Building Java Programs

Lab 5: `while` loops, `Strings`, and fencepost loops

lab document created by Whitaker Brand and Marty Stepp

Today's lab

Goals for today:

use while loops for indefinite repetition
use Strings to represent and manipulate text data
more practice with logic and logical tests
exposure to fencepost and sentinel loop patterns
Where you see this icon, you can click it to check the problem in Practice-It!

Recall: `String` Methods

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

Exercise : String Expressions

Write the results of each expression. Put Strings in "quotes".

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

`str1.length()`		13
`str1.indexOf("o")`		2
`str2.toUpperCase()`		"GANDALF THE GRAY"
`str1.toLowerCase().indexOf("B")`		-1
`str1.substring(5)`		" Baggins"
`str2.substring(3, 14)`		"dalf the GR"
`str2.replace("a", "oo")`		"Goondoolf the GRAY"
`str2.replace("gray", "white")`		"Gandalf the GRAY"
`"str1".replace("r", "range")`		"strange1"

Exercise : `while` loop basics

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 above execute?		7
What output is produced?		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) {
    int z = 0;
    while (x % y != 0) {
        x = x / y;
        z++;
        System.out.print(x + ", ");
    }

    System.out.println(z);
}

`mystery(25, 2)`		12, 1
`mystery(32, 4)`		0
`mystery(10345, 10)`		1034, 103, 10, 3
`mystery(63, 2)`		31, 15, 7, 3, 1, 0, 6

Exercise : Syntax errors

The following Java program has 11 errors. Can you find them all?

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 == Whitaker) {
            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 - answer

line 5: nextString should be next
line 9: string should be String
line 9: name should not be in quotes
line 10: Whitaker should be in quotes
line 10: cannot compare strings with ==; must use .equals
line 13: cannot call replace without specifying a string object (name)
line 14: toUppercase should be toUpperCase
line 14: name. should come before toUpperCase, not passed as a parameter to it
line 14: must say name = to store the result of toUpperCase
line 15: must say name = to store the result of substring
line 16: must use parentheses () when calling length

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.next();
        process(name);
    }

    public static void process(String "name") {
        if (name.equals("Whitaker")) {
            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 and save this program in jGRASP, 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: ");
        
        // your code goes here
        
        System.out.println("Your name is: " + name);
    }
}

Exercise - code to add

Add code to the program so that it reads the user's first and last name (read an entire line as a single string), then prints the last name followed by a comma and the first initial. (Assume that the user types a valid name.) Example:
```
Type your name: Jessica Miller
Your name is: Miller, J.
```
(Notice that the program reads an entire line of user input, not just one word.)

Exercise - answer

The following is a completed version of the program:

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: ");
        
        // your code goes here
        String name = console.nextLine();
        int space = name.indexOf(" ");
        String first = name.substring(0, space);
        String last = name.substring(space + 1, name.length());
        String firstInitial = first.substring(0, 1);
        name = last + ", " + firstInitial + ".";
        
        System.out.println("Your name is: " + name);
    }
}

Exercise : ProcessName2

Modify your 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 - answer

import java.util.*;  // for Scanner

public class ProcessName2 {
    public static void main(String[] args) {
        Scanner console = new Scanner(System.in);
        System.out.print("Type your name: ");
        String name = console.nextLine();
        
        while (name.length() < 5 || name.indexOf(" ") < 0) {
            System.out.println("Error, must be at least 5 chars with a space.");
            System.out.print("Type your name: ");
            name = console.nextLine();
        }
        
        int space = name.indexOf(" ");
        String first = name.substring(0, space);
        String last = name.substring(space + 1, name.length());
        String firstInitial = first.substring(0, 1);
        name = last + ", " + firstInitial + ".";
        System.out.println("Your name is: " + name);
    }
}

Exercise : digitSum

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.
Try solving this problem in Practice-It! using the link above.
(Hint: To extract a digit from a number, use / 10 and % 10 operations.)

Exercise : digitSum - solution

public static int digitSum(int n) {
    n = Math.abs(n);            // handle negatives
    int sum = 0;
    while (n > 0) {
        int lastDigit = n % 10;
        sum = sum + lastDigit;  // add last digit to sum
        n = n / 10;             // remove last digit from n
    }
    return sum;
}

Exercise : repl

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 less, an empty string is returned.
Try solving this problem in Practice-It! using the link above.
(Hint: This is best solved with a cumulative algorithm. Start with an empty string and build it up piece by piece.)

Exercise : ZuneBug

The following code from Microsoft's Zune music player calculates today's date from the years/days passed since 1980. But all Zune players locked up on Jan 1 2009. Why? Download ZuneBug.java and modify it to fix the bug.

int days = getTotalDaysSince1980();   // pretend this method exists
int year = 1980;
while (days > 365) {                  // subtract out years
    if (isLeapYear(year)) {           // pretend this method exists
        if (days > 366) {
            days = days - 366;
            year++;
        }
    } else {
        days = days - 365;
        year++;
    }
}

Exercise : ZuneBug - answer

The bug occurs when the current year is a leap year and there are exactly 366 days left (i.e., if today is Jan 1 on a year after a leap year). The code gets stuck in an infinite loop with days == 366 because the while test is true but the if (days > 366) is false. Here is a fixed version:

int days = getTotalDaysSince1980();   // pretend this method exists
int year = 1980;
while (days > 365 || (isLeapYear(year) && days > 366)) {
    if (isLeapYear(year)) {
        days = days - 366;
    } else {
        days = days - 365;
    }
    year++;
}

Exercise : printLetters

Consider the following flawed method printLetters, which accepts a String as its parameter and attempts to print the letters of the String, separated by dashes. For example, the call of printLetters("Rabbit") should print R-a-b-b-i-t .

public static void printLetters(String text) {
    for (int i = 0; i < text.length(); i++) {
        System.out.print(text.substring(i, i + 1) + "-");
    }
    System.out.println();   // to end the line of output
}

What is wrong with the code? Paste the code in Practice-It! and fix it.

Exercise : printLetters - answer

public static void printLetters(String text) {
    if (text.length() > 0) {
        System.out.print(text.substring(0, 1));   // fencepost
        for (int i = 1; i < text.length(); i++) {
            System.out.print("-" + text.substring(i, i + 1));
        }
        System.out.println();   // to end the line of output
    }
}

Exercise : printFactors

Write a method named printFactors that accepts an integer as its parameter and uses a fencepost loop to print the factors of that number, separated by the word "and". For example, the call printFactors(24) should print as 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.

Try solving this problem in Practice-It! using the link above.
(Hint: This is an example of a fencepost problem.)

Exercise : swapPairs

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".
Try solving this problem in Practice-It! using the link above.

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 Chapter 5 and try to solve them!

Building Java Programs

Lab 5: while loops, Strings, and fencepost loops

Building Java Programs

Lab 5: while loops, Strings, and fencepost loops

Today's lab

Recall: String Methods

Exercise : String Expressions

Exercise : while loop basics

Exercise : while loop mystery

Exercise : Syntax errors

Exercise - answer

Exercise - Corrected version

Exercise : ProcessName

Exercise - code to add

Exercise - answer

Exercise : ProcessName2

Exercise - answer

Exercise : digitSum

Exercise : digitSum - solution

Exercise : repl

Exercise : ZuneBug

Exercise : ZuneBug - answer

Exercise : printLetters

Exercise : printLetters - answer

Exercise : printFactors

Exercise : swapPairs

If you finish them all...

Lab 5: `while` loops, `Strings`, and fencepost loops

Lab 5: `while` loops, `Strings`, and fencepost loops

Recall: `String` Methods

Exercise : `while` loop basics

Exercise : `while` loop mystery