while
loops, Strings
, and fencepost
loopswhile
loops, Strings
, and fencepost
loopsExcept where otherwise noted, the contents of this document are Copyright 2010 Stuart Reges and Marty Stepp.
lab document created by Whitaker Brand and Marty Stepp
Goals for today:
String
s to represent and manipulate text data
while
loops for indefinite repetition
String
MethodsMethod name | Description |
---|---|
charAt(index)
|
character at given index |
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 |
Write the results of each expression with String
s in
"quotes" and characters in single quotes ('a'
)
// index 0123456789012345
String str1 = "Frodo Baggins";
String str2 = "Gandalf the GRAY";
str1.length() |
13 |
|
str1.charAt(7) |
'a' |
|
str2.charAt(0) |
'G' |
|
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" |
We are once again going to download a program from the textbook that we will use for a debugging exercise. To do so, follow these steps:
Hailstone.java
in jGRASP.
We are going to practice setting stops and continuing execution. We will
explore Hailstone.java
. This program computes a sequence
known as a hailstone sequence. This is related to an unsolved problem in
mathematics known as the
Collatz
Conjecture.
To complete these problems, you will want to set breakpoints and to use the "Resume" button that resumes execution. The "Resume" button is the one that uses a standard "play" icon (the sixth one from the left below):
continued on the next slide...
The first call on the method asks it to compute the sequence starting with
the value 7. Set a stop that will allow you to determine the first five
values that the variable value
takes on not counting the
initial value of 7.
Initial |
7
|
---|---|
next value |
22 |
next value |
11 |
next value |
34 |
next value |
17 |
next value |
52 |
continued on the next slide...
You should still be in the middle of the first execution of the method. Clear your old stop by clicking on it a second time and set a stop for the second method call in main. Then resume execution. You should end up back in main just before the second call is made.
Now we want to determine the sequence of values that the
variable min
takes on in this second call. It is initialized
to the first value of 7 and is reset when a new minimum is encountered.
Set a stop that will allow you to determine the first four values that the
variable min
takes on not counting the initial value of 7.
Initial |
7
|
---|---|
next value |
5 |
next value |
4 |
next value |
2 |
next value |
1 |
while
loop
basicsConsider the following loop.
int x = 1; System.out.print(x); while (x < 100) { x = x + x; System.out.println(", " + x); }
How many times does the code in the body while
loop above
execute?
7
What output is produced? 1, 2, 4, 8, 16, 32, 64, 128
while
loop
mysteryFill 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); } |
|
while
loop
mysteryFill in the boxes at right with the output produced by each method call.
public static void mystery2(int x) { int y = 1; int z = 0; while (2 * y <= x) { y = y * 2; z++; } System.out.println(y + " " + z); } |
|
while
loop
mysteryFill in the boxes at right with the output produced by each method call.
public static void mystery3(int x) { int y = 0; while (x % 2 == 0) { y++; x = x / 2; } System.out.println(x + " " + y); } |
|
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.
printLetters
that takes
a String
as its parameter and that prints the letters of the
String, separated by commas. For example, the call
of printLetters("Rabbit")
should print:
R, a, b, b, i, t
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
.)
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 } }
lastDigit
that returns the last digit
of an integer. For example, lastDigit(3572)
should
return 2
. It should work for negative numbers as well. For
example, lastDigit(-947)
should return 7
.
firstDigit
that returns the first digit
of an integer. For example, firstDigit(3572)
should
return 3
. It should work for negative numbers as well. For
example, firstDigit(-947)
should return 9
.
season
that takes two integers as
parameters representing a month and day and that returns a String
indicating the season for that month and day. Assume that months are
specified as an integer between 1 and 12 (1 for January, 2 for February,
and so on) and that the day of the month is a number between 1 and 31.
"Winter"
. If the date falls between 3/16 and 6/15,
you should return "Spring"
. If the date falls between 6/16
and 9/15, you should return "Summer"
. And if the date falls
between 9/16 and 12/15, you should return "Fall"
.
printAverage
that accepts
a Scanner
for the console as a parameter and repeatedly
prompts the user for numbers. Once any number less than zero is typed,
the average of all non-negative numbers typed is displayed. Display the
average as a double, and do not round it. For example, a call to your
method might look like this:
Scanner console = new Scanner(System.in); printAverage(console);The following is one example log of execution for your method:
Type a number: 10 Type a number: 15 Type a number: -1 Average was 12.5If the first number typed is negative, do not print an average.
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.
use / 10
and % 10
operations.)
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; }
Write a method named hopscotch
that accepts
an integer parameter for a number of "hops" and prints a hopscotch board of
that many hops.
For example, the call hopscotch(3);
would produce the
following output:
1 2 3 4 5 6 7 8 9 10
Try solving this problem in Practice-It: click on the check-mark above!
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!
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 |
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"); } } |
nextString
should be next
string
should be String
name
should not be in quotesWhitaker
should be in quotes==
; must
use .equals
replace
without specifying a string
object (name
)toUppercase
should be
toUpperCase
name.
should come
before toUpperCase
, not passed as a parameter to itname =
to store the result
of toUpperCase
name =
to store the result
of substring
()
when
calling length
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"); } }
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.println("Type your name: "); // your code goes here System.out.println("Your name is: " + name); } }
Type your name: Jessica Miller
Your name is: Miller, J.
import java.util.*; // for Scanner public class ProcessName { public static void main(String[] args) { Scanner console = new Scanner(System.in); System.out.println("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); } }
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.
import java.util.*; // for Scanner public class ProcessName2 { public static void main(String[] args) { Scanner console = new Scanner(System.in); System.out.println("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.println("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); } }
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
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 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!