CS173: Intro to Computer Science - Strings

Activity Goals

The goals of this activity are:
  1. To be able to explain that a String is a class that stores text and provides functionality that manipulates that text.
  2. To be able to explain that a String is immutable, and that new memory is allocated for a String when it is modified.
  3. To be able to create and manipulate a String.
  4. To be able to invoke the concat(), indexOf(), substring() and replace() methods of a String.

Supplemental Reading

Feel free to visit these resources for supplemental background reading material.

Warmup

Trace this algorithm to swap two int variables.

The Activity

Directions

Consider the activity models and answer the questions provided. First reflect on these questions on your own briefly, before discussing and comparing your thoughts with your group. Appoint one member of your group to discuss your findings with the class, and the rest of the group should help that member prepare their response. Answer each question individually from the activity, and compare with your group to prepare for our whole-class discussion. After class, think about the questions in the reflective prompt and respond to those individually in your notebook. Report out on areas of disagreement or items for which you and your group identified alternative approaches. Write down and report out questions you encountered along the way for group discussion.

Model 1: The String

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public class Main {
    public static void main(String[] args) {
        // These are equivalent
        // String x = "Test";
        String x = new String("Test");
         
        // So are these:
        x = "Another test";
        x = new String("Another test");
    }
}

Questions

  1. Unlike re-assigning a primitive variable, assigning a variable to an object creates a new object. A String is a class that allocates memory for and stores text. Since String text cannot be reassigned once it is allocated (and must be re-created instead), a String is called an immutable object. Why can’t a String be re-assigned like an int or double can?

Model 2: String Manipulation with substring

Manim String substring Animation

Questions

  1. What is the difference between x = x.concat(y) and x = x + y for String objects x and y?
  2. Rewrite the statement String z = "Cheese" + "Hamburger".substring(3); using the concat function.

Embedded Code Environment

You can try out some code examples in this embedded development environment! To share this with someone else, first have one member of your group make a small change to the file, then click "Open in Repl.it". Log into your Repl.it account (or create one if needed), and click the "Share" button at the top right. Note that some embedded Repl.it projects have multiple source files; you can see those by clicking the file icon on the left navigation bar of the embedded code frame. Share the link that opens up with your group members. Remember only to do this for partner/group activities!

Model 3: String Searching with indexOf

Manim String indexOf Animation

Questions

  1. What is the difference between the two indexOf() methods given above? How do you know which version you are calling from a program?

Model 4: More String Functions


Manim String replace Animation

Questions

  1. What is the difference between the two replace() methods given above? How do you know which version you are calling from a program?
  2. Write a Java statement to replace "Ham" with "Cheese" in a String "Hamburger"

Model 5: Working with Strings

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public class Main {
    public static void main(String[] args) {
        String x = "Test";
        String y = new String("Test");
         
        if(x.equals(y)) {
            System.out.println("x and y contain the same text!");
        }
         
        String z = "tan";
        System.out.println(z.substring(1, 2) + z.substring(0, 1));
    }
}

Questions

  1. Why is it necessary to write x.equals(y) instead of x == y to check if two String objects contain the same text?
  2. What does the print statement output for the string z?
  3. How might you print only the "est" characters from x using the substring method?
  4. You can compare two strings using str1.equals(str2). Why do you think str1 == str2 won't work?
  5. Java includes a equalsIgnoreCase function. How do you think this works, using other functions that the String class provides?
  6. Write a function that accepts two String parameters and returns true if the third character of each String is the same. Be careful to check the length of each String first!
  7. Using String methods, create a variable containing the word "Book" and the word "keeper", and create the word "Bookkeeper" and "Beekeeper". There are multiple ways to do this!
  8. Read a String from the console, check that it is exactly a 5-letter word, and if so, convert it to upper-case. Replace all the vowels with asterisks Print the result!
  9. Read a String from the console containing only the letters A, C, G, and/or T, and convert it to uppercase. Replace the A's to T's, and all the T's to A's. Replace the G's with C's and all the C's to G's.
  10. Challenge: there is a String function called matches that will check if a text pattern exists inside the String. It returns a boolean (true if the pattern is found, false if not). Use the matches function to see if any letter besides A, C, G, or T is in the String. Here are some hints: .* means to search for any character (the . represents this) any number of times (the *). You can check for a particular character using brackets like this: [ACGT] will match one character that is equal to A, C, G, or T. Try searching for any number of characters, followed by a character in the alphabet other than A, C, G, or T, followed by any number of characters.

Submission

I encourage you to submit your answers to the questions (and ask your own questions!) using the Class Activity Questions discussion board. You may also respond to questions or comments made by others, or ask follow-up questions there. Answer any reflective prompt questions in the Reflective Journal section of your OneNote Classroom personal section. You can find the link to the class notebook on the syllabus.

For Additional Practice

Feel free to visit these resources for additional practice exercises.