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Feb 01: Arrays vs Strings

Learning Objectives

After today's class, you should be able to:

  • Write standard and enhanced for loops that iterate over arrays.
  • Describe common patterns for looping over arrays and strings.
  • Summarize the types of questions that will be asked on Exam1.

Lesson Outline

Mini Lecture [10 min]

  • See notes below

Lab03 [45 min]

  • Lab 3: CodingBat Loops
  • Each team will be assigned a problem to solve
  • Spokesperson will present solution to the class

Exam1 Prep [20 min]

  • Watch for announcement about study material

Reminders

If you haven't already…

  • Submit Quiz03 on Canvas
  • Need to install OpenJDK 17 (for testing to work right)
  • Submit HW3 stubs to Gradescope before the weekend!

By the end of Monday

Mini Lecture

Use the following reference material during today's lab.

Operators

Increment (++) and decrement (--)

Assignment Shorthand
i = i + 1; i++;
i = i - 1; i--;

Compound assignment

Assignment Shorthand
i = i + 2; i += 2;
i = i - 2; i -= 2;
i = i * 2; i *= 2;
i = i / 2; i /= 2;
i = i % 2; i %= 2;

While Loops

Also known as a pre-test loop.

number = 1;
while (number <= 10) {
    System.out.println(number);
    number++;
}

Question

What is output by the following loop? And what mistake was made? 

int i = 0;
while (i < 3)
    System.out.println("i = " + i);
    i = i + 1;

Do-While Loops

Also known as a post-test loop.

number = 1;
do {
    System.out.println(number);
    number++;
} while (number <= 10);

Question

What is output by the following loop? Explain how the code works. 

number = 99;
do {
    System.out.println(number);
    number++;
} while (number <= 10);
System.out.println(number);

For Loops

Also known as the loop that you should use over 90% of the time.

// Loop A: count forwards
for (i = 1; i <= 10; i++) {
    System.out.println(i);
}

// Loop B: count backwards
for (i = 10; i >= 1; i--) {
    System.out.println(i);
}

Question

Rewrite each for loop above as a while loop.

Question

Change the for loops to print multiples of 3.

Array Loops

Standard for loop ("for each index")

for (int i = 0; i < array.length; i++) {
    // do something with array[i]
}
  • Note: the variable name i stands for index

Enhanced for loop ("for each value")

for (int value : array) {
    // do something with value
}
  • The : means in (i.e., "for each value in array")
  • Note: don't name the variable i (not an index)

String Loops

Standard for loop ("for each index")

for (int i = 0; i < str.length(); i++) {
    // do something with str.charAt(i)
}
  • Notice string length() uses parentheses
  • Must use charAt() method instead of []

Enhanced for loop ("for each char")

for (char c : str.toCharArray()) {
    // do something with c
}
  • c is a more appropriate variable name than i

Exam1 Prep

Coming Soon

The following materials will be available by Monday of next week.

Remember that we will not have class on Tuesday (Assessment Day).

  • Videos
    • Solution for HW3
    • Review Weeks 1–3
  • Practice
    • Sample exam (same format)
    • Additional sample questions

Question Types

  • Identify errors
    • Invalid assignments (data types)
    • Syntax (Ex: semicolons, braces)
  • Write statements
    • Declare (only)
    • Declare and initialize
    • if, while, for, etc.
    • Basic string methods
  • Evaluate expressions
    • Arithmetic (esp. / and %)
    • Logic (&&, ||, !)
  • Code tracing
    • What is the output of …?
    • What is the value of…?
  • Code writing
    • Write a method that…
    • Write a JUnit test for…