Unit 1.4: Assignment and Input¶

Scope: CS Awesome 2, Section 1.4

Learning Goals¶

By the end of this lesson, you should be able to:

Trace assignment statements.
Explain why = means assignment rather than mathematical equality.
Check type compatibility in assignments.
Use a variable's previous value to compute its next value.
Read keyboard input with Scanner.

Assignment Statements¶

An assignment statement evaluates the expression on the right and stores the resulting value in the single variable on the left.

score = points * 10 + 5;

Read = as “is assigned” or “gets.” Assignment is directional.

int x = 3;
int y = x; // copies the current value 3 into y
x = 8;     // changes x, but y remains 3

For primitive values, assigning one variable to another copies the value. It does not permanently connect the two variables.

Tracing Changes¶

int a = 2;
int b = 5;
a = b;
b = b + 1;

Statement completed	`a`	`b`
initialization	2	5
`a = b;`	5	5
`b = b + 1;`	5	6

The right side uses the old value of b; the result is then stored back into b.

Type Compatibility¶

Every local variable must be assigned a value before use, and the assigned value must be compatible with the variable's type.

double total = 4.5; // valid
double count = 4;   // int can widen to double
int result = 4.5;   // compile-time type error

Reference variables may hold an object reference or null.

String message = null;
message = "System ready";

null means that the reference is not associated with an object. Calling a method through null causes a run-time error.

Updating a Variable¶

int visitors = 10;
visitors = visitors + 1;

This is meaningful in programming: evaluate visitors + 1 using the old value, then store the new value.

To swap two values, preserve one value in a temporary variable:

int left = 4;
int right = 9;
int temp = left;
left = right;
right = temp;

Input with `Scanner`¶

Input can be text, audio, visual, or tactile. Scanner is one way to read text typed at the keyboard.

import java.util.Scanner;

public class CheckIn
{
    public static void main(String[] args)
    {
        Scanner input = new Scanner(System.in);

        System.out.print("Name: ");
        String name = input.nextLine();

        System.out.print("Group size: ");
        int size = input.nextInt();

        System.out.println("Welcome, " + name);
        System.out.println("Group size: " + size);
    }
}

Common methods include:

Method	Reads
`nextInt()`	an `int` token
`nextDouble()`	a `double` token
`next()`	one whitespace-delimited string token
`nextLine()`	the rest of the current line

Keyboard input is useful for programs, although it is not a focus of the AP exam.

Practice Missions¶

Mission 1: Trace the State¶

Create a trace table for x, y, and z after each statement.

int x = 4;
int y = 7;
int z = x;
x = y;
y = z + 2;
z = x - y;

Mission 2: Rotate Three Values¶

Given three initialized int variables a, b, and c, move the original value of a into b, b into c, and c into a. Use temporary storage and do not use numeric literals in the rotation statements.

Mission 3: Delivery Estimate¶

Write a program that reads a destination, whole number of packages, and estimated hours as a decimal. Print a labeled summary. Choose a compatible Scanner method and variable type for each input.

Mission 4: Diagnose the Type Error¶

Explain why this assignment fails and provide two valid fixes with different result types:

int items = 7;
double price = 2.5;
int total = items * price;

Key Summary¶

Concept	Core idea
Assignment	Evaluates the right side and stores it on the left.
Initialization	First assignment of a variable.
Type compatibility	The stored value must fit the declared type.
Copying a primitive	Copies its current value.
`null`	A reference associated with no object.
`Scanner`	Reads text input and converts selected tokens.

Source scope: CS Awesome 2, Unit 1.4