Loops and arrays — Java

In this lesson you will learn

Write for, while and do-while loops and choose between them
Control a loop with break and continue
Create an array, read and change its elements, and find their sum and maximum

Imagine you have to print a greeting for each of the 30 students in a class. You could write println 30 times — but what if there were 3000 students? Computers are great at repeating the same work without getting tired, and that is what loops are for. To keep many values in one place, we use arrays.

for and while loops

A for loop is handy when you know the number of steps in advance. Its header has three parts: the start (int i = 1), the condition (i <= 5) and the step (i++). A while loop repeats as long as its condition is true — it suits cases where you do not know how many repetitions there will be.

Java

public class Main {
    public static void main(String[] args) {
        for (int i = 1; i <= 5; i++) {
            System.out.print(i + " ");
        }
        System.out.println();
        int sum = 0;
        for (int i = 2; i <= 10; i += 2) {
            sum += i;
        }
        System.out.println("Sum of even numbers: " + sum);
        int balance = 100;
        int years = 0;
        while (balance < 200) {
            balance += balance / 10;
            years++;
        }
        System.out.println(years + " years, balance " + balance);
    }
}

Expected output

1 2 3 4 5 
Sum of even numbers: 30
8 years, balance 212

In the last part, Elvin puts 100 manats in a bank, and each year 10% is added to the amount. We do not know how many years he has to wait, so we use a while loop: it keeps going until the balance reaches 200. balance / 10 is integer division, so each year's interest is rounded down to whole manats.

Interactive

Loading simulation…

The exact version of the same calculation: without dropping fractions, you get a little over 214 manats after 8 years. The int in the program throws away the fractions every year.

do-while, break and continue

A do-while loop checks its condition at the end, so its body runs at least once even if the condition is false. break ends the loop immediately, while continue skips the rest of the current step and moves on to the next one.

Java

public class Main {
    public static void main(String[] args) {
        int tries = 0;
        do {
            tries++;
            System.out.println("Attempt " + tries);
        } while (tries < 0);
        for (int i = 1; i <= 10; i++) {
            if (i % 3 == 0) {
                continue;
            }
            if (i > 7) {
                break;
            }
            System.out.print(i + " ");
        }
        System.out.println();
    }
}

Expected output

Attempt 1
1 2 4 5 7

The condition tries < 0 is false from the start, yet “Attempt 1” was still printed. In the second loop, 3 and 6 were skipped by continue, and at 8 break stopped the loop completely, so 9 and 10 were never printed.

Arrays

Definition

Array

An object that holds a fixed number of values of the same type in order. Elements are accessed by an index starting at 0, and the array's length cannot change after it is created.

Java

public class Main {
    public static void main(String[] args) {
        int[] scores = {85, 92, 78, 64, 99};
        System.out.println("First: " + scores[0]);
        System.out.println("Count: " + scores.length);
        scores[3] = 70;
        int sum = 0;
        int max = scores[0];
        for (int s : scores) {
            sum += s;
            if (s > max) {
                max = s;
            }
        }
        System.out.println("Average: " + (double) sum / scores.length);
        System.out.println("Max: " + max);
        String[] days = new String[3];
        System.out.println(days[0]);
        double[] temps = new double[2];
        System.out.println(temps[1]);
    }
}

Expected output

First: 85
Count: 5
Average: 84.8
Max: 99
null
0.0

int[] scores = {85, 92, ...}; creates an array with ready values; new String[3] creates an array with 3 empty slots.
scores.length is the number of elements (no parentheses — it is a field, not a method).
for (int s : scores) is a for-each loop: it hands each element of the array to s in turn.
A new array is filled automatically: numbers get 0 or 0.0, boolean gets false, and objects (such as String) get null.

The Arrays class and 2D arrays

The java.util.Arrays class has ready-made tools for arrays: Arrays.toString() turns an array into readable text, and Arrays.sort() puts the elements in ascending order. An array's elements can themselves be arrays — such a two-dimensional array works like a table: the first index is the row and the second is the column.

Java

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        int[] nums = {42, 7, 19, 3, 25};
        System.out.println(Arrays.toString(nums));
        Arrays.sort(nums);
        System.out.println(Arrays.toString(nums));
        int[][] table = new int[3][4];
        for (int r = 0; r < table.length; r++) {
            for (int c = 0; c < table[r].length; c++) {
                table[r][c] = (r + 1) * (c + 1);
            }
        }
        for (int[] row : table) {
            System.out.println(Arrays.toString(row));
        }
        System.out.println(table[2][3]);
    }
}

Expected output

[42, 7, 19, 3, 25]
[3, 7, 19, 25, 42]
[1, 2, 3, 4]
[2, 4, 6, 8]
[3, 6, 9, 12]
12

Key points

for (start; condition; step) is used when the number of steps is known, while to repeat until a condition changes.
do-while checks its condition at the end and runs the body at least once.
break ends the loop; continue jumps to the next step.
An array has a fixed length, and its indexes go from 0 to length - 1.
Arrays.toString() helps you print an array, and Arrays.sort() sorts it.

Check yourself

10 questions. Every correct answer earns XP.

1 / 10

How many times does for (int i = 0; i < 4; i++) repeat?