While Loops, User Input, and Scanner

24au ver.

Note: this is for the Autumn 2024 iteration of CSE 121. Looking for a different quarter? Please visit https://courses.cs.washington.edu/courses/cse121/.

While Loops¶

(indefinite loop, we don’t know in advance how many times it will repeat!)

Syntax

while (condition) {
    statement;
    ...
}

Example

int num = 5;
while (num < 50) {
    num *= 5;
}
System.out.println(num);

Output

User Input and Scanner¶

(execute some behavior based on information the user gives us directly)

Scanner Methods¶

Method	Description
`next()`	one token as `String`
`nextLine()`	entire line as `String`
`nextInt()`	one token as `int` (if possible) throws execption if not
`nextDouble()`	one token as `double` (if possible) throws exception if not

Example

import java.util.*; // make sure you don't forget this!

public class ScannerExample {
    public static void main(String[] args) {
        //Create a Scanner object
        Scanner console = new Scanner(System.in);
        // Get next integer user input from console and stores in num
        int num = console.nextInt();
    }
}

Fenceposting¶

(pulling one iteration out of the loop)

Example

int num = 1;
System.out.print(num); // one number to start with
while (num < 5) {
    num++;
    System.out.print(", " + num); // commas between numbers
}