Except where otherwise noted, the contents of this document are Copyright 2012 Stuart Reges and Marty Stepp.
lab document created by Marty Stepp, Stuart Reges and Whitaker Brand
Goals for today:
String
s to represent and manipulate text datawhile
loops for indefinite repetitionString
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(4) |
"o 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" |
Copy/paste and save ProcessName.java 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); } }
continued on the next slide ...
Type your name: Jessica Miller
Your name is: Miller, J.
int sum = 0;
for (int i = 1; i <= 100; i++) {
sum = sum + i;
}
System.out.println(sum); // 5050
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.
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!
In this exercise we will download a program from the textbook for a debugging exercise.
ch04
.
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.
Hailstone.java
in jGRASP.
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...
value
printHailstoneMaxMin(7, 10);
value
as the loop executes.
# |
value
|
---|---|
first value |
7
|
second value |
22
|
third value |
11 |
fourth value |
34 |
fifth value |
17 |
sixth value |
52 |
continued on the next slide...
min
printHailstoneMaxMin(7, 20);
min
during the second call.
# |
min
|
---|---|
first value |
7
|
second value |
5 |
third value |
4 |
fourth value |
2 |
fifth value |
1 |
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 }
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
.)
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 the following output:
1 and 2 and 3 and 4 and 6 and 8 and 12 and 24
while
Loops
A while
loop repeats indefinitely until a given condition is met.
while (test) { statement(s); }
Example:
int num = 1;
while (num < 5) {
System.out.print(n + " "); // output: 1 2 3 4
n++;
}
while
loop basicsConsider 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 |
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 = 1; int z = 0; while (2 * y <= x) { y = y * 2; z++; } System.out.println(y + " " + z); } |
|
while
loop mystery
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); } |
|
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.)
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.
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 the book and try to solve them!