Today's lab
Goals for today:
- pass arrays as parameters and returns from methods
- explore the idea of reference parameters
- use arrays for tallying
- recall file Scanner usage
- Where you see this icon, you can click it to check the problem in Practice-It!
Exercise : Array reference mystery
What values are stored in the array at the comment in main?
Note that the incrementAll method returns void, but does take an int[] parameter.
public class ArrayReference {
public static void main(String[] args) {
int[] nums = {2, 4, -1, 3};
incrementAll(nums);
// HERE!
}
public static void incrementAll(int[] data) {
for (int i = 0; i < data.length; i++) {
data[i]++;
}
}
}
| index | 0 | 1 | 2 | 3 |
|---|---|---|---|---|
| value | 3 | 5 | 0 | 4 |
Exercise : Array reference mystery
Solve another reference mystery problem on PracticeIt!
Discuss it with your neighbor before submitting the answer and make sure you agree with each other's solution. There will be a problem of this type on the final!
Exercise : swapAll
Write a method named swapAll that accepts two arrays of
integers as parameters and swaps their entire contents. You may assume that
the arrays passed are not null and are the same length.
For example, if the following arrays are passed:
int[] a1 = {11, 42, -5, 27, 0, 89};
int[] a2 = {10, 20, 30, 40, 50, 60};
swapAll(a1, a2);
After the call, the arrays should store the following elements:
a1: {10, 20, 30, 40, 50, 60}
a2: {11, 42, -5, 27, 0, 89}
Exercise : copyRange
Write a method named copyRange that takes as parameters two
arrays a1 and a2, two starting
indexes i1 and i2, and a length l,
and copies the first l elements of a1 starting at
index i1 into array a2 starting at
index i2.
For example, if the following arrays are declared:
int[] a1 = {10, 20, 30, 40, 50, 60};
int[] a2 = {91, 92, 93, 94, 95, 96};
copyRange(a1, a2, 0, 2, 3);
After the preceding call, the contents of a2 would
be {91, 92, 10, 20, 30, 96}. You may assume that the
parameters' values are valid, that the arrays are large enough to hold the
data, and so on.
Exercise : append
Write a method called append that accepts two integer arrays as parameters and that returns a new array that contains the result of appending the second array's values at the end of the first array.
You should solve this problem in Practice-It!
Exercise : longestSortedSequence
Write a method called longestSortedSequence
that accepts an array of integers as a parameter and that returns the
length of the longest sorted (nondecreasing) sequence of integers in the
array.
You should solve this problem in Practice-It!
Exercise : countWords errors
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 |
// 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 = line.hasNext; wordCount++; } } } |
The above attempted solution to Practice-It problem
"countWords" has 5 errors.
Open Practice-It from the link above, copy/paste this code into it, and fix
the errors. Complete the code so that it passes the test cases.
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!
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
leads to the following console output:
how*2 you*4 I*3 Jack's*2 smirking*5 wow*2 yippee*2 yippee*2 yay*3
Exercise : mostCommonNames
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 in a row. 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 : stripHtmlTags
Write a method stripHtmlTags that accepts a Scanner for an input file containing an HTML web page, then reads that file and prints the file's text with all HTML tags removed. A tag is any text between the character < and >. For example, consider the following text:
<html> <head> <title>My web page</title> </head> </html>
If a file containing these lines were passed to your program, your program should output the following:
My web page
Exercise : wordLengths
Write a method called wordLengths that accepts a Scanner representing an input file as its argument. Your method should read from the given file, count the number of letters in each token in the file, and output a result diagram of how many words contain each number of letters. Use tabs before the asterisks so that they'll line up. If there are no words of a given length, omit that line from the output.
You should solve this problem in Practice-It!