CSA UNIT 4: Iteration | 现代数学启蒙

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Key Points

While Loops

Structure & Syntax:

Execution: – The condition is tested before each iteration. – The loop may execute zero times if the condition is false initially.
Common Pitfalls: – Forgetting to update the loop variable can lead to infinite loops. – Incorrect termination conditions cause off–by–one errors.

For Loops

Structure & Syntax:

Usage: – Ideal when the number of iterations is known. – Combines initialization, condition checking, and update in one line.
Common Pitfalls: – Off–by–one errors when using < vs. <= in the condition. – Misplaced update statements that can lead to unexpected behavior.

Developing Algorithms Using Strings

Iteration Through Strings: – Use loops (for or while) to traverse each character. – String.charAt(index) accesses individual characters.
Common Operations: – Reversing a string, counting vowels/characters, extracting substrings.
Immutability: – Strings in Java are immutable. Use concatenation or a StringBuilder when modifying.

Additional Skills Emphasized

Code Comprehension: – Determining the output of code segments (with or without method calls). – Tracing the order of execution.
Algorithm Development: – Writing code to meet method specifications using expressions, conditionals, and loops.
Code Equivalence & Analysis: – Recognizing when different loop structures (for vs. while) produce the same results.
Iteration Counting: – Being able to determine exactly how many times a loop executes.

Sample Code Examples

1. Using a While Loop

2. Using a For Loop

3. String Manipulation – Reversing a String

4. Counting Vowels in a String

AP Style Multiple–Choice Questions

Question 1:

Consider the following code segment:

What is the output?

A) 1234

B) 01234

C) 2345

D) 12345

Question 2:

What is the output of the following for loop?

A) 0 1 2 3 4

B) 1 2 3 4 5

C) 0 1 2 3 4 5

D) 1 2 3 4

Question 3:

Given the code below, what does it print?

A) apple

B) elppa

C) pplea

D) lpepa

Question 4:

Two code segments are provided below.

Segment A:

Segment B:

Which statement is true?

A) Both segments produce the same result (sum = 6).

B) Only Segment A compiles.

C) Only Segment B produces the correct sum.

D) Both segments produce an error.

Question 5:

Examine the following code:

What is the output?

A) 5

B) 0

C) No output

D) An infinite loop occurs

Question 6:

What is the output of the following code?

A) 10

B) 15

C) 5

D) 20

Question 7:

How many times does the following loop execute?

A) 5

B) 6

C) 10

D) 4

Question 8:

What is the output of this code segment?

A) ell

B) Hel

C) llo

D) Hell

Question 9:

Determine the output of the following code:

A) 24

B) 12

C) 20

D) 16

Question 10:

What is the result of executing this code?

A) 10

B) No output

C) A compile–time error

D) An infinite loop

Question 11:

Identify the problem with the following loop:

A) It causes an infinite loop.

B) It executes only 11 times.

C) It causes a compile–time error.

D) It produces no output.

Question 12:

What does the following code print?

A) 2

B) 3

C) 1

D) 0

Question 13:

Which for loop header correctly iterates over all valid indices of an array arr of length n? A) for (int i = 0; i <= arr.length; i++)

B) for (int i = 0; i < arr.length; i++)

C) for (int i = 1; i <= arr.length; i++)

D) for (int i = 1; i < arr.length; i++)

Question 14:

What is the output of the following code?

A) aaa

B) aa

C) banana

D) a

Question 15:

What is the output of this code segment?

A) 0123

B) 011223

C) 00112233

D) 012012

Question 16:

Which while loop is equivalent to the following for loop?

Question 17:

What is the output of the following do–while loop?

A) No output

B) 1

C) 12

D) The loop runs indefinitely

Question 18:

Determine the output of this code:

A) C16

B) CS6

C) C106

D) C1A

Question 19:

What is printed by the following code?

A) 9

B) 8

C) 15

D) 10

Question 20:

How many total iterations does the inner loop execute in the following nested loops?

A) 45

B) 55

C) 65

D) 50

Answer Key

the use of “continue” in Java:

In Java, the continue statement is used within loops to skip the current iteration and proceed directly to the next one. This can be very useful when you want to ignore certain conditions without breaking out of the loop entirely.

Basic Usage

When the continue statement is executed inside a loop (like for, while, or do-while), the remaining code inside the loop for that particular iteration is skipped. The loop then moves on to its next iteration.

Example:

In this example, when i is even, the continue statement is triggered, and the System.out.println(i); statement is skipped. As a result, only odd numbers from 0 to 9 are printed.

Using `continue` with Labeled Loops

When dealing with nested loops, you can use a labeled continue to specify which loop to continue. This is particularly useful when you want to skip the current iteration of an outer loop from within an inner loop.

Example:

In this example, whenever j becomes greater than i, the control jumps to the next iteration of the outerLoop, bypassing the rest of the inner loop.

Summary

Purpose: The continue statement skips the remaining code in the current iteration and continues with the next iteration of the loop.

Where it can be used: Inside for, while, and do-while loops.

With labels: In nested loops, labels can be used with continue to specify which loop should continue.

Understanding and using the continue statement effectively can help make your loops more efficient and your code cleaner by avoiding unnecessary computations for specific iterations.

the use of “break” in Java

In Java, the break statement is used to exit a loop or a switch statement immediately. Here are its common uses:

Exiting Loops Early:

In loops (for, while, or do-while), break stops the loop even if the loop’s termination condition hasn’t been met. This is useful when you want to stop processing once a certain condition is met.

Switch Statements:

In a switch statement, break prevents the program from executing subsequent cases (known as "fall-through"). Without break, once a matching case is found, the code for all following cases will execute until a break is encountered or the switch ends.

Labeled Break for Nested Loops:

When you have nested loops, you can use a labeled break to exit an outer loop directly from within an inner loop.

Key Points:

Scope: The break statement only exits from the innermost loop or switch block it is in (unless used with a label).

Use with Caution: While break can simplify some logic, overusing it might make your code harder to read. Always consider if there is a clearer way to structure your loops or switch statements.

Understanding how to use break effectively can help you control the flow of your Java programs, making them more efficient and easier to understand.