CS 177 Week 5 Recitation Slides. Loops

Transcription

1 CS 177 Week 5 Recitation Slides Loops 1

2 Announcements Project 1 due tonight at 9PM because of evac. Project 2 is posted and is due on Oct. 8th 9pm Exam 1 Wednesday Sept 30 6:30 7:20 PM Please see class web page under Course Syllabus week 6 to determine your exam location!! Exam will be 25 questions for 1 hour Mentor Program moved to Tuesday 5:45 7:45 PM instead of Wednesday due to exam Wednesday consultation hours cancelled (exam) Last semester exam posted 2

3 QUESTIONS??? 3

4 While Loops Uses repetition to perform a lot of work very fast They look a lot like if statements: int x = 0; if(x < 5) /*do work using statements*/ When here, finished int x = 0; while(x < 5) /*do work Need to assure a statement is in place to cause the expression to eventually evaluate to false using statements*/ x += 1; When here, loop back up And test again 4

5 While Braces are optional for a single statement while (i < 10) sum += i++; As with ifs, only when the expression is true will the statements within the while block be executed. The big difference is that when the last statement of a while is executed (if the expression was first true), the expression will be re-evaluated. MUST BE CAREFUL that the condition will depend on a value that changes as the loop progresses or a break will be needed (to be discussed). 5

6 Small contrived example boolean value = true; //program statements while (value) System.out.println( If here, value is true! ); value = false; System.out.println( value can change ) System.out.println( But ONLY matters at the end ); value = true; With value set to true in the end it will loop forever 6

7 boolean value = true; //program statements int count = 0; while (value) System.out.println( If here, value is true! ); value = false; // lots of work value = true; count++; if (count > 10) value = false; This causes the loop to stop After how many iterations??? i.e. how many times will If here, value is true! print? 7

8 Keep multiplying input numbers until a sentinel reached int i = 1; int product = 1; while( i!= 0 ) product *= i; Why is i initialized to 1? So that the while expression will evaluate to true to first enter the loop but also once inside the loop, multiplying by 1 does not affect the final value (as opposed to i = 2). System.out.print( Enter number: ); i = StdIn.readInt(); System.out.println( Product: + product); What is the sentinel here? 8

9 From lecture: int i = 0; double sum = 0; int count = 0; Getting the sentinel value counted as1 input, so, just calculate with (count-1) to calculate with just the actual values while( i >= 0 ) sum += i; count++; System.out.print( Enter number: ); i = StdIn.readInt(); count--; //fixes extra count for sentinel System.out.println( Average: +(sum/(count-1))); + (sum/count)); 9

10 Infinite Loops int i = 0; int sum = 0; while(i <= 10) sum += i; i++; This loop will run forever! WHY??? How do we fix this? This run-time error is a little harder to make when using for loops 10

11 for loops for(init; condition; inc) //statements init; //statement while(condition) //statements inc; //statement Without increment statements, loops can run forever (unless there is a break to-be-discussed) but with for loops, you add it right at the beginning. 11

12 Side by Side int i = 0; int sum = 0; while(i <= 10) sum += i; i++; int sum = 0; for(int i = 0; i <= 10; i++) sum += i; 12

13 for loops cont. int sum = 0; for(int i = 0; i <= 10; i++) sum += i; init performed once only at beginning of loop. condition is checked at the beginning and before each next loop iteration inc is performed at the end of every loop iteration just before condition is rechecked 13

14 Another Painful Error for Both int i = 0; while and for Loops int sum = 0; while(i <= 10); sum += i; i++; int sum = 0; for(int i = 0; i <= 10; i++); sum += i; Semicolons placed here will give you a not-so-expected result!! What will happen in the two cases and why??? Will be different for each. For loop will have a compilation error since i is declared inside the loop and i is used outside of loop 14

15 What about int sum = 0; int i; for(i = 0; i <= 10; i++); sum += i; What will sum and i be here (no error this time )? 15

16 When do we use whiles and when do we use fors? The answer is not EXACTLY clear cut, BUT there is a general use for each that seems natural: for loops are great for count controlled loops; i.e. loops that have a count with them. Loops from 1 to 100 for instance while loops are great for event controlled loops; i.e. loops that keep iterating until some event has occurred Loop until program gets correct input for instance 16

17 From lecture The following code will tell you if a number isn t prime int number = StdIn.readInt(); for( int i = 2; i < number; i++) if( number % i == 0 ) System.out.println(number + is not prime. ); What s the problem? 17

18 cont. By adding a boolean, we can keep track of whether or not the number is prime int number = StdIn.readInt(); boolean prime = true; for( int i = 2; i < number ; && i++) prime; i++) Some might think this if( number % i == 0 ) would work? (and in a prime break; = false; way it does ) BUT What about when the loop is finished and we want to know LATER if the number is actually prime without doing the loop again?? 18 18

19 do-while Work just like while loops, only execute statements once before ever checking the condition do //statements while(condition); Note here the semicolon, because it is at the end NOT at the beginning, it denotes the end of the statement. 19

20 Get a password/combination String combinationfordruidea; //anyone name the movie? do System.out.println( Please enter password: ); combinationfordruidea = StdIn.readString(); while(!(combinationfordruidea.equals( 1234 ) ); // Amazing, now I m going to have to change the //combination on my luggage! If you know the loop will have to execute the statements AT LEAST once, then you can use a do-while loop. 20

21 Side by Side for product int product = 1; System.out.print(prompt); int x = StdIn.getInt() while(x!= 0) String prompt = Please enter a number to multiply to the product Enter 0 to quit\n ); product *= x; System.out.print(prompt); x = StdIn.getInt(); int product = 1; int x; do System.out.print(prompt); x = StdIn.getInt(); product *= x; while(x!= 0) 21

22 import lejos.nxt.*; public class BasicRobotTest public static void main(string[] args) TouchSensor tsensor = new TouchSensor(SensorPort.S1); int x = 3; int x = 3; LCD.drawInt(x, 0, 0); 0, 0); Sound.beep(); pause(1000); pause(1000); LCD.drawString("Touch me to", 0, 0); LCD.drawString("Touch me to", 0, 0); LCD.drawString("shut me up!", 0, 1); me up!", 0, 1); while (!tsensor.ispressed()) while (!tsensor.ispressed()) Sound.buzz(); Motor.A.setSpeed(720); Motor.C.setSpeed(720); Motor.A.forward(); Motor.C.forward(); pause(1000); Motor.A.stop(); Motor.C.stop(); Motor.C.stop(); Motor.A.setSpeed(720); Motor.C.setSpeed(720); Motor.A.forward(); Motor.C.forward(); Next week s Lego Lab Starter Displays a 3 and beeps for a second Displays Touch me to shut me up! Loop to buzz forever UNTIL the sensor is pressed Sets the speed of the motors Tells the motors to move forward at that speed for a second then to stop. 22

23 Last Bit on Loop Errors BE CAREFUL!! When using loops, you have to follow the syntax and the logic you want EXACTLY or a lot can go wrong. If you have a loop and the program just seems to sit there, you may have an infinite loop. Infinite Loops can be caused for various reasons In cases, your value can be off by 1 if you re not careful. Sometimes, a condition can look right but is actually wrong, a loop may never execute And the misplaced semicolon can make life miserable! 23

24 Infinite Loops int x = 0; do System.out.println(x); while(x < 10) How would we fix either? for(int x = 0; x < 10; x++) x-- System.out.println(x); /*code that make you feel you ve accomplished something */ x--; //even more code What about? 24

25 Off by 1 int ranthrough = 0; for(int i = 1; i < 40; i++) System.out.println(i); ranthrough++; System.out.println( The loop ran through + ranthrough + times ); What will be the printed value of i on the LAST print statement? ranthrough is 39! What will print here? 25

26 Problems? for(int i = 1; i >= 40; i++) System.out.println(i); This will never print anything. WHY? Looks right but a logic problem with expression int i; for(int i = 1; i <= 40; i++); for(i = 1; i <= 40; i++); System.out.println(i); System.out.println(i); //gives compilation error! What is the problem? //i used outside loop What will actually print and //where how it many was times? declared. 26

27 Final QUESTIONS??? 27