nestedloops-mod7-part31 two-dimensional arrays and nested loops – part 3 bugs in the garden...

NestedLoops-Mod7-part3 1

Two-Dimensional Arrays and Nested Loops – part 3

Bugs in the garden

Originally by Barb EricsonGeorgia Institute of Technology


Learning Goals

• Understand at a conceptual and practical level– How to copy pixels from one picture to

another– How to declare, initialize and change several

variables in a for loop– How to copy pixels from one picture to a

specified location in another picture


Copying Pixels to a New Picture

• Need to track the source picture x and y – And the target picture

x and y

• We can use any picture – As the target picture

• Several blank pictures are available

– 640x480.jpg– 7inX95in.jpg

1 2

3 4

1 2

3 4

source

target


For loops

• A for loop can have multiple statements in each section with semicolons in betweenfor(intialization; condition; increment){

// do something

}

• For example:for(int a=0, b=0; a<5; a++,b++){

System.out.println(“(a,b)=”+a+”, “+b);

}

(a,b)=0,0

(a,b)=1,1

(a,b)=2,2

(a,b)=3,3

(a,b)=4,4


Copy Picture Algorithm

• Copy a picture to another picture– Create the target picture object– Create the source picture object

• Loop through the source picture pixels– Get the source and target pixels– Set the color of the target pixel to the color of

the source pixel


CopyOver Method – in a NEW class.public class Duplicate {

public static void copyOver () { String sourceFile = “name your source file here”; String targetFile = “name the target file here”; Picture sourcePicture = new Picture(sourceFile); Picture targetPicture = new Picture(targetFile);

Pixel sourcePixel = null; Pixel targetPixel = null;

Convert the Algorithm into code

//Create variables to hold the pixels while you // transfer the color over

Create the target picture objectCreate the source picture object


Convert the Algorithm to Code

• Loop through the source picture pixels


CopyOver Method - Continued

What goes here? Where do you get the target pixel from?

This code goes inside the nested loops so every pixel in the source file gets copied over into the target file.

Be sure your source file is smaller than your target file!

Get the source and target pixelsSet the target pixel color to the source pixel color


Copying Pixels to a New Picture

• What if we want to copy the target to a different location in the source than 0,0– Say startX and startY

• What is an algorithm that will do this?– Use the same for loops

going through all pixels in the source picture but add startX and startY to the target x,y position

1 2

3 4

1 2

3 4

0

1

0

1

0 1 2 3

0

1

2

3

source

target


What if you want to eliminate the background of the source picture?

• Blue backgrounds were made to eliminate

• Before changing the color of the target pixel, ask if the color of the source picture is blue

• Only change the target picture if the source is NOT blue.

Color src = sourcePixel.getColor( ):

if(!src.equals(Color.blue))

/// change the color of the target pixel

to src which is the color of the source pixel

NOTE: you must import the Color class to use Colors

import java.awt.Color;


Copy to Position Exercise• First add a ladybug to

position 0,0 in the garden without any changes.

• Next: • Add code to eliminate any

blue in the source picture

• Last:• Add ladybugs to 3

different locations in the garden by with 3 different x and y values eliminating the blue background.

Use these pictures: ladybug.gif and garden.jpg

nestedloops-mod7-part31 two-dimensional arrays and nested loops – part 3 bugs in the garden...

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