Lab 2: Strings and arrays; a first look at reference types — CS 112, Boston University CS 112 Spring 2022 Home Syllabus Schedule Lectures Labs Problem Sets Staff Office Hours Resources Collaboration Policies Blackboard Piazza Gradescope Lab 2: Strings and arrays; a first look at reference types Creating the necessary folder Task 1: Working with Java Strings Task 1.1: String objects and their methods Task 1.2: Using String methods Task 2: Arrays and references Task 3: Array methods Task 4: Challenge: More practice with array methods Extra Practice! Creating the necessary folder If you haven’t already created a folder named cs112 for your work in this course, follow these instructions to do so. Then create a subfolder called lab2 within your cs112 folder, and put all of the files for this lab in that folder. Task 1: Working with Java Strings Task 1.1: String objects and their methods In Problem Set 2, there are several problems in which you will need to work with String objects in Java. It turns out that Java lacks many of Python’s built-in operators and functions for strings. Instead, most operations involving strings – including indexing and slicing – are done using methods that are inside the string object. If you would like to review, this is an example of a code trace using Java Strings. The table below compares the ways in which some common string operations would be accomplished in Python and Java: operation in Python in Java assigning to a variable (and, in Java, declaring the variable) s1 = 'Boston' s2 = "University" String s1 = "Boston"; String s2 = "University"; concatenation s1 + s2 s1 + s2 finding the number of characters len(s1) s1.length() indexing (note: Java does not have negative indices) s1[0] s2[-1] s1.charAt(0) s2.charAt(s2.length()-1) (see the notes below the table for a key fact about this method) slicing s1[1:4] s2[3: ] s1[ :5] s1.substring(1, 4) s2.substring(3) s1.substring(0, 5) converting all letters to uppercase s1.upper() s1.toUpperCase() converting all letters to lowercase s2.lower() s2.toLowerCase() determining if a substring is in a string s2 in s1 'sit' in s2 s1.contains(s2) s2.contains("sit") finding the index of the first occurence of a character or substring s1.find("o") s2.find('ver') s1.indexOf('o') s2.indexOf("ver") testing if two strings are equivalent s1 == s2 s1 == 'Boston' s1.equals(s2) s1.equals("Boston") Additional notes: Java has a separate type called char for values that consist of a single character. Literals of this type are surrounded by single quotes (e.g., the 'o' in the call s1.indexOf('o')), whereas string literals must be surrounded by double quotes. (In Python, there is no separate char type, and string literals can be surrounded by either single or double quotes.) The charAt() method that we use for indexing in Java returns a value of type char. For example, the call s1.charAt(0) in the table above would return the char value 'B', since 'B' is the first character in the string "Boston". Task 1.2: Using String methods To ensure that you understand how the above string operations work in Java, let’s write a simple Java test program that you can use to play around with the different string operations. Getting started If you haven’t already done so, create a folder named lab2 for your work on this lab. You can find instructions for creating folders here. As needed, open your lab2 folder using the File->Open Folder or File->Open menu option in VS Code. Select File->New File, which will open up an empty editor window. Select File->Save, and give the new file the name StringTest.java. Here are the tasks you should perform as part of your StringTest class: Create a class header and a main method. In the main method, begin by initializing two string variables that we will use in the later steps: String s1 = "Boston";
String s2 = "University";
If you have done everything correctly, the statements: System.out.println(s1);
System.out.println(s2);
should output: Boston
University
If for some reason you don’t see this output, make whatever changes are needed before proceeding. We’re now going to solve a series of puzzles in which we construct an expression involving operations on s1 and/or s2 to produce a specified target value. For example, let’s say that we want to construct an expression involving s1 or s2 that produces the following value: "ton"
One possible expression that works for this puzzle is s1.substring(3, 6). To see this, we can add the line: System.out.println( s1.substring(3, 6) );
which should output: ton
Now solve the puzzles below. As you do so, add a println statement to your main method to test each expression that you construct. Construct an expression involving s1 or s2 that produces the following value: "Uni"
Construct an expression involving s1 or s2 that produces the following value: "UNI"
Hint: Chain two method calls together. For example: System.out.println( s1.toLowerCase().substring(0, 3) );
should output: bos
Construct an expression involving s1 or s2 that produces the following value: 'v'
Note that the single quotes mean that you must produce a char, not a String. See the notes under the table above that discuss the char type. Construct an expression involving s1 or s2 that produces the following value: "STONE"
Write a static method called replaceStart that takes two strings s1 and s2, and does the following: If the length of s1 (call it s1Len) is less than the length of s2, the method returns a string in which the first s1Len characters of s2 are replaced by s1. For example: System.out.println( replaceStart("abc", "xxxxxxx") );
should output: abcxxxx
and, System.out.println( replaceStart("hello", "xxxxxxx") );
should output: helloxx
If the length of s1 is greater than or equal to the length of s2, the method just returns s1. For example: System.out.println( replaceStart("hello", "bye") );
should output: hello
Why does the method replaceStart need to be called from within the println method? In other words, what would you expect to happen if our program just made the call to replaceStart on its own line, rather than calling it as part of a println? Make sure you understand. Task 2: Arrays and references To get the most out of this task, draw out a memory diagram of the arrays on a piece of paper (follow examples we did in lecture, and the instructions of your TA). This is the best way to understand the correct answer to each of the questions, and why. Let’s draw what things look like in memory for the each of the following code snippets: // snippet A
int[] a1 = {1, 2, 3, 4};
int[] a2 = a1;
// snippet B
int[] b1 = {1, 2, 3, 4};
int[] b2 = {1, 2, 3, 4};
Given the two snippets above, what is the value of the following expression? a1 == a2
Given the two snippets above, what is the value of the following expression? b1 == b2
Consider the following static method: public static void mystery(int[] vals) {
for (int i = 0; i < vals.length; i++) {
vals[i] *= -1;
}
int[] vals2 = new int[vals.length];
for (int i = 0; i < vals.length; i++) {
vals2[i] = vals[i];
}
vals = vals2;
for (int i = 0; i < vals.length; i++) {
vals[i] += 2;
}
}
What does the array a1 look like after the execution of the following code snippet, which you many assume is part of the main method of a valid Java class? int[] a1 = {1, 2, 3, 4};
mystery(a1);
Draw pictures to show what things look like in memory. You may find it helpful to use the following template, which shows separate method frames for main() and mystery(): +--------------+
| mystery |
| ------- |
| +----+ |
| vals2 | | |
| +----+ |
| +----+ |
| vals | | |
| +----+ |
+--------------+
| main |
| ---- |
| +----+ |
| a1 | | |
| +----+ |
+--------------+
Task 3: Array methods Getting started As needed, open your lab2 folder using the File->Open Folder or File->Open menu option in VS Code. Select File->New File, which will open up an empty editor window. Select File->Save, and give the new file the name ArrayPractice.java. Here are the tasks you should perform as part of your ArrayPractice class: Before the class header, you should include the following statement, which will give us access to the methods in the Arrays class: import java.util.*;
Create a class header and a main method, and add the following initial statements to your main method: int[] a = {2, 4, 6, 8};
System.out.println( a );
// What do you expect to see?
System.out.println( Arrays.toString(a) );
// Now, what do you expect to see?
Write a static method named equals that takes two arrays of integers and determines if the two arrays are identical (i.e. same elements in the same positions). The method should return true or false accordingly. For this (and each of the following methods), you will need to add calls to your main method to test them. Example: int[] a1 = {1, 2, 3, 4};
int[] a2 = {1, 2, 3, 4};
System.out.println( a1 == a2 );
System.out.println( equals(a1, a2) );
You should expect to see: false
true
Congratulations! You just wrote your own equals method for arrays! However the Arrays class contains many methods, one of which is the equals method. To use the built-in Arrays method, try this: Example: System.out.println( Arrays.equals(a1, a2) );
You should expect to see: true
Write a static method named square that takes an array of integers and squares each element of the array. This method does not need to return anything. (If you’re not sure why, please ask!) Example: int[] a1 = {1, 2, 3, 4};
square(a1);
System.out.println( Arrays.toString(a1) );
You should expect to see: [1, 4, 9, 16]
Task 4: Challenge: More practice with array methods Write a static method named shiftLeft that takes an array of integers and shifts each element one position to the left. The last element of the array should end up being 0, and the original first element should be returned by the method. For example: int[] a3 = {1, 2, 3, 4, 5, 6};
System.out.println( shiftLeft(a3) );
System.out.println( Arrays.toString(a3) );
You should expect to see: 1
[2, 3, 4, 5, 6, 0]
Hint: To determine the necessary logic, you may find it helpful to begin by writing down examples of specific assignments statements that the method will need to perform as part of the shifting. Then look for the general pattern in these assignments, and use it to write the necessary loop. Other hints: You will need to store the original first element in a variable before you do the shifting, so that you can return the first element at the end of the method. Don’t forget to put a 0 in the last position of the array. Write a static method named replace that takes an array of integers and two integer variables, val1 and val2 and replaces all occurences of val1 with val2. The method should return how many values were replaced. If no value was replaced (i.e. there were no occurrences of val1 in the array), the method should return a zero. For example: int[] a4 = {1, 2, 3, 2, 2, 6};
int numReplaced;
// first test
numReplaced = replace(a4, 2, 9);
System.out.println( Arrays.toString(a4) );
System.out.println( "numReplaced = " + numReplaced );
You should expect to see: [1, 9, 3, 9, 9, 6]
numReplaced = 3
Adding these statements: // second test
numReplaced = replace(a4, 2, 9);
System.out.println( Arrays.toString(a4) );
System.out.println( "numReplaced = " + numReplaced);
You should expect to see: [1, 9, 3, 9, 9, 6]
numReplaced = 0
How could I produce the exact same output as above without needing to declare the variable numReplaced? What is the output of the following code segment? int[] a4 = {1, 2, 3, 2, 2, 6};
replace(a4, 2, 9);
System.out.println( replace(a4, 2, 9) );
Write a static method named interleave that takes two arrays of integers, creates and returns a third array of integers which contains all the elements of the two arrays, but in interleaved order. For example: int[] a5 = {1, 2, 3, 4, 5};
int[] a6 = {6, 7, 8, 9, 10};
int[] a7 = interleave(a5, a6);
System.out.println( Arrays.toString(a7) );
This should produce: [1, 6, 2, 7, 3, 8, 4, 9, 5, 10]
Note: If the arrays passed to the method are of two different sizes, only interleave elements until you run out of elements from the smallest array. Write a static method named isMirror that takes two arrays of integers and determines if the second array is an exact mirror image of the first array. The method should return true or false accordingly. Example 1: int[] a8 = {1, 2, 3, 4, 5};
int[] a9 = {5, 4, 3, 2, 1};
System.out.println( isMirror(a8, a9) );
This should produce: true
Example 2: int[] a10 = {1, 4, 5, 2, 1};
System.out.println( isMirror(a8, a10) );
This should produce: false
Write a new static method named isMirror that takes two arrays of strings and determines if the second array is an exact mirror image of the first array. The method should return true or false accordingly. (Note: It’s okay for two methods to have the same name, as long as the number and/or types of the parameters are different.) Example: String[] s1 = { "abc", "def", "ghi" };
String[] s2 = new String[3];
s2[0] = new String("ghi");
s2[1] = new String("def");
s2[2] = new String("abc");
System.out.println( isMirror(s1, s2) );
// what do you expect to see?
Hint: If you don’t obtain the expected result, consider how you are testing for equality on strings. Extra Practice! If you get through the exercises above, congratulations! For extra practice, you can try some exercises from an excellent site called Practice-It. Go to Practice-It. Create an account if needed. Select the group of problems entitled Building Java Programs, 3rd edition. Try problems from: BJP3 Chapter 7: Arrays The system will test your solution and tell you whether it is correct. Last modified on February 3, 2022.
