AP Computer Science A Unit 3

Repetition

A form of iteration

Control

Iteration statements change the flow of this by repeating a segment of code zero or more times

True

What the boolean expression controlling the loop must evaluate to in order to repeat the segment of code

One

What is iteration?

Repetition of code controlled by a Boolean expression.

When does a while loop check its condition?

Before executing the loop body.

What causes infinite loops?

Condition never becomes false.

What is an off-by-one error?

Loop runs one time too many or too few.

3 parts of a for-loop header?

Initialization, condition, update.

How many times is initialization executed in a for-loop?

Once before first check.

When is the update executed?

After each loop body execution.

Can loop control variable be used outside loop?

Only if declared before loop header.

Convert for-loop to while-loop?

Move init before, condition in while, update inside.

What is a nested loop?

A loop inside another; inner runs fully each outer iteration.

How to compute total iterations of nested loops?

Outer count × inner count.

Math.random() returns?

Double in [0.0,1.0).

How to generate int in [a,b]?

(int)(Math.random()*(b-a+1))+a

Import for File class?

import java.io.File;

What must be added to main when using File?

throws IOException

What happens if file name invalid?

FileNotFoundException.

Construct File object?

File f = new File("data.txt");

Construct Scanner for file?

Scanner input = new Scanner(f);

What does hasNext() do?

Checks if more tokens exist.

Difference: hasNextInt() vs nextInt()

hasNextInt checks, nextInt consumes.

Why loop for input validation?

To repeatedly prompt until correct type.

What does validation loop on p14 do?

Repeats until integer is entered.

What to print on invalid input?

Error message; consume token with next().

What does while(true) mean?

Creates an intentional infinite loop until a break occurs.

When should you use a while loop?

When number of iterations is unknown beforehand.

When should you use a for loop?

When number of iterations is known or count-controlled.

What does break do?

Immediately exits the nearest loop.

What does continue do?

Skips rest of loop body and proceeds to next iteration.

Common cause of logic errors in loops?

Incorrect condition, update, or variable scope.

What is loop accumulation?

Repeatedly updating a total inside a loop.

Why initialize sum to 0?

So accumulation starts with a neutral element.

Why initialize product to 1?

1 is the multiplicative identity.

What is integer division used for?

Truncating decimal results when dividing ints.

What is casting?

Forcing a value to another type, e.g., (double) num.

Why cast when computing averages?

To avoid integer division and keep decimal precision.

Scanner next() reads what?

A single token up to whitespace.

Scanner nextLine() reads what?

The entire line including spaces.

Common bug mixing nextInt() and nextLine()?

Leftover newline requires consuming nextLine first.

What does FileNotFoundException indicate?

File path or name is incorrect or missing.

Difference between relative and absolute path?

Relative is from project folder; absolute is full system path.

What is a token?

Smallest meaningful unit read by Scanner (word, number).

What happens if nextInt reads a non-int?

InputMismatchException.

What does hasNextDouble check?

Whether next token is a valid double.

What is loop tracing?

Simulating loops step-by-step to predict output.

Purpose of nested loop tracing tables?

Helps determine variable values each iteration.

What is time complexity of nested loops?

Usually O(n^2) when loops scale together.

What is the loop invariant?

A condition that remains true throughout loop execution.

What is the purpose of a loop counter?

Tracks iterations and controls loop progress.

What happens if you modify loop variable inside the loop?

May cause unintended behavior or infinite loops.

What is accumulation?

Adding values repeatedly inside a loop (e.g., sum += x).

What is concatenation in loops?

Building strings by repeatedly adding substrings.

Difference between ++i and i++?

++i increments then returns; i++ returns then increments.

What is the scope of a variable declared in a loop header?

It exists only inside the loop.

What is the purpose of using {} around loops?

Groups statements; without them only first line is inside loop.

What does return do inside a loop?

Immediately exits the method.

What is sentinel-controlled input?

Loop continues until a special sentinel value is entered.

What is flag-controlled input?

Loop continues based on a boolean variable toggled when a condition occurs.

Why should floating-point comparisons avoid == ?

Precision errors make exact comparison unreliable.

What does System.out.print vs println do?

print stays on same line; println moves to next line.

Why avoid hard-coded values?

Reduces flexibility and makes code harder to maintain.

What happens if you open a Scanner on a file twice?

May cause errors or read pointer resets depending on implementation.

Why close a Scanner reading a file?

Releases file resources.

Does closing Scanner(System.in) close System.in?

Yes; can cause errors on further input.

Why must hasNext be checked before next?

Calling next without a next token throws an exception.

What does nextLine return if last input was nextInt?

An empty line due to leftover newline.

What is string parsing?

Using methods to read specific characters/substrings.

What does substring(begin,end) do?

Returns substring from begin up to end-1.

What is charAt used for?

Accessing a single character at a specific index.

What type does compareTo return?

An int:

What is escape sequence \n?

Represents a newline.

What is escape sequence \t?

Represents a tab.

Why use equals instead of == for Strings?

#ERROR!

What is the difference between compile-time and runtime errors?

Compile-time found before running; runtime occurs during execution.

What is an algorithm?

Finite set of steps to solve a problem.

What is pseudocode?

High-level description of an algorithm.

What does trace table help with?

Tracking variable values across iterations.

What is input sanitization?

Ensuring input meets expected format before processing.

What is defensive programming?

Code written to handle unexpected inputs and errors.

Why use constants (`final`)?

Improves clarity and prevents accidental changes.

What is variable shadowing?

Local variable hides a variable declared outside.

What does % operator compute?

Remainder of integer division.

How do you check if a number is even?

num % 2 == 0

How do you check if a number is odd?

num % 2 != 0