Input and Repetition: Scanner + While Loop

The SayHello program

Scanner is a Java class that makes it easy to get input from the user. SayHello asks for your name, then greets you. When you run it, it prints a prompt (a question), then waits. In BlueJ’s terminal there is a line at the bottom where you type. You type Java, press Enter, and it replies.

// Tell Java we want to use the Scanner class.
import java.util.Scanner;

// The class (the box that holds this program).
public class SayHello
{
    // run() holds this program's instructions.
    public void run()
    {
        // Ask the user for their name (the prompt).
        System.out.print("What is your name? ");

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

        // Wait for the user to type a word; keep it in name.
        String name = myInput.next();

        // Greet the user by name, then end the line.
        System.out.println("Hello " + name + ", nice to meet you");

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

    // Java starts here: make a SayHello object, call run().
    public static void main(String[] args) {
        new SayHello().run();
    }
}

Every piece of this program, explained:

New word: class (public class SayHello)

class SayHello:
    ...

Curly brackets { }

def run():
    print("inside run")   # indented = inside
print("outside run")

New word: method (public void run())

def run():
    print("Hi")

run()

main, where Java starts (and arrays)

if __name__ == "__main__":
    SayHello().run()

new

SayHello().run()

import

import random          # use a library module
from math import sqrt  # or one thing from it

Scanner myInput = new Scanner(System.in);

# Python needs no Scanner:
# input() reads the keyboard directly

next(): reading one word

name = input()

print versus println

print("What is your name? ", end="")  # stay on the line
print("Hello")                       # new line after

+ to join text

print("Hello " + name + ", nice to meet you")

Semicolon ;

name = input()   # the end of the line ends it

close()

# Nothing needed for input() in Python.
# (Files are similar: f.close())

Variables again, and compiling

Variables again. SayHello uses two variables. myInput holds a Scanner, made with new Scanner(...). name holds a String, filled by asking the Scanner for the user’s answer.

Tracer 1: SayHello (you type Java)

Press Next step. The highlighted line is the one Java is running. Watch the variables and the terminal change. Green lines are comments; Java skips them. In the terminal, what you type appears after the question, and pressing Enter starts a new line.

The TripAdvisor program

Last week StarRating used ratings that were fixed in an array (“hard-coded”). TripAdvisor asks the user instead: where did you go, and how many stars? Then it prints the place and its stars using a switch where each case falls through (no break), so the right number of stars comes out.

// Ask for the place (prompt stays on the same line).
System.out.print("Where have you just visited? ");
// Make the Scanner.
Scanner myInput = new Scanner(System.in);
// Read one word: the place.
String place = myInput.next();
// Ask for the stars.
System.out.print("How many stars would you give it (1-5)? ");
// Read one word: the rating, kept as TEXT, e.g. "3".
String starNumber = myInput.next();
// Show the place and the rating, same line.
System.out.print(place + " (" + starNumber + " stars): ");

// Switch with fall-through: Java jumps to the case that
// matches starNumber and then runs every case below it,
// because there is no break. "3" prints 3 stars.
// Anything else (like "7") jumps straight to default,
// which just 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:
        // No more stars to print: just end the line.
        System.out.println("");
}

// Close the Scanner.
myInput.close();

switch, case and default

match starNumber:          # Python 3.10+
    case "3":
        print("***", end="")
    case _:                # like default
        pass

break, and falling through

# The nearest Python version of the fall-through trick:
if starNumber in ["1", "2", "3", "4", "5"]:
    print("*" * int(starNumber), end="")
print("")

The cases have quotes ("3") because next() always gives you text, not a number. Combining Scanner with a switch lets the user control what the program does, instead of fixed data. Tracer 2, just below, shows it step by step.

Tracer 2: TripAdvisor (you type Brighton, then 3)

Press Next step. The highlighted line is the one Java is running. Watch the variables and the terminal change. Green lines are comments; Java skips them. In the terminal, what you type appears after the question, and pressing Enter starts a new line.

What will it print? Paris and 7

Guess first. Write your guess down. Then open the answer.

next, nextLine, nextInt, nextDouble

Once you have a Scanner (here myInput), you can read different kinds of input. The slides call this slightly advanced; the first four are the ones you will use most.

// Read one word (up to a space or Enter), as a String.
String word = myInput.next();
// Read the whole rest of the line, spaces included.
String line = myInput.nextLine();
// Read a whole number.
int age = myInput.nextInt();
// Read a decimal number.
double price = myInput.nextDouble();

int and double

age = int(input())      # whole number
price = float(input())  # decimal number
word = input()          # text

The HelloEverybody program

HelloEverybody adds a while loop to SayHello. It keeps asking for names until the name is the word “bye”. It starts like SayHello: print a prompt, read the first answer into name. Then it goes into the while loop, where name is the loop variable.

// Ask for the first name, before the loop.
System.out.print("What is your name (bye to exit)? ");
// Make a Scanner that reads the keyboard.
Scanner myInput = new Scanner(System.in);
// Read the whole line typed: the first name.
// name is the loop variable.
String name = myInput.nextLine();

// While loop: repeat while name is NOT "bye".
// It stops when the user types bye (in lower case).
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)? ");
    // IMPORTANT: update the loop variable, so bye can stop it.
    name = myInput.nextLine();
}

// After the loop: close the Scanner.
myInput.close();

The last line of the loop body asks for another name and stores it in name. That updates the loop variable. When the user types bye, the test becomes false and the loop stops. Without that line, it would never stop. Tracer 3, just below, shows it step by step.

! (not)

while name != "bye":
    ...
# or: while not name == "bye":

.equals versus ==

print(name == "bye")   # Python == compares the letters

Tracer 3: HelloEverybody (you type Tom, Harry, bye)

Press Next step. The highlighted line is the one Java is running. Watch the variables and the terminal change. Green lines are comments; Java skips them. In the terminal, what you type appears after the question, and pressing Enter starts a new line.

What will it print? The missing line

Guess first. Write your guess down. Then open the answer.

// Read the first name.
String name = myInput.nextLine();
// While loop: repeat while name is not "bye".
while (!name.equals("bye")) {
    // Greet the current name.
    System.out.println("Hello " + name + ", nice to meet you");
    // Ask again (but the answer is never read!).
    System.out.print("What is your name (bye to exit)? ");
}

The Santa program

Loops based on a calculation are often simpler. Santa prints Ho! Ho! Ho! Ho! Ho!. The loop variable is x, a whole number. Before the loop, x is set to 5. The condition is x > 0, so the loop stops when x reaches 0. The body prints “Ho!” and takes 1 off x.

// The class that holds the Santa program.
public class Santa
{
    // run() holds the program's instructions.
    public void run()
    {
        // The counter: number of times Santa says "Ho!".
        int x = 5;

        // While loop: repeat while x is bigger than 0.
        // x goes down by 1 each time, so it stops at 0.
        while (x > 0)
        {
            // print keeps all the Ho!s on one line.
            System.out.print("Ho! ");
            // Take 1 off x, so the loop can stop.
            x = x - 1;
        }

        // After the loop: end the line.
        System.out.println();
    }

    // Java starts here and calls run().
    public static void main(String[] args){new Santa().run();}
}

print keeps all the Ho!s on the same line; the println() after the loop moves to a new line at the end. Tracer 4, just below, shows it step by step.

int and >

x = 5
print(x > 0)   # True

x = x - 1;

x = x - 1

x-- and x++

x -= 1   # Python has no x--
x += 1   # or x++

Tracer 4: Santa (counting down)

Press Next step. The highlighted line is the one Java is running. Watch the variables and the terminal change. Green lines are comments; Java skips them. In the terminal, what you type appears after the question, and pressing Enter starts a new line.

What will it print? Three Ho!s

Guess first. Write your guess down. Then open the answer.

// The counter starts at 3.
int x = 3;
// While loop: repeat while x is bigger than 0;
// x goes down by 1 each time.
while (x > 0) {
    // print: stay on the same line
    System.out.print("Ho! ");
    // take 1 off x
    x = x - 1;
}
// After the loop: print a message and end the line.
System.out.println("Merry Christmas");

What will it print? Counting up

Guess first. Write your guess down. Then open the answer.

// The counter starts at 0.
int x = 0;
// While loop: repeat while x is less than 3;
// x goes up by 1 each time.
while (x < 3) {
    // print x and a space, same line
    System.out.print(x + " ");
    // add 1 to x
    x++;
}

Input-based and calculation-based loops

• Input-based while loops are unpredictable: they depend on what the user types.
• Calculation-based while loops are predictable: they always run the same way.
• General rule: code is predictable unless it uses something from outside, such as user input.
• You can combine them: a calculation loop that runs a number of times the user chooses (see the DrawSquare exercise).
• Java also has a random number generator, which makes programs unpredictable even without input.

Just for fun: tell Santa how many Ho!s

Challenge part 1: ask the user how many times Santa should say “Ho!”, instead of using a fixed number. Part 2: keep asking until the user enters 0 or a negative number. Hint: use two while loops: one updated by user input, the other by calculation. Don’t worry if it feels hard. The slide notes give the code for part 1 (shown here with comments):

// Tell Java we want to use the Scanner class.
// (The slide notes leave this line out; you need it.)
import java.util.Scanner;

// The class that holds the challenge program.
public class Santa1 {
    // run() holds the program's instructions.
    public void run() {
        // Ask how many Ho!s (print keeps the answer on this line).
        System.out.print("How many Hos do you want to Santa to say? ");
        // Make a Scanner that reads the keyboard.
        Scanner myInput = new Scanner(System.in);
        // Read a whole number and keep it in number.
        int number = myInput.nextInt();

        // Calculation loop: repeat while number is above 0.
        // number goes down by 1 each time, so it stops at 0.
        while(number>0){
            // Say Ho! once, staying on the same line.
            System.out.print("Ho! ");
            // Take 1 off number, so the loop can stop.
            number=number-1;
        }
    }

    // Java starts here and calls run().
    public static void main(String[] args) {
        new Santa1().run();
    }
}

number = int(input("How many Hos? "))