Decisions and Loops

Introduction

Programs become useful when they can decide and repeat. This lesson covers both. If you know another language, the ideas will be familiar, and only the exact spelling differs.

Comparisons and logic

Comparison operators give a boolean: ==, !=, <, >, <=, >=. Combine them with && (and), || (or) and ! (not).

Java

public class Main {
    public static void main(String[] args) {
        int age = 15;

        System.out.println(age >= 12);
        System.out.println(age == 15);
        System.out.println(age > 12 && age < 18);
        System.out.println(age < 5 || age > 65);
        System.out.println(!(age > 12));
    }
}

true
true
true
false
false

A single = stores a value. A double == compares. (For text, never use ==, as the strings lesson explains.)

if, else if, else

Java

public class Main {
    public static void main(String[] args) {
        int temperature = 28;

        if (temperature > 30) {
            System.out.println("Hot");
        } else if (temperature > 20) {
            System.out.println("Warm");
        } else {
            System.out.println("Cool");
        }
    }
}

Warm

The condition goes in round brackets, and the code in curly brackets. Java checks from the top and runs the first branch whose condition is true. Unlike Python, indentation is not required: the braces matter. Still, indent consistently, because it is how humans read the code.

Quick quiz

Which operator means 'and' in Java conditions?

The ternary operator

A compact if-else that produces a value: condition ? valueIfTrue : valueIfFalse.

Java

public class Main {
    public static void main(String[] args) {
        int score = 72;
        String result = score >= 50 ? "pass" : "fail";
        System.out.println(result);
    }
}

pass

switch

When one value is compared to many options, switch is tidier. Modern Java (14 and newer) has an arrow form that is shorter and avoids the old "fall through" bug.

Java

public class Main {
    public static void main(String[] args) {
        int day = 3;

        String name = switch (day) {
            case 1 -> "Monday";
            case 2 -> "Tuesday";
            case 3 -> "Wednesday";
            case 6, 7 -> "Weekend";
            default -> "Another day";
        };

        System.out.println(name);
    }
}

Wednesday

switch here is an expression that produces a value, and default handles everything else. Several values can share one case, like case 6, 7.

for loops

A for loop has three parts in its brackets: where to start, when to keep going, and what to do after each round.

Java

public class Main {
    public static void main(String[] args) {
        for (int i = 1; i <= 5; i++) {
            System.out.println("Round " + i);
        }
    }
}

Round 1
Round 2
Round 3
Round 4
Round 5

Read for (int i = 1; i <= 5; i++) as: start with i = 1, continue while i <= 5, and add 1 to i after each round.

while and do-while

while checks the condition before each round. do-while checks after, so it always runs at least once.

Java

public class Main {
    public static void main(String[] args) {
        int count = 3;
        while (count > 0) {
            System.out.println(count);
            count--;
        }
        System.out.println("Liftoff!");

        int n = 10;
        do {
            System.out.println("n is " + n);
            n++;
        } while (n < 10);
    }
}

3
2
1
Liftoff!
n is 10

The do-while body ran once even though n < 10 was false from the start. If nothing in a while loop ever changes the condition, it runs forever, which is an infinite loop.

break and continue

break leaves the loop. continue skips to the next round.

Java

public class Main {
    public static void main(String[] args) {
        for (int i = 1; i <= 10; i++) {
            if (i % 2 == 0) {
                continue;
            }
            if (i > 7) {
                break;
            }
            System.out.println(i);
        }
    }
}

Nested loops

A loop inside a loop. The inner loop runs completely for each round of the outer one.

Java

public class Main {
    public static void main(String[] args) {
        for (int row = 1; row <= 3; row++) {
            for (int col = 1; col <= 4; col++) {
                System.out.print(row * col + "\t");
            }
            System.out.println();
        }
    }
}

1	2	3	4	
2	4	6	8	
3	6	9	12

Quick quiz

How is do-while different from while?

FizzBuzz

A classic exercise combining everything: for numbers 1 to 15, print Fizz for multiples of 3, Buzz for multiples of 5, FizzBuzz for both, otherwise the number.

Java

public class Main {
    public static void main(String[] args) {
        for (int n = 1; n <= 15; n++) {
            if (n % 15 == 0) {
                System.out.println("FizzBuzz");
            } else if (n % 3 == 0) {
                System.out.println("Fizz");
            } else if (n % 5 == 0) {
                System.out.println("Buzz");
            } else {
                System.out.println(n);
            }
        }
    }
}

1
2
Fizz
4
Buzz
Fizz
7
8
Fizz
Buzz
11
Fizz
13
14
FizzBuzz

Try it yourself

On your computer, write a program that uses a loop to add up all the numbers from 1 to 100 and prints 5050. Then change it to add only the even numbers, which gives 2550.

Summary

Comparisons give boolean. Combine with &&, || and !.
if / else if / else choose a branch. switch with arrows picks among many values.
for, while and do-while repeat code. break leaves a loop and continue skips a round.
Braces mark blocks in Java. Indentation is for readers, not for the compiler.

Decisions and Loops

In this lesson you will

Introduction

Comparisons and logic

if, else if, else

The ternary operator

switch

for loops

while and do-while

break and continue

Nested loops

FizzBuzz

Try it yourself

Summary

Lesson complete