Tue Nov 11 & Thu Nov 13 : complete Ch 5 Thu Nov 13/Fri Nov 14 : lab exercises Ch 5. Tue Nov 18 : TERM TEST #3 (Ch.4,5)

Transcription

1 CSE1710 Click to edit Master Week text 10, styles Lecture 18 Second level Third level Fourth level Fifth level Fall 2013! Tuesday, Nov 11, Overview Tue Nov 11 & Thu Nov 13 : complete Ch 5 Thu Nov 13/Fri Nov 14 : lab exercises Ch 5 Tue Nov 18 : TERM TEST #3 (Ch.4,5) Thu Nov 20/Fri Nov 21 : Labtest #3 (Ch 4,5) 3 lectures allotted for Ch 6 2 lab sessions allotted for Ch 6 2 1

2 Forms of Iteration the "enhanced" for loop the "boolean" for loop 3 RasterImage services how to create instances? lab exercises demonstrated several different ways default constructor constructor with 2 int parameters constructor with single string parameter (switch display over to Eclipse, to demonstrate the different constructors) 4 2

3 Iterating over the pixels in a RasterImage instance (3 methods) 1. the RasterImage object itself is a collection this means we can use it in an enhanced for loop 2. use the service for accessing the collection of pixels, namely getpixelarray() this provides us with an array data structure; we can iterate over the elements in two ways using an enhanced for loop access the elements by index; iterate over all the indices 5 Iterating over the pixels in a RasterImage instance (3 methods) 3. use the service for accessing a particular pixel, by column and row indices, namely getpixel(int, int) we can access each pixel by column and row position iterate over all columns and row indices using two boolean for loops, one nested inside the other 6 3

4 The Enhanced For Loop!! a collection is simply a bunch of elements, possibly in a particular order, but not necessarily"! the elements must have a type (e.g., Pixel)"! there are certain sub categories for collections"! a set is a collection in which duplicates are not permitted"! a list is a collection in which the elements are ordered"! an array is a specific kind of list " collection, set, list ": "abstractions, not specific to Java" array " " ": "a Java programming element " 7 The Enhanced For Loop!! for ( Type-of-Element e : Identifier-of-Collection ) // here is the body of the loop This is called an enhanced for loop 8 8" 4

5 The Enhanced For Loop! It just so happens that a RasterImage object can function as a collection of Pixel elements for (Pixel p : myimage) // here is the body of the loop 9 9" The Boolean For Loop! for (; boolean expression ;) // here is the body of the loop 10 10" 5

6 The Boolean For Loop! for ( initial ; boolean expression ; bottom ) // here is the body of the loop 11 11" The Boolean For Loop What do we need in order to use the boolean for loop?" let's start with the array version" RasterImage myimage = Pixel[] thepixels = myimage.getpixelarray(); now we need a way to visit each pixel in the array once and only once 12 6

7 Some arbitrary element in an array RasterImage myimage = Pixel[] pixels = myimage.getpixelarray(); Pixel p = thepixels[60]; this retrieves the 61 tst element in the array! What happens it there is no 61 st element?! 13 The first and final indices for an array RasterImage myimage = Pixel[] pixels = myimage.getpixelarray(); int firstpixelindex = 0; int lastpixelindex = thepixels.length-1; length is a special final attribute of arrays in Java! WHY minus 1?! 14 7

8 The first and final elements of an array RasterImage myimage = Pixel[] pixels = myimage.getpixelarray(); int firstpixelindex = 0; int lastpixelindex = thepixels.length-1; Pixel pfirst = thepixels[0]; // the first element Pixel plast = thepixels[lastpixelindex]; // the final //element 15 java.lang.arrayindexoutofboundsexception Pixel[] thepixels = mypict.getpixelarray(); int firstpixelindex = 0; int lastpixelindex = thepixels.length-1; Pixel p1 = thepixels[firstpixelindex -1]; // runtime error Pixel p2 = thepixels[lastpixelindex+1]; // runtime error 16 8

9 Exercise #1 change the colour of each pixel to green RasterImage myimage = Pixel[] pixels = myimage.getpixelarray(); int firstpixelindex = 0; int lastpixelindex = thepixels.length-1; thepixels[firstpixelindex].setcolor(color.green); // thepixels[lastpixelindex].setcolor(color.green); 17 Exercise #1 (partial solution) thepixels[firstpixelindex].setcolor(color.green); thepixels[firstpixelindex+1].setcolor(color.green); thepixels[firstpixelindex+2].setcolor(color.green); thepixels[firstpixelindex+3].setcolor(color.green); // keep going until firstpixelindex+i=lastpixelindex so how could we automate this? 18 9

10 Exercise #1 (solution) Pixel[] thepixels = mypict.getpixels(); int firstpixelindex = 0; int lastpixelindex = thepixels.length-1; int currentindex = firstpixelindex; boolean iswithinbounds = currentindex <= lastpixelindex; for( ; iswithinbounds ; ) Pixel currentpixel = thepixels[currentindex]; currentpixel.setcolor(color.red); mypict.repaint(); currentindex++ iswithinbounds = currentindex <= lastpixelindex; 19 Exercise #1 (solution) Pixel[] thepixels = mypict.getpixels(); int firstpixelindex = 0; int lastpixelindex = thepixels.length-1; int currentindex = firstpixelindex; for( ; currentindex <= lastpixelindex; ) Pixel currentpixel = thepixels[currentindex]; currentpixel.setcolor(color.red); mypict.repaint(); currentindex

11 Exercise #1 (solution) Pixel[] thepixels = mypict.getpixels(); int firstpixelindex = 0; int lastpixelindex = thepixels.length-1; for(int currentindex = firstpixelindex; currentindex <= lastpixelindex; ) Pixel currentpixel = thepixels[currentindex]; currentpixel.setcolor(color.red); mypict.repaint(); currentindex++ 21 Exercise #1 (solution) Pixel[] thepixels = mypict.getpixels(); int firstpixelindex = 0; int lastpixelindex = thepixels.length-1; for(int currentindex = firstpixelindex; currentindex <= lastpixelindex; currentindex++) Pixel currentpixel = thepixels[currentindex]; currentpixel.setcolor(color.red); mypict.repaint(); 22 11

12 Exercise #2 change the colour of each pixel to green, using getpixel(int,int) RasterImage myimage = int finalcolindex = int finalrowindex = myimage.getpixel(0,0).setcolor(color.green); // myimage.getpixel(finalcolindex,finalrowindex).setcolor(color.green); 23 Exercise #2 (partial solution) Pixel[] thepixels = myimage.getpixels(); int finalcolindex = myimage.getwidth(); int finalrowindex = myimage.getheight(); for(int currentcolindex = 0; this will iterate over all of the currentcolindex <= finalcolindex; columns, and will change the currentcolindex++) first pixel of each column Pixel currentpixel = myimage(currentcolindex, 0); currentpixel.setcolor(color.green); mypict.repaint(); 24 12

13 Exercise #2 (solution) Pixel[] thepixels = myimage.getpixels(); int finalcolindex = myimage.getwidth(); int finalrowindex = myimage.getheight(); for(int currentcolindex = 0; currentcolindex <= finalcolindex; currentcolindex++) for(int currentrowindex = 0; currentrowindex <= finalrowindex; currentrowindex++) Pixel currentpixel = myimage(currentcolindex, currentrowindex); currentpixel.setcolor(color.green); mypict.repaint(); Flow of Control! I t e r a t i o n loop body S B 1 B 2 X 26 13

14 5.2.2 The for statement! Flow: Syntax : false 27 S for X initial condition body bottom condition true Statement -S for (initial; condition; b ottom) body; Statement -X Algorithm: 1. Start the for scope 2. Execute initial 3. If condition is false go to 9 4. Start the body scope 5. Execute the body 6. End the body scope 7. Execute bottom 8. If condition is true go to 4 9. End the for scope Example! final int MAX = 10; final double SQUARE_ROOT = 0.5; for (int i = 0; i < MAX; i = i + 1) double sqrt = Math.pow(i, SQUARE_ROOT); output.print(i); output.print("\t"); // tab output.println(sqrt); 28 14

15 for (initial; condition; bottom)! for (int i = 0; i < MAX; i = i + 1)... int i; for (; i < MAX; i = i + 1) for (initial; condition; bottom)! Can it be omitted?! Can it be set to the literal true?! What if it were false at the beginning?! Is it monitored throughout the body?! 30 15

16 for (initial; condition; bottom)! Can it be any statement?! Will the loop be infinite if it is omitted?! Flow of Control What is control; how does it flow? Sequence versus Selection Flow (a) (b) S 1 S 2 A 1 B 1 C 1 X S n 32 16

17 5.1.2 The if Statement S Statement -S A 1 true false if ( condition ) Statement -A1 A 2 Statement -A2 X Statement -X 33 Example Write a fragment that reads an int and outputs its abs value without using Math.abs

18 Example Write a fragment that reads an int and outputs its abs value without using Math.abs. output.print("enter an integer... "); int entry = input.nextint(); int absvalue = entry; if (entry < 0) absvalue = -entry; output.println(absvalue); 35 Pitfall What is wrong with this? output.print("enter an integer... "); int entry = input.nextint(); int absvalue = entry; if (entry < 0); absvalue = -entry; output.println(absvalue); 36 18

19 The if-else Statement S Statement -S A 1 A 2 true false B 1 B 2 if ( condition ) Statement -A1 Statement -A2... X else Statement -B1 Statement -B2... Statement -X 37 Example Rewrite the abs value fragment using if-else 38 19

20 Example Rewrite the abs value fragment using if-else int absvalue; if (entry < 0) absvalue = -entry; else absvalue = entry; 39 Pitfall What is wrong with this? if (entry < 0) int absvalue = -entry; else int absvalue = entry; 40 20

21 5.1.3 Building the Condition Relational Expression if (k < 0) Boolean Variable boolean b = k < 0 if (b) Boolean Expression if (k < 0 b && m > h) 41 Uses boolean operators: &&,, and! Operator Precedence ! cast * / % + - < <= > >= ==!= && = op= 21

22 Examples Express the condition x [a, b) Express the condition x [a, b) 43 Examples Express the condition x [a, b) if (x >= a && x < b) Express the condition x [a, b) if (!(x >= a && x < b)) Can use demorgan s Law to convert negated conjunctions to disjunctions. if (x < a x >= b) 44 22

23 Example Write a fragment to determine if the objects of two given references x and y are equal. if (x.equals(y)) But what if x were null? 45 Example Write a fragment to determine if the objects of two given references x and y are equal. if (x!= null && x.equals(y)) But wouldn t this crash anyway? Lazy (short-circuit) evaluation of && and

24 5.1.4 Multiway Branching Reduce an n-way branch to n-1 2-way branches A 1 A2 S true X false true false B 1 C1 B 2 C2 Statement -S if (condition -1) Statement -A1 Statement -A2... else if (condition -2) Statement -B1 Statement -B2... else Statement -C1 Statement -C2... Statement -X 47 Example Write a fragment to determine the full letter grade given the mark (out of 100) in a course

25 Example char grade; if (mark >= 80) grade = 'A'; else if (mark >= 70) grade = 'B'; else if (mark >= 60) grade = 'C'; else if (mark >= 50) grade = 'D'; else grade = 'F'; 49 25