Control Statements. if for while

Transcription

1 Control Structures

2 Control Statements if for while

3 Control Statements if for while

4 This This is is called called the the initialization initialization statement statement and and is is performed performed before before the the loop loop starts. starts. This This is is called called the the step step or or increment increment and and is is performed performed at at the the end end of of each each loop loop iteration. iteration. for (int i = 0; i < N; i++) { This This is is called called the the loop loop condition condition or or termination termination condition. condition. The The loop loop will will check check whether whether this this statement statement is is true true before before each each iteration iteration of of the the loop. loop.

5 for (int i = 0; i < 3; i++) { println("baby"); println("whoa");

6 for (int i = 0; i < 3; i++) { println("baby"); println("whoa");

7 for (int i = 0; i < 3; i++) { println("baby"); println("whoa");

8 for (int i = 0; i < 3; i++) { println("baby"); println("whoa"); int i 0

9 for (int i = 0; i < 3; i++) { println("baby"); println("whoa"); int i 0

10 for (int i = 0; i < 3; i++) { println("baby"); println("whoa"); int i 0

11 for (int i = 0; i < 3; i++) { println("baby"); println("whoa"); int i 0 Baby

12 for (int i = 0; i < 3; i++) { println("baby"); println("whoa"); int i 0 Baby

13 for (int i = 0; i < 3; i++) { println("baby"); println("whoa"); int i 1 Baby

14 for (int i = 0; i < 3; i++) { println("baby"); println("whoa"); int i 1 Baby

15 for (int i = 0; i < 3; i++) { println("baby"); println("whoa"); int i 1 Baby

16 for (int i = 0; i < 3; i++) { println("baby"); println("whoa"); int i 1 Baby Baby

17 for (int i = 0; i < 3; i++) { println("baby"); println("whoa"); int i 1 Baby Baby

18 for (int i = 0; i < 3; i++) { println("baby"); println("whoa"); int i 2 Baby Baby

19 for (int i = 0; i < 3; i++) { println("baby"); println("whoa"); int i 2 Baby Baby

20 for (int i = 0; i < 3; i++) { println("baby"); println("whoa"); int i 2 Baby Baby

21 for (int i = 0; i < 3; i++) { println("baby"); println("whoa"); int i 2 Baby Baby Baby

22 for (int i = 0; i < 3; i++) { println("baby"); println("whoa"); int i 2 Baby Baby Baby

23 for (int i = 0; i < 3; i++) { println("baby"); println("whoa"); int i 3 Baby Baby Baby

24 for (int i = 0; i < 3; i++) { println("baby"); println("whoa"); int i 3 Baby Baby Baby

25 for (int i = 0; i < 3; i++) { println("baby"); println("whoa"); int i 3 Baby Baby Baby

26 for (int i = 0; i < 3; i++) { println("baby"); println("whoa"); int i 3 Baby Baby Baby Whoa

27 for (int i = 0; i < 3; i++) { println("baby"); println("whoa"); int i 3 Baby Baby Baby Whoa

28 for (int i = 5; i > 0; i--) { println(i + "..."); println("lift-off!"); Lift-off!

29 An Actual Rocket Launch (Start at 9:25)

30 T-31 Seconds: Handoff has occurred T-17 Seconds: Firing chain armed T-10 Seconds: Hydrogen burn T-0 Seconds: Liftoff! for (int i = 40; i > 0; i--) { println("t-" + i + "..."); println("lift-off!");

31 public void run() { /* Do the launch countdown! */ for (int i = 40; i > 0; i--) { println("t-" + i + " seconds."); /* Specific mission commands. */ if (i == 31) { println("handoff has occurred."); else if (i == 17) { println("firing chain is armed."); else if (i == 10) { println("hydrogen burn."); println("lift-off!");

32 public void run() { /* Do the launch countdown! */ for (int i = 40; i > 0; i--) { println("t-" + i + " seconds."); /* Specific mission commands. */ if (i == 31) { println("handoff has occurred."); else if (i == 17) { println("firing chain is armed."); else if (i == 10) { println("hydrogen burn."); println("lift-off!");

33 public void run() { /* Do the launch countdown! */ for (int i = 40; i > 0; i--) { println("t-" + i + " seconds."); /* Specific mission commands. */ if (i == 31) { println("handoff has occurred."); else if (i == 17) { println("firing chain is armed."); else if (i == 10) { println("hydrogen burn."); println("lift-off!");

34 Magic Numbers A magic number is a number written in a piece of code whose meaning cannot easily be deduced from context. double weight = 9.8 * (mass 14.3); Magic numbers are a Bad Thing; they make code harder to read.

35 Constants Not all variables actually vary. A constant is a name for a value that never changes. Syntax (defined outside of any method): private static final type name = value; By convention, constants are named in UPPER_CASE_WITH_UNDERSCORES to differentiate them from variables.

36 General Rules for Constants You can usually use 0 and 1 in loops without needing constants. When computing averages, it's fine to just use the number 2. Anything more complex than this should probably be made into a constant.

37 Time-Out for Announcements!

38 Friday Four Square! Today at 4:15PM, Outside Gates

39 Announcements Programming Assignment #1 due right now. Due on next Wednesday if using a late day. LaIR closed on Sunday but open on Monday. due on Sunday night. Programming Assignment #2 out today, due Friday, January 31 at 3:15PM. Play around with Java statements and control structures! Make some pretty pictures! Explore your creative potential!

40 Casual Dinner for Women in CS Next Wednesday, January 22 at 6:00PM on the fifth floor of the Gates building. Good food, great company, and everyone is invited! RSVP through link (sent out Tuesday).

41 Control Statements for if while

42 Control Statements for if while

43 The while Loop while (condition) { statements Checks condition before each iteration and executes statements if true. Does not check condition in the middle of the loop.

44 Example: int x = 15; while (x > 1) { x /= 2; println(x); while loop Based on slides by Mehran Sahami

45 Example: int x = 15; while (x > 1) { x /= 2; println(x); while loop Based on slides by Mehran Sahami

46 while loop Example: int x = 15; while (x > 1) { x /= 2; println(x); 15 int x Based on slides by Mehran Sahami

47 while loop Example: int x = 15; while (x > 1) { x /= 2; println(x); 15 int x Based on slides by Mehran Sahami

48 while loop Example: int x = 15; while (x > 1) { x /= 2; println(x); 15 int x Based on slides by Mehran Sahami

49 while loop Example: int x = 15; while (x > 1) { x /= 2; println(x); 7 int x Based on slides by Mehran Sahami

50 while loop Example: int x = 15; while (x > 1) { x /= 2; println(x); 7 int x Based on slides by Mehran Sahami

51 while loop Example: int x = 15; while (x > 1) { x /= 2; println(x); 7 int x 7 Based on slides by Mehran Sahami

52 while loop Example: int x = 15; while (x > 1) { x /= 2; println(x); 7 int x 7 Based on slides by Mehran Sahami

53 while loop Example: int x = 15; while (x > 1) { x /= 2; println(x); 7 int x 7 Based on slides by Mehran Sahami

54 while loop Example: int x = 15; while (x > 1) { x /= 2; println(x); 3 int x 7 Based on slides by Mehran Sahami

55 while loop Example: int x = 15; while (x > 1) { x /= 2; println(x); 3 int x 7 Based on slides by Mehran Sahami

56 while loop Example: int x = 15; while (x > 1) { x /= 2; println(x); 3 int x 7 3 Based on slides by Mehran Sahami

57 while loop Example: int x = 15; while (x > 1) { x /= 2; println(x); 3 int x 7 3 Based on slides by Mehran Sahami

58 while loop Example: int x = 15; while (x > 1) { x /= 2; println(x); 3 int x 7 3 Based on slides by Mehran Sahami

59 while loop Example: int x = 15; while (x > 1) { x /= 2; println(x); 1 int x 7 3 Based on slides by Mehran Sahami

60 while loop Example: int x = 15; while (x > 1) { x /= 2; println(x); 1 int x 7 3 Based on slides by Mehran Sahami

61 while loop Example: int x = 15; while (x > 1) { x /= 2; println(x); 1 int x Based on slides by Mehran Sahami

62 while loop Example: int x = 15; while (x > 1) { x /= 2; println(x); 1 int x Based on slides by Mehran Sahami

63 while loop Example: int x = 15; while (x > 1) { x /= 2; println(x); 1 int x Based on slides by Mehran Sahami

64 Greatest Common Divisors Given two integers a and b, the greatest common divisor (or gcd) of a and b is the largest number that divides a and b. Examples: The gcd of 12 and 8 is 4. The gcd of 100 and 10 is 10. The gcd of 137 and 42 is 1.

65 Euclid's Algorithm 45 35

66 Euclid's Algorithm 45 35

67 Euclid's Algorithm 10 35

68 Euclid's Algorithm 10 35

69 Euclid's Algorithm 10 5

70 Euclid's Algorithm 10 5

71 Euclid's Algorithm 45 35

72 Euclid's Algorithm To compute the gcd of a and b: If b = 0, the gcd is a. Otherwise: Divide a by b and obtain the remainder r. Set a equal to b and b equal to r. Repeat. This procedure was known to the Greeks as anthyphairesis; it's almost always referred to now as Euclid's algorithm. It is one of the oldest algorithms still in use today.