Arrays and Loops

Arrays and Loops
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Basic math
Simple decision making
Doing things over and over again very, very fast
And storing data

So far we’ve been talking about single data values.

Java primitive types allow you to represent single numbers (integers or floating point), truth values (true or false), and characters.
But what about representing multiple values?

Can you think of some real things that could be represented as a series of Java’s primitive types?

Text, or what we call strings in computer science.
DNA, which we can represent as just a limited kind of string.
Time series data, like a series of temperatures taken at regular intervals.
Music, as a time series of air pressure measurements.

Arrays are a simple yet powerful data structure.

A Java array represents a series of zero or more values of the same type.

Arrays are our first example of a data structure.
Arrays put values in order, one after another.
Values in an array also have an index, their position in the array.

Just like other variables, arrays have a name and type.

But when we declare them we use brackets to declare an array instead of a single value.

// A single integer named single
int single;
// An array of integers named multiple
int[] multiple;

// A single character named one
char one;
// An array of characters named all
char[] all;

When it is declared an array is empty. To use it we have to tell Java how many elements it has.

The size cannot be changed once the array is initialized. All arrays have a .length property that we can use to get their size.

// An array of 8 integers named multiple
int[] multiple = new int[8];
System.out.println(multiple.length); // Prints 8

// An array of characters named all
char[] all;
// Initialize all to hold 4 characters
all = new char[4];

We can also assign values to an array when it is initialized:

// An array of the 4 integers 1, 2, 5, 10
int[] multiple = {1, 2, 5, 10};

// An array of the characters c, s, !
char[] awesome = {'c', 's', '!'};

Here we don’t have to specify the size. Why not?

We use the bracket syntax to both get and set array values by index.

int[] twos = {1, 2, 4};
System.out.println(twos[0]); // Prints 1
twos[0] = 2;
System.out.println(twos[0]); // Prints 2
System.out.println(twos[2]); // Prints 4

We’ll discuss many data structures this semester. Arrays are the first, and the simplest: but still extremely useful.

Data structures associate metadata—or data about data—with the original data.

For an array:

The data is the array contents
What is the metadata?
The order of the items in the array.

Basic math
Simple decision making
Doing things over and over again very, very fast
And storing data

We call the process of repeatedly doing the same thing over and over again a loop.

Loops are a common way to work with data stored in an array.

Here’s the simplest loop, although it’s not that useful:

while (true) {
  // do something
}

The while loop has two parts:

A block of code to repeat
A condition that must be true each time the loop is entered

As soon as the condition becomes false, the block of code is not executed and execution continues after the loop

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Now that we’ve acquired the power to make the computer do things repeatedly, we need to make sure it stops…

An unterminated loop will cause your code to hang, or hit a timeout if you are running our tests.

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int index = 0;
while (index < 4) {
  System.out.println(index);
  index++;
}

The pattern above is very common:

Initialize a variable (int index = 0)
Loop until a condition is met (index < 4)
Update the variable inside the block (index++)

So common that it has its own special syntax.

int index = 0;
while (index < 4) {
  // do something
  index++;
}

The while loop above is equivalent to this for loop:

for (int index = 0; index < 4; index++) {
  // do something
}

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for loops are a bit more complex than while loops—but they are also more common.

Here’s what to keep in mind:

Initialization only happens once when the loop is first executed
The conditional is evaluated every time the loop block is executed, including the first time
The update is performed after each time the block is executed and before the condition is checked

Initialize the loop variable.

Then:

Check the condition.
If the condition is false, continue execution after the for loop
If the condition is true, execute the loop block
After the block finishes, update the loop variable
Repeat

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All three parts of a for loop are optional.

int i = 0;
for (; i < 10; i++) {
  // do something
}
for (; i < 10; ) {
  // do something else
}
for (;;) {
  // do something forever
}

Don’t do this unless you have a good reason.

If you get confused by a for loop, try rewriting it as a while loop.

That may help clear things up.

Class participation will be counted starting Monday. We have scores out now for the past three classes, and maybe some of you are upset.

But it’s not that hard:

Arrive on time
Follow along
Keep your computer connected to the WiFi
Don’t leave early

I’ve released the Lab 1 homework for you to complete in case you didn’t get it done in lab yesterday.
Homework continues today—and the entire first set of homework is due Sunday at midnight.
We have office hours today starting at noon. Please plan on coming in this week to finish setting up Android Studio.
Everything is on the calendar.
Please fill out the initial student survey. 1% extra credit for anyone who does by Sunday 2/2/2020.

What Are Computers Good At?

Multiple Data Values

Why Multiple Values?

This is About Data

Arrays

Declaring Arrays

Initializing Arrays

Initializing Arrays With Values

Getting And Setting Array Values

Data Structures and Metadata

Questions About Arrays?

What Are Computers Good At?

Loops

`while`

Unterminated Loops

Common Loop Pattern

`for` Loops

Understanding `for` Loops

`for` Loop Algorithm

Incomplete `for` Loops

If You Get Confused

Questions About Loops?

Class Participation Guidelines

Announcements

What Are Computers Good At?

Multiple Data Values

Why Multiple Values?

This is About Data

Arrays

Declaring Arrays

Initializing Arrays

Initializing Arrays With Values

Getting And Setting Array Values

Data Structures and Metadata

Questions About Arrays?

What Are Computers Good At?

Loops

while

Unterminated Loops

Common Loop Pattern

for Loops

Understanding for Loops

for Loop Algorithm

Incomplete for Loops

If You Get Confused

Questions About Loops?

Class Participation Guidelines

Announcements

`while`

`for` Loops

Understanding `for` Loops

`for` Loop Algorithm

Incomplete `for` Loops