Input and Repetition: Scanner + While Loop

1Quick recap of Week 1

The lecture started with a recap. Last week you met:

• the three basic structures: sequence, selection and repetition (loops);
• two ways to choose: if / else and switch / case / break / default;
• the enhanced for loop, which runs its body once for each item in an array (a group of values of the same type, stored under one name);
• variables: named boxes. Each box has a name and a type, and holds one value at a time.

Stage: the very start of the lecture.

2Input and output (IO)

Programs need to talk to the outside world. That is called input and output, or IO.

• Input: information coming into the program, for example from the keyboard, mouse, touch screen, a file or another program.
• Output: information the program sends out, for example to the screen, a file or another program.

Humans do IO too. Your senses are inputs; talking, moving and writing are outputs. In a conversation, my output is your input.

Java gives you two basic channels: System.out (output to the screen, which you used with System.out.println) and System.in (input from the keyboard).

Stage: early in the lecture.

3Scanner: reading what the user types

Scanner is a class that comes with Java. It makes reading keyboard input easy. Using it takes a few steps, all shown in SayHello:

// What this class does: asks for your name, then greets you by name.
// import tells Java we want to use the Scanner class, which lives in java.util
import java.util.Scanner;

public class SayHello {
    public void run() {
        // The prompt: print (not println) so you type on the same line
        System.out.print("What is your name? ");

        // Make a new Scanner that reads from the keyboard (System.in),
        // and keep it in a variable called myInput
        Scanner myInput = new Scanner(System.in);

        // Wait for the user to type a word and press Enter.
        // Store that word in a new String variable called name
        String name = myInput.next();

        // Greet the user, using the value stored in name
        System.out.println("Hello " + name + ", nice to meet you");

        // Close the Scanner because we have finished with it
        myInput.close();
    }

    // Start the program (BlueJ: right-click, void main)
    public static void main(String[] args) {
        new SayHello().run();
    }
}

1. import java.util.Scanner; goes at the very top. import tells Java to fetch a ready-made class from its library. Without it, Java does not know what Scanner is.
2. Print a prompt: a message that tells the user what to type. Use print, so the typing appears on the same line.
3. Scanner myInput = new Scanner(System.in); makes the Scanner. new creates a new object (a working copy of a class). You can choose the variable name (myInput), but the rest must be exactly like this. The lecturer said: just remember this line for now; you will understand why later.
4. myInput.next() calls a method on the Scanner. The dot means “use this method of that object”. The program stops and waits until the user types and presses Enter.
5. myInput.close(); at the end, when you have finished with it.

In BlueJ, the Terminal Window has an input line at the bottom. Type there and press Enter. While the program waits, BlueJ’s “running” bar stays striped: the program has not crashed, it is waiting for you.

name = input("What is your name? ")    # prompt and read in one step
print("Hello " + name + ", nice to meet you")

Stage: early to middle. The lecturer live-coded SayHello, starting from what the Terminal Window should look like.

4next, nextLine, nextInt, nextDouble

The lecturer’s demo: if you type New York, nextLine() gets "New York", but next() only gets "New". Also, next() always gives you a String: if you type 42, you get the text "42", not a number you can do sums with. Use nextInt() for that.

// What this class does: asks three questions and reads a line of text,
// a whole number and a decimal number.
import java.util.Scanner;                       // we need the Scanner class

public class AskThree {
    public void run() {
        // Make the Scanner that reads from the keyboard
        Scanner myInput = new Scanner(System.in);

        // Read a whole line, so "Tom Edwards" is kept in full
        System.out.print("What is your full name? ");
        String name = myInput.nextLine();

        // Read a whole number into an int box
        System.out.print("How old are you? ");
        int age = myInput.nextInt();

        // Read a decimal number into a double box
        System.out.print("What did lunch cost? ");
        double price = myInput.nextDouble();

        // Use all three values
        System.out.println(name + " is " + age + " and spent " + price);

        // Finished with the Scanner
        myInput.close();
    }

    // Start the program
    public static void main(String[] args) {
        new AskThree().run();
    }
}

name = input("What is your full name? ")       # always a string
age = int(input("How old are you? "))           # turn the text into a whole number
price = float(input("What did lunch cost? "))   # turn the text into a decimal
print(name, "is", age, "and spent", price)

Stage: the middle of the lecture, after SayHello.

5ScannerTemplate and Duplicate

The lab project has a class called ScannerTemplate. It holds just the basic Scanner code: the import, making the Scanner, and closing it. To start a new program that needs input, right-click ScannerTemplate in BlueJ, choose Duplicate…, give the copy a name that starts with a capital letter, and add your code in the middle.

6TripAdvisor: Scanner plus switch

TripAdvisor brings back last week’s StarRating. Instead of ratings fixed in an array, it asks the user for a place and a star rating, then uses a switch with fall-through (no break between cases) to print that many stars.

// What this class does: asks where you went and how many stars (1-5)
// you give it, then prints the place and that many stars.
import java.util.Scanner;                       // we need the Scanner class

public class TripAdvisor {
    public void run() {
        // Prompt for the place
        System.out.print("Where have you just visited? ");

        // Make the Scanner that reads from the keyboard
        Scanner myInput = new Scanner(System.in);

        // Read one word for the place
        String place = myInput.next();

        // Prompt for the rating, then read it. next() gives text, e.g. "3"
        System.out.print("How many stars would you give it (1-5)? ");
        String starNumber = myInput.next();

        // Show the place and the rating, staying on the same line
        System.out.print(place + " (" + starNumber + " stars): ");

        // Choice with fall-through: jump to the matching case, then keep going
        // down, printing one star per case. default ends the line.
        switch (starNumber) {
            case "5": System.out.print("*");
            case "4": System.out.print("*");
            case "3": System.out.print("*");
            case "2": System.out.print("*");
            case "1": System.out.print("*");
            default:
                System.out.println("");
        }

        // Finished with the Scanner
        myInput.close();
    }

    // Start the program
    public static void main(String[] args) {
        new TripAdvisor().run();
    }
}

The lab file uses ★ stars; this copy uses *. Because starNumber is a String, the case labels are in quotes: case "5":. In the live demo the lecturer read the rating as a whole number with nextInt() instead; then the labels would be case 5: with no quotes.

Stage: the middle of the lecture, just before the break.

7The while loop

The enhanced for loop needs an array: it runs once per item. A while loop needs no list. It uses a condition (a test that is true or false), like an if statement.

The pattern is while (condition) { body }. Read it as: “while the condition is true, run the body again”. Java checks the condition before each run of the body. When the condition is false, the loop stops and Java carries on after the closing }. If it is false the very first time, the body never runs at all.

// The loop variable: a counter that starts at 0
int x = 0;

// Loop: keep running the body while x is less than 5.
// x goes 0, 1, 2, 3, 4, so the body runs 5 times. When x is 5 the test is false and it stops.
while (x < 5) {
    // Say Ho! and stay on the same line
    System.out.print("Ho! ");
    // Add 1 to x, so the loop moves towards stopping
    x = x + 1;
}
// After the loop: finish the line
System.out.println();

x = 0
while x < 5:            # same test, no round brackets needed
    print("Ho! ", end="")
    x = x + 1
print()

Stage: right after the mid-lecture break.

8Making sure the loop stops

The condition’s code stays the same each time, but the values it checks can change. That is what lets a loop stop.

There are two common ways to update the loop variable:

• User input: ask the user for a new value inside the loop (HelloEverybody).
• A calculation: add or subtract from a counter (Santa).

Stopping a runaway program in BlueJ: right-click the striped “running” bar at the bottom left of the main BlueJ window and choose Reset Java Virtual Machine. Then fix the loop and compile again.

9HelloEverybody: stopping with user input

// What this class does: keeps asking for names and greeting each one,
// until the user types bye.
import java.util.Scanner;                       // we need the Scanner class

public class HelloEverybody {
    public void run() {
        // First prompt, before the loop
        System.out.print("What is your name (bye to exit)? ");

        // Make the Scanner and read the first name (a whole line)
        Scanner myInput = new Scanner(System.in);
        String name = myInput.nextLine();

        // Loop: keep going while name is NOT "bye".
        // ! means "not". It stops as soon as the user types bye.
        while (!name.equals("bye")) {
            // Greet the current name
            System.out.println("Hello " + name + ", nice to meet you");

            // Ask again...
            System.out.print("What is your name (bye to exit)? ");
            // ...and UPDATE the loop variable. Without this line it never stops
            name = myInput.nextLine();
        }

        // Runs once the loop has stopped
        System.out.println("Goodbye!");

        // Finished with the Scanner
        myInput.close();
    }

    // Start the program
    public static void main(String[] args) {
        new HelloEverybody().run();
    }
}

! means not. name.equals("bye") is true when the name is bye, so !name.equals("bye") is true while the name is not bye. (The lab version has no “Goodbye!” line; the lecturer added a goodbye message in the live demo.)

name = input("What is your name (bye to exit)? ")
while name != "bye":              # Java: !name.equals("bye")
    print("Hello " + name + ", nice to meet you")
    name = input("What is your name (bye to exit)? ")
print("Goodbye!")

Stage: after the break. The lecturer first left out the update line, and the program printed “Hi Tom” forever until it was stopped.

10Santa: stopping with a calculation

// What this class does: Santa says Ho! five times on one line.
public class Santa {
    public void run() {
        // The loop variable: how many Ho!s are left to say
        int x = 5;

        // Loop: repeat while x is greater than 0.
        // x goes 5, 4, 3, 2, 1, so 5 Ho!s. When x reaches 0 it stops.
        while (x > 0) {
            // Say Ho! and stay on the same line
            System.out.print("Ho! ");
            // Take 1 away from x so the loop can stop
            x = x - 1;
        }

        // After the loop: move to a new line
        System.out.println();
    }

    // Start the program
    public static void main(String[] args) {
        new Santa().run();
    }
}

x = x - 1; looks like a maths equation, but it is an instruction: “take the value in x, subtract 1, and store the result back in x”. Java reads the right-hand side first.

Shortcuts: x++ adds 1 (increment) and x-- subtracts 1 (decrement). x++; does the same as x = x + 1;. The lecturer used the long form in class because it is easier to read when you start.

x = 5
while x > 0:
    print("Ho! ", end="")
    x = x - 1          # or x -= 1. Python has no x-- or x++
print()

Stage: after HelloEverybody. In the live demo the lecturer counted up from 0 with x < 5 (the version in idea 7); the lab file counts down from 5.

11Predictable and unpredictable loops

• Input-based loops are unpredictable: they depend on what the user types.
• Calculation-based loops are predictable: they run the same way every time.
• General rule: code is predictable unless it uses something from outside, like user input. You can mix the two: a counting loop that runs as many times as the user asks (see the DrawSquare exercise). Java also has random numbers, which add unpredictability even without input.

12Optional: a loop inside a loop

The “Tell Santa” challenge uses two while loops, one inside the other. The outer loop keeps asking the user for a number until they type 0. The inner loop counts that many Ho!s. The inner loop finishes completely each time before the outer loop carries on.

// What this class does: asks how many Ho!s Santa should say, says them,
// and keeps asking until the user types 0.
import java.util.Scanner;                       // we need the Scanner class

public class Santa2 {
    public void run() {
        // Prompt, make the Scanner, read the first number
        System.out.print("How many Hos do you want Santa to say (0 exit)? ");
        Scanner myInput = new Scanner(System.in);
        int number = myInput.nextInt();

        // OUTER loop: keep going while the user has not typed 0.
        // Its loop variable (number) is updated by user input at the end.
        while (number != 0) {

            // INNER loop: say Ho! and count number down to 0.
            // Its loop variable is updated by a calculation.
            while (number > 0) {
                System.out.print("Ho! ");
                number = number - 1;
            }

            // Move to a new line, then ask again (this updates the outer loop variable)
            System.out.println("");
            System.out.print("How many Hos do you want Santa to say (0 exit)? ");
            number = myInput.nextInt();
        }

        // Finished with the Scanner
        myInput.close();
    }

    // Start the program
    public static void main(String[] args) {
        new Santa2().run();
    }
}

number = int(input("How many Hos do you want Santa to say (0 exit)? "))
while number != 0:                 # outer loop: until the user types 0
    while number > 0:              # inner loop: count down
        print("Ho! ", end="")
        number = number - 1
    print()
    number = int(input("How many Hos do you want Santa to say (0 exit)? "))

Stage: near the end of the lecture.