1. Barbara Ericson Georgia Tech Sept 2005
Loops - Review
Barbara Ericson
Georgia Tech
Sept 2005
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2. Georgia Institute of Technology
Purpose of Loops
Repeat execution of a single statement or block of code
Often with at least one variable changing each time through the loop
If I ask you to print out “I love Java!” 100 times
Would you want to type 100 statements of
System.out.println(“I love Java!”);
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Types of Loops
For each loop (1.5 or 5.0)
Do body of loop for each element in a collection
An Array, List, or Set
While loop
Loop while a boolean test is true
Declare and initialize variables before the test
Change loop variable in the body of the loop
For loop
The same as the while loop but with one place to
Declare and initialize variables
Specify the boolean test
Change variables
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For Each Loop Syntax
for (type name : arrayName) {
// do these statements for each element }
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Decrease Red – For Each
public void decreaseRed() {
Pixel[] pixelArray = this.getPixels();
int value = 0;
// loop through all the pixels
for (Pixel pixelObj : pixelArray)
// get the red value
value = pixelObj.getRed();
// decrease the red value
value = (int) (value * 0.5);
// set the red value
pixelObj.setRed(value); }
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While Loop Syntax
while (test) {
// statements to repeat while test is true }
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While Loop Example
// declare variables
int count = 0;
// check if should do body
while (count < 100) {
System.out.println(“I love Java”);
count = count + 1; // change variables }
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Decrease Red - While
public void decreaseRed() {
Pixel[] pixelArray = this.getPixels();
Pixel pixel = null;
int value = 0;
int index = 0;
// loop through all the pixels
while(index < pixelArray.length)
// get the current pixel
pixel = pixelArray[index];
// get the value
value = pixel.getRed();
// decrease the red value
value = (int) (value * 0.5);
// set the red value
pixel.setRed(value);
// increment the index
index = index + 1; }
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For Loop Syntax
for (declare and init; test; change) {
// statements to execute while test is true }
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Decrease Red - For
public void decreaseRedFor() {
Pixel[] pixelArray = this.getPixels();
Pixel pixel = null;
int value = 0;
// loop through all the pixels
for (int i = 0;
i < pixelArray.length; i++)
// get the current pixel
pixel = pixelArray[i];
// get the value
value = pixel.getRed();
// decrease the red value
value = (int) (value * 0.5);
// set the red value
pixel.setRed(value); }
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Nested Loops
Put one loop inside of the block of another
Useful for walking through 2-dimensional arrays
Outer loop for rows
Inner loop for columns
Useful for keeping track of the row and column values
For use in the loop
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Nested Loop Example
public void lighten() {
Pixel pixel = null;
Color color = null;
// loop through the columns
for (int x = 0; x < getWidth(); x++)
// loop through the rows
for (int y = 0; y < getHeight(); y++)
// get pixel at the x and y location
pixel = getPixel(x,y);
// get the current color
color = pixel.getColor();
// get a lighter color
color = color.brighter();
// set the pixel color
pixel.setColor(color); }
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Mirror Vertical
public void mirrorVertical() {
int width = this.getWidth();
int mirrorPoint = width / 2;
Pixel leftPixel = null;
Pixel rightPixel = null;
// loop through all the rows
for (int y = 0; y < getHeight(); y++)
// loop from 0 to the middle (mirror point)
for (int x = 0; x < mirrorPoint; x++)
leftPixel = getPixel(x, y);
rightPixel = getPixel(width x, y);
rightPixel.setColor(leftPixel.getColor()); }
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Ways to Teach This
Ask students to repeat some action for a set number of times
Jump up and down
Clap
How do we know if they did it right?
Walk through what is happening with a nested loop
Each student do the next time through the loop
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