Processing arrays using loops
A convenient way to process an array is to iterate over the array using a loop. The property length of an array can help us to avoid ArrayIndexOutOfBoundsException.
Example 1. Filling an array with the squares of indexes of its elements.
int n = 10; // the size of an array
int[] squares = new int[n]; // creating an array with the specified size
System.out.println(Arrays.toString(squares)); // [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
/* iterating over the array */
for (int i = 0; i < squares.length; i++) {
squares[i] = i * i; // set the value by the element index
}
System.out.println(Arrays.toString(squares)); // [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]In this code, an array with the size 10 and filled with zeros is created. Then the value of each element of the array is set to the square of the element's index. Then the program converts the array to the string representation (with square brackets) and prints it to the standard output.
Example 2. Checking the order of elements.
The following program checks the given array is sorted ascending and prints "OK", otherwise it prints "BROKEN".
int[] numbers = { 1, 2, 3, 4, 5, 10, 6 }; // the order is broken
boolean broken = false; // suppose the array is well-ordered
/* iterating over the array */
for (int i = 1; i < numbers.length; i++) {
if (numbers[i] < numbers[i - 1]) { // if the order is broken
broken = true; // write a result
break; // terminate the loop
}
}
System.out.println(broken ? "BROKEN" : "OK");For the given array the program prints "BROKEN".
To iterate over arrays while and do-while loops are also admissible, but they are used less often.
Reading an array from the standard input
Using a loop we can read all elements of an array from the standard input.
For example, the input consists of two lines. The first line contains the length of an array, the second line - all elements of the array.
5
101 102 504 302 881Let's read these numbers using the Scanner (you can use another tool for reading) and then output all read numbers.
import java.util.Scanner;
import java.util.Arrays;
public class ReadingArrayExample {
public static void main(String args[]) {
Scanner scanner = new Scanner(System.in);
int len = scanner.nextInt(); // reading a length
int[] array = new int[len]; // creating an array with the specified length
for (int i = 0; i < len; i++) {
array[i] = scanner.nextInt(); // read the next number of the array
}
System.out.println(Arrays.toString(array)); // output the array
}
}The program outputs:
[101, 102, 504, 302, 881]Iterating over arrays using the for-each loop
Since Java 5 there is a special form of the for-loop called for-each. It is used to iterate through each element of an array, string or a collection (we will learn them in next lessons) without indexes.
Let's write code for calculating the number of 'a' letters in the given character array. To iterate over the array we'll use for-each loop.
char[] characters = { 'a', 'b', 'c', 'a', 'b', 'c', 'a' };
int counter = 0;
for (char ch : characters) {
if (ch == 'a') {
counter++;
}
}
System.out.println(counter); // it outputs "3"As you can see, the absence of indexes makes the code more readable. It also allows you to avoid
ArrayIndexOutOfBoundsException.