common/conf common/conf CSE 142 Lab 5: Strings, cumulative algorithms, while loops, fencepost loops

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

Today's lab

Goals for today:

String methods

Method name Type Returns....
charAt(index) char the character at the given index
indexOf(str) int the index where the start of the given String appears in this string (or -1 if not found)
length() int the number of characters in this String
replace(str1, str2) String a new string with all occurrences of str1 changed to str2
substring(index1, index2)
or substring(index1)
String the characters in this string from index1 (inclusive) to index2 (exclusive); if index2 is omitted, grabs till end of string
toLowerCase() String a new string with all lowercase letters
toUpperCase() String a new string with all uppercase letters

Exercise : String expressions

Write the results of each expression with Strings in "quotes" and characters in 'single quotes'.
Hint: Strings index starting at 0. A String with 10 characters has the indices 0-9!

//       index 0123456789012345
String str1 = "Frodo Baggins";
String str2 = "Gandalf the GRAY";
str1.length()
13
str2.charAt(0)
'G'
str1.indexOf("o")
2
str2.toUpperCase()
"GANDALF THE GRAY"
str1.substring(4)
"o Baggins"
str2.substring(3, 14)
"dalf the GR"
str2.replace("a", "oo")
"Goondoolf the GRAY"

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";
String str3 = str2.replace("gray", "white") // str3    = ?
"Gandalf the GRAY"
str2.toLowerCase() // str2    = ?
"Gandalf the GRAY"
str2 = str2.toLowerCase() // str2    = ?
"gandalf the gray"
int index = str1.toLowerCase().indexOf("B") // index = ?
-1

Exercise : Syntax errors

The following Java program has several errors. Can you find them all?
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public class StringOops {
    public static void main(String[] args) {
        Scanner console = new Scanner(System.in);
        System.out.print("Type your name: ");
        String name = console.nextString();
        process(name);
    }

    public static void process(string "name") {
        if (name == Brett) {
            System.out.println("You must be really awesome.");
        }
        replace("a", "e");
        toUppercase(name);
        name.substring(0, 3);
        System.out.println(name + " has " + name.length + " letters");
    }
}
Copy and paste the code into jGrasp and see if you can fix the errors.

Exercise - Corrected version

public class StringOops {
    public static void main(String[] args) {
        Scanner console = new Scanner(System.in);
        System.out.print("Type your name: ");
        String name = console.nextString();
        process(name);
    }

    public static void process(String "name") {
        if (name.equals("Brett")) {
            System.out.println("You must be really awesome.");
        }
        name = name.replace("a", "e");
        name = name.toUpperCase();
        name = name.substring(0, 3);
        System.out.println(name + " has " + name.length() + " letters");
    }
}

Exercise : ProcessName

Copy/paste the following code into jGRASP, or icon download it. Then go to the next slide.

import java.util.*;  // for Scanner

public class ProcessName {
    public static void main(String[] args) {
        Scanner console = new Scanner(System.in);
        System.out.print("Type your name: ");
        
        // read the entire input as a single line
        String input = console.nextLine();
        
        // your code goes here
        
        System.out.println("Your name is: " + name);
    }
}

continued on the next slide ...

Exercise - code to add

Add code to the program so that it reads the user's first and last name (reading the entire line of input as a single string), then prints the last name followed by a comma and the first initial. Assume that the user types a single first name, a space, and then a single last name.

Example:
Type your name: Jessica Miller
Your name is: Miller, J.
If you get stuck, ask a TA for help!

Exercise : ProcessName Solution

import java.util.*;  // for Scanner

public class ProcessName {
    public static void main(String[] args) {
        Scanner console = new Scanner(System.in);
        System.out.print("Type your name: ");
        
        // read the entire input as a single line
        String input = console.nextLine();
        
        int spaceIndex = input.indexOf(" ");
        String lastName = input.substring(spaceIndex + 1);
        char firstInitial = input.charAt(0);
        
        String name = lastName + ", " + firstInitial + ".";
        
        System.out.println("Your name is: " + name);
    }
}

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.

Example: Cumulative algorithm to sum the numbers 1-100:
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

Exercise : repl practice-it

Write a method named repl that accepts a String and a number of repetitions as parameters and returns the String concatenated that many times. For example, the call

repl("hello", 3)
returns
"hellohellohello"
If the number of repetitions is 0 or fewer, an empty string is returned.

(Hint: This is best solved with a cumulative algorithm. Start with an empty string and build it up piece by piece.)

Exercise : Getting textbook code files

In this exercise we will download a program from the textbook for a debugging exercise.

  1. Go to the course web page and click the Textbook sidebar link.
  2. Find the section labeled Code Files and click the "code files" link.
  3. Click on the 4ed link. This will bring you to a listing of all chapters. Click the link for ch04.
  4. Download and save Hailstone.java. Right-click the file name and choose the option to Save the Link in the folder you have been using for lab work.
  5. Compile and run Hailstone.java in jGRASP.

Exercise : jGRASP Debugger

We are going to practice using the jGRASP debugger with Hailstone.java. This program computes a sequence of integers called a hailstone sequence. (This is related to an unsolved problem in mathematics known as the Collatz Conjecture.)

continued on the next slide...

Exercise - Values of value

# value
first value 7
second value 22
third value
11
fourth value
34
fifth value
17
sixth value
52

continued on the next slide...

Exercise - Values of min

# min
first value 7
second value
5
third value
4
fourth value
2
fifth value
1

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
}

Exercise : printLetters practice-it

Write a method named printLetters that takes a String as its parameter and that prints the letters of the String, separated by dashes. For example, the call of printLetters("Rabbit") should print:
R-a-b-b-i-t
(Hint: This is a fencepost problem. Remember that each String object has a length method that tells you how many characters are in the String and a charAt method that gets you individual characters of the String.)

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 : while loop basics

Consider the following loop.

int x = 1;
System.out.print(x);
while (x < 100) {
    x = x + x;
    System.out.print(", " + x);
}
How many times does the code in the while loop execute? 7
What output is produced by the overall code? 1, 2, 4, 8, 16, 32, 64, 128

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);
}
mystery(1);
4
mystery(6);
8
mystery(19);
20
mystery(40);
44

Exercise : while loop mystery practice-it

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

public static void mystery(int x) {
    int y = 1;
    int z = 0;
    while (2 * y <= x) {
        y = y * 2;
        z++;
    }
    System.out.println(y + " " + z);
}
mystery(1);
1 0
mystery(6);
4 2
mystery(19);
16 4
mystery(39);
32 5
mystery(74);
64 6

Exercise : while loop mystery practice-it

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

public static void mystery2(int x, int y) {
    int z = 0;
    while (x % y != 0) {
        x = x / y;
        z++;
        System.out.print(x + ", ");
    }

    System.out.println(z);
}
mystery2(25, 2);
12, 1
mystery2(32, 4);
0
mystery2(10345, 10);
1034, 103, 10, 3
mystery2(63, 2);
31, 15, 7, 3, 1, 0, 6

Exercise : ProcessName2 practice-it

Modify your previous ProcessName program so that it re-prompts until the user types a name that is at least 5 letters total in length and has at least one space in it. Example:

Type your name: Joe
Error, must be at least 5 chars with a space.
Type your name: O K!
Error, must be at least 5 chars with a space.
Type your name: what
Error, must be at least 5 chars with a space.
Type your name: Tyler Durden
Your name is: Durden, T.

Exercise : digitSum practice-it

Write a method named digitSum that accepts an integer as a parameter and returns the sum of the digits of that number. For example, the call digitSum(29107) returns 2+9+1+0+7 or 19. For negative numbers, return the same value that would result if the number were positive. For example, digitSum(-456) returns 4+5+6 or 15. The call digitSum(0) returns 0.

(Hint: This is a cumulative algorithm. To extract a digit from a number, use / 10 and % 10 operations.)

Exercise : longestName practice-it

Write a method named longestName that reads names typed by the user and prints the longest name (the name that contains the most characters) in the format shown below. Your method should accept a console Scanner and an integer n as parameters and should then prompt for n names.

A sample execution of the call longestName(console, 4) might look like the following:

name #1? roy
name #2? DANE
name #3? sTeFaNiE
name #4? Erik
Stefanie's name is longest

Try to solve this problem in Practice-It: click on the check-mark above!

Exercise : printFactors practice-it

Write a method called printFactors that accepts an integer as its parameter and that prints the factors of that number, separated by the word "and". Recall that a factor is a number that goes evenly into another number. For example, the call printFactors(24) should print the following output:

1 and 2 and 3 and 4 and 6 and 8 and 12 and 24

You may assume that the parameter value passed is greater than 0.

Exercise : swapPairs practice-it

Write a method named swapPairs that accepts a String as a parameter and returns that String with each pair of adjacent letters reversed. If the String has an odd number of letters, the last letter is unchanged. For example, the call swapPairs("forget") should return "ofgrte" and the call swapPairs("hello there") should return "ehll ohtree".

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!