# 1.4 Assignment and Input

Each coding exercise has its own folder. Open its `.java` file and press **Run** to try it. **Run Tests** checks every graded exercise in this lesson at once.

## Assignment Statements

**Assignment statements** initialize or change the value stored in a variable using the assignment operator `=`. An assignment statement always has a single variable on the left hand side. The value of the **expression** (which can contain math operators and other variables) on the right of the `=` sign is assigned to and stored in the variable on the left side.

![Assignment Statement (variable = expression;)](images/assignment.png)

*Assignment Statement (variable = expression;)*

Instead of saying equals for the = in an assignment statement, say “gets” or “is assigned” to remember that the variable gets or is assigned the value on the right. In the figure above score is assigned the value of the expression 10 times points (which is another variable) plus 5.

The following [video](https://www.youtube.com/watch?v=MZwIgM__5C8&ab_channel=colleenlewis) by Dr. Colleen Lewis shows how variables can change values in memory using assignment statements.

As we saw in the video, we can set one variable’s value to a *copy* of the value of another variable like `y = x;`. This won’t change the value of the variable that you are copying from.

Let’s step through the following code in the Java visualizer to see the values in memory ([step through it in the Java visualizer](http://www.pythontutor.com/visualize.html#code=public+class+Test2%0A%7B%0A+++public+static+void+main(String%5B%5D+args%29%0A+++%7B%0A+++++int+x+%3D+3%3B%0A+++++int+y+%3D+2%3B%0A+++++System.out.println(x%29%3B%0A+++++System.out.println(y%29%3B%0A+++++x+%3D+y%3B%0A+++++System.out.println(x%29%3B%0A+++++System.out.println(y%29%3B%0A+++++y+%3D+5%3B%0A+++++System.out.println(x%29%3B%0A+++++System.out.println(y%29%3B%0A+++%7D%0A%7D&mode=display&origin=opt-frontend.js&cumulative=false&heapPrimitives=false&textReferences=false&py=java&rawInputLstJSON=%5B%5D&curInstr=0)).

```java
public class Test2
{
  public static void main(String[] args)
  {
    int x = 3;
    int y = 2;
    System.out.println(x);
    System.out.println(y);
    x = y;
    System.out.println(x);
    System.out.println(y);
    y = 5;
    System.out.println(x);
    System.out.println(y);
  }
}
```

> Step through this code on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/topic-1-4-assignment.html#codelensAsgn

> Check your understanding: What are the values of x, y, and z after the following code executes? You can step through this code by clicking on this [Java visualizer](http://www.pythontutor.com/visualize.html#code=public+class+Test2%0A%7B%0A+++public+static+void+main(String%5B%5D+args%29%0A+++%7B%0A+++++int+x+%3D+0%3B%0A+++++int+y+%3D+1%3B%0A+++++int+z+%3D+2%3B%0A+++++x+%3D+y%3B%0A+++++y+%3D+y+*+2%3B%0A+++++z+%3D+3%3B%0A+++++System.out.println(x%29%3B%0A+++++System.out.println(y%29%3B%0A+++++System.out.println(z%29%3B%0A+++%7D%0A%7D&mode=display&origin=opt-frontend.js&cumulative=false&heapPrimitives=false&textReferences=false&py=java&rawInputLstJSON=%5B%5D&curInstr=0) link.
>
> ```java
> int x = 0;
> int y = 1;
> int z = 2;
> x = y;
> y = y * 2;
> z = 3;
> ```
>
> Answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/topic-1-4-assignment.html#qasgn1

> Check your understanding: The following has the correct code to ‘swap’ the values in x and y (so that x ends up with y’s initial value and y ends up with x’s initial value), but the code is mixed up and contains one… — answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/topic-1-4-assignment.html#swapParsons

## Data Types in Assignments

Every variable must be assigned a value before it can be used in an expression. That value must be from a compatible data type. A variable is **initialized** the first time it is assigned a value. During execution, an expression is evaluated to produce a single value. The value of an expression has a type based on the types of the values and operators evaluated in the expression. For example, an arithmetic expression that uses at least one `double` value will evaluate to a `double` value and must be saved in a `double` variable, as seen in the exercise below. In the next lesson, we will see how to change a variable or expression from one type to another.

### Exercise 1

The code looks okay at first glance, but if you run it, you will see that there is an error of incompatible types. Change the data type of one of the variables to fix the error.

> Open the `01-assignmentTypes` folder and edit `Asgn.java`. Press **Run** to try it; **Run Tests** checks it (1 test).

Reference types like `String` can be assigned a new object or null if there is no object. The literal **null** is a special value used to indicate that a reference is not associated with any object.

```java
// String variables can be assigned null
// or a new String object
String str = null;
str = "new object";
```

## Adding 1 to a Variable

If you use a variable to keep score, you would probably increment it (add one to the current value) whenever score should go up. You can do this by setting the variable to the current value of the variable plus one (`score = score + 1`) as shown below. The formula would look strange in math class, but it makes sense in coding because it is assigning a new value to the variable on the left that comes from evaluating the arithmetic expression on the right. So, the score variable is set to the previous value of score plus 1.

### Exercise 2

Try the code to see how score is incremented by 1. Try substituting 2 instead of 1 to see what happens.

> Open the `02-incrementScore` folder and run `Test1.java`. It is an example, so it has no tests.

## Input with Variables

Variables are a powerful abstraction in programming because the same algorithm can be used with different input values saved in variables. Input can come in a variety of forms, such as **tactile** for example by clicking on a button, **audio** with speech, **visual** using a webcam, or the most common form, **text** that the user types in. The `Scanner` class in Java is one way to obtain text input from the keyboard.

The code below using the `Scanner` class will say hello to anyone who types in their name and will have different results for different name values. Try again with a friend’s name. The code works for any name: behold, the power of variables!

### Exercise 3

The code will say hello to anyone who types in their name. Try again with a friend’s name.

> Open the `03-inputName` folder and run `Main.java`. It is an example, so it has no tests. **Run** feeds it the input in `input.txt`; change that file to try other input.

Although you will not be tested in the AP CSA exam on using the Java input from the keyboard, learning how to do input in Java is very useful and fun. For more information on using the `Scanner` class, go to [https://www.w3schools.com/java/java_user_input.asp](https://www.w3schools.com/java/java_user_input.asp), and for the newer `Console` class, [https://howtodoinjava.com/java-examples/console-input-output/](https://howtodoinjava.com/java-examples/console-input-output/).

## Coding Challenge: Dog Years

![dog](images/dog-free.png)

In this coding challenge, you will calculate your age, and your pet’s age from your birthdates, and your pet’s age in dog years. In the code below, type in the current year, the year you were born, the year your dog or cat was born (if you don’t have one, make one up!) in the variables below. Then write formulas in assignment statements to calculate how old you are, how old your dog or cat is, and how old they are in dog years which is 7 times a human year. Finally, print it all out. If you are pair programming, switch drivers (who has control of the keyboard in pair programming) after every line of code.

### Exercise 4

Calculate your age and your pet’s age from the birthdates, and then your pet’s age in dog years.

> Open the `04-challenge1-4` folder and edit `Challenge1_4.java`. Press **Run** to try it; **Run Tests** checks it (6 tests).

## Summary

- (AP 1.4.A.2) The **assignment operator** (`=`) allows a program to initialize or change the value stored in a variable. The value of the expression on the right is stored in the variable on the left.
- (AP 1.4.A.1) Every variable must be assigned a value before it can be used in an expression. That value must be from a compatible data type.
- (AP 1.4.A.1) A variable is **initialized** the first time it is assigned a value.
- (AP 1.4.A.1) Reference types can be assigned a new object or `null` if there is no object. The literal `null` is a special value used to indicate that a reference is not associated with any object.
- (AP 1.4.A.3) During execution, an expression is evaluated to produce a single value. The value of an expression has a type based on the types of the values and operators used in the expression.
- (AP 1.4.B.1) Input can come in a variety of forms, such as tactile, audio, visual, or text. The `Scanner` class is one way to obtain text input from the keyboard, although input from the keyboard will not be on the AP exam.

## AP Practice

> Check your understanding: Consider the following code segment.
>
> ```java
> int a = 5;
> int b = a/2;
> double c = a/2.0;
> double d = 5 + a / b * c - 2;
> System.out.println(d);
> ```
>
> What is printed when the code segment is executed?
>
> Answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/topic-1-4-assignment.html#apcsa_sample1