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Intro to Java

Loops

Another concept in the area of control flow are loops. Loops are used in your program to run the same block of code several times.

In Java there are different kinds of loops:

  1. for
  2. while
  3. do … while
  4. for each

In this lesson we will focus on for and while loops. However, please note, most of these loop types are interchangeable. It does not really matter if you use a for or a while loop to solve your problem. It is even possible to convert a for loop into a while loop and vice versa. But in some cases—for better readability for example—it can be more applicable to use one type of loop instead of another.

Given the syntax is slightly simpler, let’s have a look at while loops first:

While loops

The structure of a while loop is very similar to the structure of an if statement:

while (<CONDITION>) {
    <CODE TO EXECUTE>
}

Let’s deconstruct this block of code:

  1. Like the if keyword the while keyword is a part of the Java language, you just have to learn it by heart.
  2. Like you learned studying the if statement, the <CONDITION> represents a boolean expression, which evaluates to true or false.
  3. The block between the curly braces—{}—will be executed as long as the <CONDITION> evaluates to true.

An example for the use of a while loop is a simple number guessing game. The game works like this:

  1. A random number of type int will be stored in a variable.
  2. The player guesses the number.
  3. The guess will be compared to the random number variable declared in step one.
  4. In every loop a secondary variable will be increased to count the tries the player needed to guess the number.
  5. If the player correctly guessed the given number, the loop stops.
  6. The program then prints the tries the player needed to guess the number.
import java.util.Scanner;

public class NumberGuessingGame {

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        int numberToGuess = 19;
        boolean numberGuessed = false;
        int neededTries = 0;

        while (!numberGuessed) {
            neededTries++;
            System.out.println("Please guess a number between 0 and 20");

            int guessedNumber = input.nextInt();

            if (guessedNumber == numberToGuess) {
                numberGuessed = true;
            }
        }

        System.out.println("Congratulations! You needed " + neededTries + " tries to guess the given number!");
    }
}

For loops

The structure of a for loop is a little more complicated:

for (<INITIAL_VALUE>; <CONDITION>; <INCREMENT>) {
    <CODE TO EXECUTE>
}

Where a while loop is mostly used to run statements until a condition evaluates to false, for loops are often used to run a code block—represented by <CODE TO EXECUTE> between the {}—a given amount of times.

For this kind of loop a few values must be defined.

  1. <INITIAL_VALUE>: The starting value.
  2. <CONDITION>: When to stop.
  3. <INCREMENT>: How much to add to the <INITIAL_VALUE> on each loop.

A very simple example of a for loop is increasing an integer by 1 for every loop and printing the value of the counting variable, in our case, the int i = 0:

public class SimpleForLoopExample {

    public static void main(String[] args) {

        for (int i = 0; i < 10; i++) {
            System.out.println("i is " + i);
        }
    }
}

This example does not do a lot, given it’s just printing the value of i from 0 to 9. To see the power of for loops, let’s convert our earlier example of the number guessing game and substitute the while loop for a for loop. The goal here is to limit the tries a player has to guess the number:

import java.util.Scanner;

public class NumberGuessingGameForLoop {

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        int numberToGuess = 19;
        boolean numberGuessed = false;
        int neededTries = 0;
        int maximumTries = 5;

        for (int i = 0; i < maximumTries; i++) {

            if (!numberGuessed) {
                neededTries++;
                System.out.println("Please guess a number between 0 and 20");

                int guessedNumber = input.nextInt();

                if (guessedNumber == numberToGuess) {
                    numberGuessed = true;
                }
            }
        }

        if (numberGuessed) {
            System.out.println("Congratulations! You needed " + neededTries + " tries to guess the given number!");
        }
        else {
            System.out.println("Sorry but you lost the game!");
        }
    }
}

Another example is the programming game we developed in our first lesson, where we tried to find the tallest person in our group.

If you remember, we learned how to compare the height (or just two numbers) of two people in our previous class about decision statements:

Recap:

public class TallestPerson {

    public static void main(String[] args) {
        int currentHeightInCm = 185;
        int newHeightInCm = 187;        

        if (newHeightInCm > currentHeightInCm) {
            currentHeightInCm = newHeightInCm;
        }

        System.out.println("Right now the tallest person in the group is " + currentHeightInCm + "cm tall!");
    }
}

This program will run once and compare both numbers stored in currentHeightInCm and newHeightInCm and save the largest of both numbers in the variable currentHeightInCm. When the program is finished the variable currentHeightInCm will contain the largest of these two numbers. So far so good. This is a good start, but imagine we now need to execute these statements for every person in our class. How would we do this?

What we need to do:

  1. Ask the user how many people are members of our class.
  2. Store this number.
  3. Create a new variable to store the height of the current tallest person (right now it’s 0 because the tallest person is not yet evaluated).
  4. Ask the next person for his/her height.
  5. Store this height in a second variable.
  6. Compare the new variable with the variable which contains the height of the person who is the tallest person so far.
  7. If the new person is taller than the current tallest person, store the new height in the variable for the current tallest person.
  8. Repeat steps 4 -7 for all students in the class.
import java.util.Scanner;

public class TallestPersonEvaluator {

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        System.out.println("Tell me: How many people are in your class?");

        int numberOfPeople = input.nextInt();

        int tallestPersonInCm = 0;

        for (int i = 0; i < numberOfPeople; i++) {
            System.out.println("How tall is the next person?");

            int currentHeight = input.nextInt();

            if (currentHeight > tallestPersonInCm) {
                tallestPersonInCm = currentHeight;
            }
        }

        System.out.println("The tallest person in your class is " + tallestPersonInCm + " cm tall!");
    }
}

Mostly for loops are used in combination with collections like an ArrayList. What an ArrayList is and how to iterate over it using a for loop will be discussed in a later chapter.

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