BASIC_TYPES.JAVA

Maths and user input

Doing sums in Java, the one division rule that trips everyone, and asking the user questions.

Download week 2 Unzip for DogYears.java and Greeter.java.

Maths

Symbol	Does	Example
`+`	Add	`5 + 2` is `7`
`-`	Subtract	`5 - 2` is `3`
`*`	Multiply	`5 * 2` is `10`
`/`	Divide	`6 / 2` is `3`
`%`	Remainder after dividing	`7 % 2` is `1`

The division trap

Divide two whole numbers and Java throws the remainder away. Drag it.

a 5

whole 5 / 2 = 2 .5 thrown away

decimal 5 / 2.0 = 2.5

The fix is to make one side a decimal. Any of these work:

5 / 2.0
5.0 / 2
(double) 5 / 2

Why this matters

When a number comes out 0.0 and the reason isn't clear, this is almost always it.

Casting

Casting changes a value from one type to another. Put the desired type in brackets in front of it.

(int) 3.4      // 3
(int) 3.9      // 3, not 4
(double) 4    // 4.0

It chops, it does not round

(int) 3.9 is 3. Casting a decimal to a whole number cuts the decimal part off and keeps what is left.

User input

A Scanner reads what the user types. Three parts.

// 1. at the very top of the file
import java.util.Scanner;

public class Greeter {
    public static void main(String[] args) {
        // 2. make one Scanner
        Scanner scanner = new Scanner(System.in);

        System.out.println("What is your name?");

        // 3. read whatever they type
        String name = scanner.nextLine();

        System.out.println("Hello, " + name + "!");
    }
}

To read	Use
A line of words	`scanner.nextLine()`
A whole number	`scanner.nextInt()`
A decimal	`scanner.nextDouble()`

Make the Scanner once, at the top of main. A new one is not needed for each question.

Problems

1. Work them out on paper

Write the answer for each, then run them and check.

5 / 2            2.5 * 2
5 / 2.0          (int) (2.5 * 3)
(int) (5.0 / 2.0)    2.5 + 2.5
(int) (5 / 2)        3.0 + 7
5 * 2            (int) (2.5 - 5)

Hint

For each one ask two questions. Are both sides whole numbers? And is there a cast on the outside? The cast happens last, after everything in the brackets.

2. Dog years

DogYears.java

Store a human age in an int, convert it to dog years, and print the result. One dog year is about seven human years.

Example output

A 12 year old human is 84 in dog years.

Hint 1

Two variables are needed: one for the human age, one for the dog age. The second is worked out from the first.

Hint 2

Build the sentence by joining pieces with +, the same way "Hello, " + name + "!" works.

3. Ask first

DogYears.java

Modify the dog years program so it asks for the user's age instead of having it fixed in the code.

Example run

How old are you?
12
A 12 year old human is 84 in dog years.

Hint 1

Three things: the import line at the top, one Scanner at the start of main, and nextInt() where the fixed number used to be.

Hint 2

nextInt() returns a number, so it goes on the right of an =, like int age = scanner.nextInt();

4. Split the change

Greeter.java

Start from Greeter.java, which already has a Scanner set up. Ask for a number of sweets and a number of people. Print how many each person gets and how many are left over.

Example run

Sweets? 17
People? 5
3 each, 2 left over.

Hint

This is the one place whole-number division is what's needed. / gives the share, % gives the leftovers.

NEXT Week 3: Conditionals