For Loops

          The enhanced for loop is a special type of for loop used with arrays. It's just a simpler way of writing for loops. See the example below for the syntax of an enhanced for loop!

How Enhanced For Loops Work

          The enhanced for loop only requires a temporary variable in which every single element of the array is assigned one by one. In the example below, each element of the names array is assigned to the name variable, and then gets printed.

Comparison of For Loops

          Let's practice changing a standard for loop to an enhanced for loop. We used index numbers to get the elements in a standard for loop, but with enhanced for loops we can get the elements directly. Remember how to use enhanced for loops to make your code simpler!

The Final Project

By using what we've learned so far, let's calculate the sum of odd numbers and the sum of even numbers in an array of integers.