CS 106A, Lecture 18 Practice with 1D and 2D Arrays · 2017-07-27 · 3 Plan for Today •Recap: 2D...

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CS106A,Lecture18Practicewith1Dand2DArrays

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Midterm!Arrays

HW5:ImageShop

Wearehere

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Plan for Today•Recap:2DArraysandImages•Practice:Shrink•Practice:Histogram•Practice:Tic-Tac-Toe

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5Imageusedunder“fairuse”foreducationalpurposes.Source:https://www.themarysue.com/decoding-the-transgender-matrix-the-matrix-as-a-transgender-coming-out-story/

The Matrix

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2D Arrays (“Matrices”)

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2D Arraystype[][] name = new type[rows][columns];

int[][] a = new int[3][5];

0 1 2 3 4

0 a[0][0] a[0][1] a[0][2] a[0][3] a[0][4]

1 a[1][0] a[1][1] a[1][2] a[1][3] a[1][4]

2 a[2][0] a[2][1] a[2][2] a[2][3] a[2][4]

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2D Arrays = Arrays of Arrays!int[][] a = new int[3][4];int[] firstRow = a[0];

a[0][0] a[0][1] a[0][2] a[0][3]

a[1][0] a[1][1] a[1][2] a[1][3]

a[2][0] a[2][1] a[2][2] a[2][3]

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Summary: 2D Arrays• Makeanew2Darray

type[][] name = new type[rows][columns];

• Getandsetvaluesusingbracketnotationname[row][col] // get elem at row,colname[row][col] = value; // set elem at row,col

• Getthenumberofrowsandcolumnsarr.length // # rowsarr[0].length // # columns

• Iterateovera2Darrayusingadoublefor-loopfor (int row = 0; row < arr.length; row++) {

for (int col = 0; col < arr[0].length; col++) {// do something with arr[row][col];

}}

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Limitations of 2D Arrays• Unlike1Darrays,youcannotcompare2DarrayswithArrays.equals.YoumustuseArrays.deepEquals.

int[][] a1 = ...int[][] a2 = ...if (Arrays.deepEquals(a1, a2)) { ... }

• A2Darraydoesnotknowhowtoprintitself:int[][] a = new int[rows][cols];println(a); // [[I@8cf420println(Arrays.toString(a)); // [[I@6b3f44,[I@32c2a8]...

// [[0, 1, 2, 3, 4], [1, 2, ...println(Arrays.deepToString(a));

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Images

Imagesarejustgrids(2Darrays!)ofpixels!Pixelsarejustintegervaluesfrom0-255.

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Images as 2D ArraysWecangetaGImage asa2Darrayofpixels.

GImage img = new GImage(“res/daisy.jpg”);int[][] pixels = img.getPixelArray();int pixel = pixels[0][0]; // top-left pixel

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Example: PointillismPointillismisanartstylewheremanysmalldotsofcolorarecombinedtomakealargerimage.

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Red, Green and Blue in one int?

Imagesencode theR,G,andBvaluesofapixelintoasingleintegerbetween0and255.YoucanconvertbetweenthispixelvalueandtheindividualRGBvalues.

int[][] pixels = image.getPixelArray();int px = pixels[0][0];int red = GImage.getRed(px);int green = GImage.getGreen(px); int blue = GImage.getBlue(px);

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Creating New PixelsImagesencode theR,G,andBvaluesofapixelintoasingleintegerbetween0and255.YoucanconvertbetweenthispixelvalueandtheindividualRGBvalues.

YoucanalsocreatepixelswithyourownRGBvalues.

int r = ...int g = ...int b = ...int pixel = GImage.createRGBPixel(r, g, b);

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Images as 2D ArraysWecangetaGImage asa2Darrayofpixels,andmodifyitanywaywewant.Then,wecancreateanewGImage withthemodifiedpixels.

GImage img = new GImage(“res/daisy.jpg”);int[][] pixels = img.getPixelArray();... (modify pixels)img.setPixelArray(pixels); // update image

// or make a new GImageGImage newImg = new GImage(pixels);

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Modifying Image Pixels• Therearemanycoolimagealgorithmsbasedaroundmodifyingindividualpixelsinanimage:grayscale,brighten,normalize,removered-eye…

grayscale zoom

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GImage Pixel MethodsGImage img = new GImage("res/daisy.jpg");

Methodname Descriptionimg.getPixelArray() returnspixelsas2Darrayofints,where

eachintinthearraycontainsall3ofRed,Green,andBluemergedintoasingleinteger

img.setPixelArray(array); updatespixelsusingthegiven2Darrayofints

GImage.createRGBPixel(r, g, b) returnsanintthatmergesthegivenamountsofred,greenandblue(each0-255)

GImage.getRed(px)GImage.getGreen(px)GImage.getBlue(px)

returnstheredness,greenness,orbluenessofthegivenpixelasanintegerfrom0-255

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Recap: Modifying Pixels• Extract pixelRGBcolorswithGImage.getRed/Blue/Green.

int red = GImage.getRed(pixels[0][0]); // 0-255int green = GImage.getGreen(pixels[0][0]); // 0-255int blue = GImage.getBlue(pixels[0][0]); // 0-255

• Modify thecolorcomponentsforagivenpixel.red = 0; // remove redness

• Combine theRGBbacktogetherintoasingleint.pixels[0][0] = GImage.createRGBPixel(red, green, blue);

• Update theimagewithyourmodifiedpixelswhenfinished.image.setPixelArray(pixels);

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ShrinkLet’swriteaprogramthatcanshrink animageto½itsoriginalsize.

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Shrink

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Shrink

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ShrinkGivenapixel(x,y)inoursmallerimage,howdoweknowwhichpixelinourlargerimageshouldgothere?

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Shrink

(0 ,0) (0,2)

(2,0) (2,2)

(0,0) (0, 1)

(1,0) (1, 1)

Givenapixel(x,y)inoursmallerimage,howdoweknowwhichpixelinourlargerimageshouldgothere?

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ShrinkGivenapixel(x,y)inoursmallerimage,howdoweknowwhichpixelinourlargerimageshouldgothere?

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Shrinkint[][] pixels = image.getPixelArray();int[][] result =

new int[pixels.length/2][pixels[0].length/2];

for (int r = 0; r < result.length; r++) {for (int c = 0; c < result[0].length; c++) {

result[r][c] = pixels[r*2][c*2];}

}

image.setPixelArray(result);

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}


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}


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}


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}


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}


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Givenafileoftemperatures(int),suchas:8266796383

WriteaHistogram programthatwillprintahistogramofstarsindicatingthe#ofdayswitheachuniquetemperature.

85: *****86: ************87: ***88: *

Weather Histogram

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Idea: Array of Counters• Forproblemslikethis,wherewewanttokeepcountofmanythings,afrequencytable(ortallyarray)canbeacleversolution.– Idea: Theelementatindexi willstoreacounterforthedigitvaluei.

– example:countoftemperatures:

index … 55 56 57 58 59 60 61 62 …

value 1 0 4 1 0 0 1 1

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Tic-Tac-ToeLet’suse2DarraystocreateaConsoleProgram versionofTic-Tac-Toe.

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Recap•Recap:2DArraysandImages•Practice:Shrink•Practice:Histogram•Practice:Tic-Tac-Toe

NextTime:Moredatastructures

CS 106A, Lecture 18 Practice with 1D and 2D Arrays · 2017-07-27 · 3 Plan for Today •Recap: 2D...

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