Arrays and Algorithms

Arrays and Algorithms
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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 integer;
// 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', '!' };

You’re computer scientist now. You start counting at zero.

An array’s first value is a[0]
An array’s last value is a[a.length - 1]

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Java arrays are fixed size, meaning that we need to know how large they are during initialization.

This can complicate our programs.
(If your NetID is janedoe2, now you know why.)
Java has other array-like data structures that are more flexible—we’ll discuss them later.

Loops and arrays are frequently used together.

This is probably the most common for loop. It iterates over all members of the array: from 0 to primes.length - 1.

int[] primes = { 2, 3, 5, 7, 11, 13 };

for (int i = 0; i < primes.length; i++) {
  System.out.println(primes[i]);
}

int[] primes = { 2, 3, 5, 7, 11, 13 };
for (int i = 0; i < primes.length; i++) {
  System.out.println(primes[i]);
}

This is so common that Java has a shortcut for it, the enhanced for loop:

int[] primes = { 2, 3, 5, 7, 11, 13 };
for (int prime : primes) {
  System.out.println(prime);
}

If you don’t care about the index, only the value, this is a useful loop.

Fall 2018 Q1: 94%
Spring 2019 Q1: 92%

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

Now we understand how to harness our computers innate abilities. But how do we get it to do what we want?

Algorithm: a process or set of rules to be followed in calculations or other problem-solving operations, especially by a computer.

As computer scientists, we implement algorithms by having computers:

Perform simple calculations
Store the results
Make simple decisions
Do things over and over again as fast as possible

Algorithms are not a new idea—but are heavily associated with a new technology.

For the next few weeks we will focus on implementing simple algorithms.

This will allow us to practice our problem-solving abilities while we learn new strategies for structuring our programs.

Given three integers, find the maximum value.

First, what is our algorithm?

Determine if first is the largest—if it is, we’re done
Otherwise we need to consider second and third…

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Homework continues today and this weekend.
MP0 will be released on Monday.
We have office hours today and over the weekend.
I will hold office hours MWF from 1–3PM in Siebel 2227. Please come by and say hi!
Everything is on the calendar.
Please fill out the initial student survey. 1% extra credit for anyone who does by Sunday 02/03/2019.

Review: Arrays

Review: Declaring Arrays

Review: Initializing Arrays

Review: Initializing Arrays With Values

0 Indexing

Array Limitations

Made For Each Other

Enhanced `for` Loop

Questions About Arrays?

You Can Do This

What Are Computers Good At?

Algorithms

Algorithm Word Usage

All Algorithms All the Time

Maximum of Three Values

Next Time: Functions

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