What would be the output for the following code? If there would be an
error, write error .
Scanner input = new Scanner(new File("readme.txt"));
while (!input.hasNextInt()) {
input.next();
}
System.out.println(input.nextInt()); // 10
Exercise -D: Scanner practice
Input file: readme.txt
6.7 This file has
several input
LINES!
10 20
What would be the output for the following code? If there would be an
error, write error .
Scanner input = new Scanner(new File("readme.txt"));
System.out.println(input.nextLine()); // 6.7 This file has
System.out.println(input.nextLine()); // several input
System.out.println(input.nextLine()); // LINES!
Exercise : Words
Download the following files
Words.java
and
wordinput.txt
to your machine (and into the same folder!) and open them with jGrasp.
The program is supposed to read the input file and count how many words
it contains. Finish the program so that it runs properly. Don't forget
to add import statements and a declaration that the method might throw a FileNotFoundException to
the code.
Exercise - answer
import java.io.*; // for File
import java.util.*; // for Scanner
public class Words {
public static void main(String[] args) throws FileNotFoundException {
int wordCount = 0;
Scanner input = new Scanner(new File("wordinput.txt"));
// your code goes here ...
while (input.hasNext()) {
String word = input.next();
wordCount++;
}
System.out.println("Total words = " + wordCount);
}
}
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 : Debug ZipCode Case Study
In this exercise we will practice the jGRASP debugger using the Case Study example from the end of Chapter 6.
To download this example, follow these steps:
Download and save the files ZipLookup.java and zipcode.txt.
Right-click the file names and choose the option to save the link in whatever folder you have been using for lab work.
Make sure to save them in the same folder.
Compile and run ZipLookup.java in jGRASP.
You might try using your own ZIP code and a relatively small radius like 0.5 miles.
The program takes a while to run because it has to search a large data file.
continued on the next slide...
Exercise - jGRASP Debugger
The debugger can be particularly helpful to understand a program that processes a large data file.
In this exercise are going to debug the ZipLookup program.
This program makes two passes through the data file.
First it looks for the target zip code in the method named find.
Then it shows all matches in the method named showMatches.
We want to debug showMatches.
The data file has a total of 43,191 zip codes in it, so it is not practical to have it display all of the data.
That's why it's important to have good debugging skills to be able to selectively stop the program at certain points to see what is going on.
continued on the next slide...
Exercise - jGRASP Debugger
We'll debug the program as it searches for all zip codes with 0.3 miles of the White House, at zip code 20500.
Run the program with those input values:
What zip code are you interested in? 20500
And what proximity (in miles)? 0.3
20500: Washington, DC
zip codes within 0.3 miles:
20045 Washington, DC, 0.26 miles
20500 Washington, DC, 0.00 miles
20501 Washington, DC, 0.27 miles
20502 Washington, DC, 0.27 miles
Set a break point on the while loop itself.
Then enter the the values of lat1 and long1 (latitude and longitude of the White House ZIP code).
lat1
38.894781
long1
-77.036122
continued on the next slide...
Exercise - jGRASP Debugger
Clear your previous break point and set a new break point inside on the printf inside the if.
Then hit the resume button that looks like a play button and fill in the table below with the values for zip, lat2, and long2.
zip
lat2
long2
20045
38.896599
-77.0319
20500
38.894781
-77.036122
20501
38.89872
-77.036198
20502
38.89872
-77.036198
Exercise : runningSum
Write a static method called runningSum that
accepts as a parameter a Scanner holding a sequence of real
numbers and that outputs the running sum of the numbers followed by the
maximum running sum. For example if the Scanner contains the
following data:
3.25 4.5 -8.25 7.25 3.5 4.25 -6.5 5.25
Your method should produce the following output:
running sum = 3.25 7.75 -0.5 6.75 10.25 14.5 8.0 13.25
max sum = 14.5
Click on the check-mark above to try out your solution in Practice-it!
Exercise : flipLines
Write a method named flipLines that accepts
a Scanner holding an input file and writes to the console the same
file's contents with each pair of lines reversed in order. If the input file has an odd number of lines, the last line should be printed in its original position! For example, if
the file contains:
Twas brillig and the slithy toves
did gyre and gimble in the wabe.
All mimsey were the borogroves,
and the mome raths outgrabe.
The End
Your method should produce the following output:
did gyre and gimble in the wabe.
Twas brillig and the slithy toves
and the mome raths outgrabe.
All mimsey were the borogroves,
The End
Exercise : countWords errors
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// Counts the total lines and words in the given input scanner.
public static void countWords(Scanner input) {
Scanner input = new Scanner(new File("example.txt"));
int lineCount = 0;
int wordCount = 0;
while (input.nextLine()) {
String line = input.line(); // read one line
lineCount++;
while (line.next()) { // count tokens in line
String word = lineScan.hasNext;
wordCount++;
}
}
}
The above attempted solution to Practice-It problem
"countWords" has a few errors.
Open Practice-It, copy/paste this code into it, and fix
the errors. Complete the code so that it passes the test cases.
// Counts the total lines and words in the given input scanner.
public static void countWords(Scanner input) {
Scanner input = new Scanner(new File("example.txt"));
int lineCount = 0;
int wordCount = 0;
while (input.hasNextLine()) {
String line = input.nextLine(); // read one line
lineCount++;
Scanner lineScan = new Scanner(line);
while (lineScan.hasNext()) { // count tokens in line
String word = lineScan.next();
wordCount++;
}
}
System.out.println("Total lines = " + lineCount);
System.out.println("Total words = " + wordCount);
System.out.printf("Average words per line = %.3f\n", (double) wordCount / lineCount);
}
Exercise : coinFlip
Write a method named coinFlip that accepts
a Scanner for an input file of coin flips that are heads (H)
or tails (T). Consider each line to be a separate set of coin flips and
output the number and percentage of heads in that line. If it is more than
50%, print "You win!". Consider the following file:
H T H H T
T t t T h H
h
For the input above, your method should produce the following output:
3 heads (60.0%)
You win!
2 heads (33.3%)
1 heads (100.0%)
You win!
Note: to print a "%" using printf, say System.out.printf("%%");
Exercise : printDuplicates
Write a method printDuplicates that accepts a Scanner for an input file.
Examine each line for consecutive occurrences of the same token on the same line and print each duplicated token along how many times it appears consecutively. For example the file:
hello how how are you you you you
I I I am Jack's Jack's smirking smirking smirking smirking smirking revenge
one fish two fish red fish blue fish
bow wow wow yippee yippee yo yippee yippee yay yay yay
Write a method mostCommonNames that accepts a Scanner for an input file with names on each line separated by spaces. Some names appear multiple times, if they do, they are listed consecutively. For example:
Benson Eric Eric Marty Kim Kim Kim Jenny Nancy Nancy Nancy Paul Paul
Stuart Stuart Stuart Ethan Alyssa Alyssa Helene Jessica Jessica Jessica Jessica
Jared Alisa Yuki Catriona Cody Coral Trent Kevin Ben Stefanie Kenneth
For each line, print the most commonly occurring name. If there's a tie, use the first name that had that many occurrences.
Most common: Kim
Most common: Jessica
Most common: Jared
Also return the total number of unique names in the whole file (e.g. 23 for the above input).
Exercise : frequentFlier
Write a method frequentFlier that accepts a Scanner for an input file of ticket type / mileage pairs and reports how many frequent-flier miles the person earned.
1 frequent flyer mile is earned for each mile traveled in coach.
2 frequent flyer miles are earned for each mile traveled in first class.
0 frequent flyer miles are earned on a discounted flight.
For example, given the input below, your method should return 15600 (2*5000 + 1500 + 100 + 2*2000).
Words.java
and
wordinput.txt
to your machine (and into the same folder!) and open them with jGrasp.