University of Washington, CSE 142

Lab 5: Strings, cumulative algorithms, while loops, fencepost loops

Except where otherwise noted, the contents of this document are Copyright 2013 Stuart Reges and Marty Stepp.

lab document created by Marty Stepp, Stuart Reges and Whitaker Brand

Basic lab instructions

• Mouse over highlighted words if you're not sure what they mean!
• Talk to your classmates for help.
• You may want to bring your textbook to future labs to look up syntax and examples.
• Stuck? Confused? Have a question? Ask a TA for help, or look at the book or past lecture slides.
• Complete as much of the lab as you can within the allotted time. You don't need to keep working on these exercises after you leave.
• Feel free to complete problems in any order.
• Make sure you've signed in on the sign-in sheet before you leave!

Today's lab

Goals for today:

• practice using String methods
• practice cumulative algorithms
• exposure to fencepost and sentinel loop patterns
• use while loops for indefinite repetition

String methods

Exercise : Catching String method returns

Write the value of the variable as prompted. Put Strings in "quotes" and characters in 'single quotes'.

//       index 0123456789012345
String str1 = "Frodo Baggins";
String str2 = "Gandalf the GRAY";

"Gandalf the GRAY"

"Gandalf the GRAY"

"gandalf the gray"

-1

Exercise : equalsIgnoreCase()

equalsIgnoreCase() is identical to .equals(), except it ignores the case of the Strings being compared.
Enter true or false for the following:

"abc".equalsIgnoreCase("abc");

true

"abc".equalsIgnoreCase("ABC");

true

"abc".equals("ABC");

false

"iLoveHalloween!".equalsIgnoreCase("iLoVeHaLlOwEeN!");

true

Cumulative algorithms

A cumulative algorithm involves incrementally accumulating a value by repeatedly adding, multiplying, dividing, etc., while storing the result in a variable.

Key aspects of a cumulative algorithm: A loop, and a variable declared outside the loop whose value is modified inside the loop.

int sum = 0;                // safe default value, 0 doesn't affect a sum
for (int i = 1; i <= 100; i++) {
    sum = sum + i;
}
System.out.println(sum);    // 5050

Fencepost Loops

A fencepost loop is a common algorithmic pattern where you want to perform N tasks with N-1 things between them. It's like a fence with N posts with N-1 wires between the posts.

To achieve this, place one "post" outside your loop, then alternate between "wires" and "posts" inside the loop.

Example:

System.out.print(1);                 // |==|==|==|==| fence
for (int i = 2; i <= 5; i++) {
    System.out.print(".." + i);      // 1..2..3..4..5
}

while Loops

for loops are fantastic for when we know how many times we want to repeat something. But sometimes we won't know how many times we'll want to repeat something in advance! while loops repeat indefinitely while a given condition is true.

while (test) {
    statement(s);
}

Example:

int num = 1;
while (num < 5) {
    System.out.print(num + " ");     // output: 1 2 3 4
    num++;
}

Exercise : Password matching

import java.util.*; // lets us use the Scanner

public class Password {
    public static void main(String[] args) {
        Scanner console = new Scanner(System.in);
    
        String password = "password"; // secure
        System.out.print("Enter your password: ");
        String input = console.next();
        
        while (!input.equals(password)) {
            System.out.print("Wrong! Try again: ");
            input = console.next();
        }
        
        System.out.println("Welcome to DefCon!");
    }
}

Exercise : while loop mystery

Fill in the boxes at right with the output produced by each method call.

public static void mystery(int x) {
    int y = 4;
    while (y <= x) {
        y = y + 4;
    }
    System.out.println(y);
}

4

8

20

44

Checkpoint: Congratulations!

Nice job making it this far--labs are tough!

These next problems get a little more challenging as we explore earlier concepts further.

We put a lot of problems in here so that you have plenty to refer back to later when working on homework. Don't feel bad if you don't finish all of them--Brett can't finish them all in a 50 minute lab, either! :)

If you finish them all...

If you finish all the exercises, try out our Practice-It web tool. It lets you solve Java problems from our Building Java Programs textbook.

You can view an exercise, type a solution, and submit it to see if you have solved it correctly.

Choose some problems from the book and try to solve them!