arrays - david r. cheriton school of computer sciencedvogel/cs105f16/materials/lectures/08...

Arrays

• Object Creation and "dot syntax"

• Array Operation Idiom

CS 105 | 08 Arrays 1

circleracers (one racer)


// the x-position of the racerfloat x = 0;

void draw() {background(245); // white

drawRacer(x, 40, 1);x += 1;

}

// draw a racer at position x,y// draw the racer numbervoid drawRacer(float x, float y, int number) {...

circleracers (three racers)


// the x-position of the racersfloat x1 = 0;float x2 = 0;float x3 = 0;


drawRacer(x1, 40, 1);x1 += random(0, 3);




float x1 = 0;float x2 = 0;float x3 = 0;float x4 = 0;float x5 = 0;float x6 = 0;float x7 = 0;float x8 = 0;float x9 = 0;float x10 = 0;float x11 = 0;float x12 = 0;float x13 = 0;float x14 = 0;float x15 = 0;float x16 = 0;float x17 = 0;float x18 = 0;float x19 = 0;float x20 = 0;

drawRacer(x1, 40, i);x1 += random(0, 3);



















drawRacer(x20, 800, i);x20 += random(0, 3);;

Array

§ An array is a special variable that holds multiple values.

§ Each value location is called an element.

§ Each element is accessed using a unique index - The index is a whole number: 0, 1, 2, …


23

a b[]

15 7 -10 66 5

Variable Array

b[0] b[1] b[2] b[3] b[4]


Declaring an Array


int[] b;

int a;

array of intstype

int variable type

variable name

array name

Variable (for comparison)

Array declare an array of ints

Creating an Array

§ new is an operator that means “create”

§ we need to “create” the elements so they can hold values- “normal” variables have a single value, no need to create


int[] b;

b = new int[5];

create an array of 5 ints

typeof values

lengthof array

“create”

Declare and Create an Array in One Step


int[] b = new int[5];

create an array of 5

ints

declare an array of ints

called b

Declare and Create Array Examples

CS 105 | 08 Arrays 10

boolean[] boxStates;boxStates = new boolean[4];

float[] scores = new float[4];

int[] hues = new int[10];

int num = 5;float[] x = new float[num];float[] y = new float[num];

boolean[] s = new boolean[num * 2];

Valid or Invalid Array Declaration and Creation?

CS 105 | 08 Arrays 11

int[] a = new int[2 * 10];

float[] b = new int[5];

boolean c = new boolean[5];

float[5] d;

int[] e; e = new int[123];

float[] f = float[12];

int[] g = new int[1];

int[] h = new int[0];

Assigning Values to an Array

§ You assign values to each array element using the index

CS 105 | 08 Arrays 12


b[3] = 66;

elementindex

assign 66 to element at

index 3

b[3] is the 4th

element

Initializing an Array

§ Initializing an array is when you assign a value to each element after creating the array

§ All elements are initialized to a default value when created- float and int are 0, boolean is false, String is null

CS 105 | 08 Arrays 13


b[0] = 15;b[1] = 7;b[2] = -10;b[3] = 66;b[4] = 5;

b[]

15 7 -10 66 5

b[0] b[1] b[2] b[3] b[4]

initi

aliz

ing

Creating and Initializing an Array in One Step

CS 105 | 08 Arrays 14

int[] b = {15, 7, -10, 66, 5};

b[]

15 7 -10 66 5

b[0] b[1] b[2] b[3] b[4]

create an array of 5 intsand initialize it to these values

Using Arrays

§ Use array elements just like variables

CS 105 | 08 Arrays 15

int[] b = new int[5];b[0] = 15;b[1] = random(-100, 100);println(b[2]);float c = b[0] * 100;b[3] = b[3] + 5;ellipse(b[0], b[1], b[2], b[2]);if (b[3] > 5) { ...

You must create an array before using it

§ if an array hasn't been created, it has no elements- trying to access an "uncreated array", is a runtime error:

CS 105 | 08 Arrays 16

int[] b;b[0] = 666; // errorb[4] = 678; // error

b[]

?????

Valid Index Values

§ index must be between 0 and one less than array length - if array length is 5, index can be 0, 1, 2, 3, or 4

- otherwise, it's a runtime error:

CS 105 | 08 Arrays 17

int[] b = new int[5];b[-1] = 123; // errorb[5] = 123; // error

b[]

15 7 -10 66 5

b[0] b[1] b[2] b[3] b[4]

Valid Index Types

§ an index must be an int type

§ otherwise, it's a syntax error

CS 105 | 08 Arrays 18

int[] b = new int[5];b[1.0] = 15; // errorfloat f = 2;b[f] = 7; // error

b[]

15 7 -10 66 5

b[0] b[1] b[2] b[3] b[4]

§ you can convert an int using int() b[int(f)] = 7; // ok

(error demos)

CS 105 | 08 Arrays 19

int[] b = new int[5];float f = 1;

void setup() {println("start"); println(b[5]); // try -1, 1.0, f println("stop");

}

What is printed to the console ?

CS 105 | 08 Arrays 20

A. 1

B. 2

C. 3

D. 4

E. 5

int[] bar = new int[4];

void setup() {bar[0] = 5;bar[1] = 4;bar[2] = 3;bar[3] = 2;

println(bar[1]); }


CS 105 | 08 Arrays 21

A. 1

B. 2

C. 3

D. 4

E. 5

int[] bar = new int[5];

void setup() {bar[0] = 1;bar[1] = 2;bar[2] = 3;bar[3] = 4;bar[4] = bar[bar[1]];

println(bar[4]); }

circleracers (with array)

CS 105 | 08 Arrays 22

// the x-position of the racersfloat[] x = new float[3];


drawRacer(x[0], 40, 1);x[0] += random(0, 3);



}

Variables for Indices

§ Use variables or return values for indices

CS 105 | 08 Arrays 23

int[] b = new int[5];int i = 0;b[i] = 15;b[i + 1] = 7;b[int(random(0, 5))] = 1000000;

int() function converts to int,

otherwise a syntax error

circleracers (with array)

CS 105 | 08 Arrays 24



int i = 0;

drawRacer(x[i], 40, i + 1);x[i] += random(0, 3);i++;

drawRacer(x[i], 80, i + 1);x[i] += random(0, 3);i++;

drawRacer(x[i], 120, i + 1);x[i] += random(0, 3);

}

circleracers (with array and loop)

CS 105 | 08 Arrays 25



int i = 0;while (i < 3) {// the y position is a "function of" ifloat y = (i + 1) * 40;drawRacer(x[i], y, i + 1);x[i] += random(0, 3);i++;

}}

How many Xs are printed to the console?

CS 105 | 08 Arrays 26

A. 0

B. 1

C. 4

D. 5

E. 6

for (int i = 0; i < 5; i++ ) {print("X");

}


CS 105 | 08 Arrays 27

A. 0

B. 1

C. 4

D. 5

E. nothing, it’s an error

int[] arr = new int[5];

for (int i = 0; i < 5; i++) {arr[i] = i;

}

println(arr[4]);

Array Length

§ Arrays know their own length- Arrays are objects- you access array length using object “dot syntax”

CS 105 | 08 Arrays 28

int[] b = new int[5];println(b.length); // prints 5

b[b.length – 1] = 123; // okb[b.length] = 123; // error

say “b dot length”

circleracers (with array and loop)

CS 105 | 08 Arrays 29



int i = 0;while (i < x.length) {// the y position is a "function of" ifloat y = (i + 1) * 40;drawRacer(x[i], y, i + 1);x[i] += random(0, 3);i++;

}}

Array Operation Idiom

§ Looping through an entire array to apply some operation to all elements is a common operation.

CS 105 | 08 Arrays 30

// initialize array elements to same valuefor (int i = 0; i < myArray.length; i++) {myArray[i] = 50;

}

// initialize array elements to random valuefor (int i = 0; i < myArray.length; i++) {myArray[i] = random(0, 100);}

Array Operation: Sum all Elements

§ Compute the total sum for all elements in an array by adding them one by one to an accumulator variable.

CS 105 | 08 Arrays 31

// compute the sum of all elementsfloat total = 0; for (int i = 0; i < myArray.length; i++) {total += myArray[i];

}println("total:", total);

Array Operation: Average Element Value

§ Sum all elements in an array and divide total by the length of the array to compute the average element size.

CS 105 | 08 Arrays 32

// calculate the average value float total = 0;for (int i = 0; i < myArray.length; i++) {total += myArray[i];

}float average = total / myArray.length;println("average:", average);

Array Operation: Find Largest Element Value

§ Search the array to find the largest element value- Start by guessing that the largest element is the first one- check all other elements one-by-one to see if any are larger- if a larger one is found, use it as the largest element

CS 105 | 08 Arrays 33

// find the largest element// (assuming myArray has at least 1 element)float largest = myArray[0];for (int i = 1; i < myArray.length; i++) {if (myArray[i] > largest) {largest = myArray[i];

}}println("largest:", largest);

Array Operation: Find Largest Element Index

§ Search the array to find the index with largest element value- Start by guessing that index 0 is the largest element index - check all other elements one-by-one to see if any are larger- if larger one is found, use it’s index as the largest element index

CS 105 | 08 Arrays 34

// find the index of the largest element// (assuming myArray has at least 1 element)int indexOfLargest = 0;for (int i = 1; i < myArray.length; i++) {if (myArray[i] > myArray[indexOfLargest]) {indexOfLargest = i;

}}println("index of largest:", indexOfLargest);println("largest:", myArray[indexOfLargest]);

Asymptotic Runtime of Algorithms

§ O(1) and O(N) … “big Oh”

§ How many operations does it take to print one element?

print(myArray[i]);

§ How many operations does it take to add all elements?// compute the sum of all elementsfloat total = 0; for (int i = 0; i < myArray.length; i++) {total += myArray[i];

}println("total:", total);

CS 105 | 08 Arrays 35

Arrays as Function Parameters and Arguments

§ float means a single floating point value

§ float[] means an array of floating point values

CS 105 | 08 Arrays 36

void func(float[] a) {…

}

void func(float f) {…

}

largestValue (in arrayoperations code)

CS 105 | 08 Arrays 37

// return the largest element in array a// (assuming a has at least 1 element)float largestValue(float[] a) {

float largest = a[0];for (int i = 1; i < a.length; i++) {if (a[i] > largest) {largest = a[i];

}}return largest;

}

arrayforest

CS 105 | 08 Arrays 38

float[] treeX = {166, 90, 77, 30, 110, 131, 55 };float[] treeLeafColour = new float[treeX.length];

void setup() {... // initialize to random tint of greenfor (int i = 0; i < treeLeafColour.length; i++ ) {

treeLeafColour[i] = random(64, 130); }

}

void draw() {... // draw trees for forestfor (int i = 0; i < treeX.length; i++) {

tree(treeX[i], 95, treeLeafColour[i]); }

}

EXTRA: Two Arrays, Shifting Elements in Array

§ Textbook Example 9.6

CS 105 | 08 Arrays 39

EXTRA: snake (LP Example 9-8)

CS 105 | 08 Arrays 40

// Shift array valuesfor (int i = 0; i < xpos.length - 1; i++) {xpos[i] = xpos[i + 1];ypos[i] = ypos[i + 1];

}

// New location at end of arrayxpos[xpos.length - 1] = mouseX; ypos[ypos.length - 1] = mouseY;

arrays - david r. cheriton school of computer sciencedvogel/cs105f16/materials/lectures/08...

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