Chapter 5 Control Structures

Transcription

1 Chapter 5 Control Structures I. Scott MacKenzie 1 Outline 2

2 Flow of Control 3 if Statement 4

3 if Statement Example IO.print("Is is raining? (1=yes, 2=no)... "); int response = IO.readInt(); SE.require(response >= 1); SE.require(response <= 2); if (response == 1) IO.println("Don't forget your umbrella"); IO.println("Have a nice day."); Example09.java 5 if-else Statement 6

4 if-else Statement Example IO.println("What day is it?"); IO.println("1=sun 2=mon 3=tue 4=wed 5=thu 6=fri 7=sat"); IO.print("Enter... "); int response = IO.readInt(); SE.require(response >= 1); SE.require(response <= 7); if (response == 1 response == 7) IO.println("Aren't weekends great?"); else IO.println("Enjoy your day at school or work!"); Example10.java 7 boolean Operators boolean operators take boolean operands and produce a boolean result Java includes three: Operator Name Result OR true if either operand is true, false otherwise && AND true if both operands are true, false otherwise! NOT true if the operand is false, false otherwise 8

5 boolean Operator Example int a = 1; int b = 2; int c = 3; IO.println(a > b && b > c); IO.println(!(c > a)); IO.println(a == 3 b == 3 c == 3); Example11.java 9 De Morgan s Theorem!(a && b) =!a!b!(a b) =!a &&!b 10

6 Lazy Evaluation As soon as the outcome of a boolean expression is certain, the VM (virtual machine) stops evaluating and produces the result This is called lazy evaluation Consider... boolean a = true; boolean b = true; boolean c = a b; In evaluating the expression a b above, the VM never tests b, because true or anything must be true 11 Lazy Evaluation Example Consider... int a = IO.readInt(); int b = IO.readInt(); if (a / b > 100)... If b is zero, a divide-by-zero runtime error occurs This is prevented using... if (b!= 0 && a / b > 100)... If b is zero, this is never evaluated. There is no need to because the result of the entire expression must be false. 12

7 Lazy Evaluation Example (2) Consider... boolean b =... int a = 1; if (b a++ < 0) IO.println( true, a = + a); else IO.println( false, a = + a); If b is true the output is... true, a = 1 If b is false, the output is false, a = 2 13 Multi-way Branch 14

8 Flow Problem 15 Pitfalls: Using == to Test Real Numbers The following produces not equal for most inputs IO.print( Enter a real ); double x = IO.readDouble(); double y = Math.pow(Math.pow(x, 0.5), 2); if (x == y) IO.println( equal ); else IO.println( not equal ); Continued 16

9 Instead, compare the absolute value of x y to EPSILON a value considered infinitesimally small in the application s context final double EPSILON = ; IO.print( Enter a real... ); double x = IO.readDouble(); double y = Math.pow(Math.pow(x, 0.5), 2); if (Math.abs(x y) < EPSILON) IO.println( equal ); else IO.println( not equal ); 17 Pitfalls: Using == to Test Objects If the arguments are objects, == tests the object references, not the objects themselves This is probably not what is intended Use the equals method instead (but make sure the references are not null) Stock s1 = new Stock( ); Stock s2 = new Stock( ); if (s1.equals(s2)) IO.println( equal ); 18

10 Pitfalls: Using = to assign a boolean Reflects poor style; e.g.,... boolean valid; If (x > a && y <= b) valid = true; else valid = false; should be coded as... boolean valid = (x > a && y <= b); 19 Pitfalls: Using Math Notations Might make sense in algebra, but not in Java: if ((0 < x < 1) (x && y) > 1)... should be coded as if ((x > 0 && x < 1) (x > 1 && y> 1))... 20

11 Iteration 21 Flow Diagram - for Statement 22

12 for Statement Example // output the squares of the numbers 1 through 10 for (int i = 1; i <= 10; ++i) int square = i * i; IO.print(square); IO.print(" "); Example12.java 23 for Statement Example (2) // Output the exponents for all powers of 2 less // than a million. (see text, p. 191) final int MILLION = ; for (int expo = 0; Math.pow(2, expo) < MILLION; ++expo) IO.print(expo + " "); IO.println(); Note: Only one statement, therefore braces are not required. Example12a.java 24

13 for Statement Example (3) // Output the exponent of the greatest power of 2 // smaller than a million. (see text, p. 191) final int MILLION = ; int expo = 0; for (; Math.pow(2, expo) < MILLION; ++expo) ; IO.println(expo - 1); Note: Empty statement. Example12b.java 25 for Statement Example (4) // Read integers from the user and output some stats on them. // Keep reading until a sentinel (-1) is inputted. (see p. 192) final int SENTINEL = -1; int count = 0; int sum = 0; IO.println("Enter integers (1 per line)... "); int num = IO.readInt(); int max = num; for (; num!= SENTINEL; num = IO.readInt()) ++count; sum += num; if (num > max) max = num; double average = (double)sum / count; IO.println("count: " + count); IO.println("sum: " + sum); IO.println("average: " + average); IO.println("max: " + max); Example12c.java 26

14 for Statement Example (5) // As before, except reading the integers from a file and using the // end of file (eof) marker as the sentinel. (text, p. 191) UniReader ur = new UniReader("data.txt"); final int SENTINEL = -1; int count = 0; int sum = 0; See API IO.println("Reading integers from 'data.txt'... "); int num = ur.readint(); int max = num; for (;!ur.eof(); num = ur.readint()) ++count; sum += num; if (num > max) max = num; double average = (double)sum / count; IO.println("count: " + count); IO.println("sum: " + sum); IO.println("average: " + average); IO.println("max: " + max); Example12d.java 27 Variable Scope There is a subtle but important difference between the following two code segments... int i; for (i = 0; i < 10; ++i) IO.println("Hello"); IO.println(i);... for (int i = 0; i < 10; ++i) IO.println("Hello"); IO.println(i);... OK! Generates a compile error because the variable i does not exist outside the scope of the for statement. 28

15 Nested Loops Note: These braces are optional 29 Nested Loop Example for (int i = 1; i <= 10; ++i) for (int j = 1; j <= 10; ++j) int tableentry = i * j; IO.print(tableEntry, "4.0"); IO.println(); Example13.java Example14.java 30

16 Other Constructs do while switch? : continue break 31 Syntax... do Statement do body is executed at least once! body while (condition); The semicolon following the while condition in a do statement is required. 32

17 do Statement Example int x; do IO.print("Enter a positive integer (or -1 to quit)... "); x = IO.readInt(); if (x > 0) IO.print("You entered "); IO.println(x); while (x!= -1); IO.println("Thanks!"); Example15.java 33 Syntax... while Statement while (condition) body 34

18 while Statement Example IO.println("Squares of numbers 1 through "); int i = 1; while (i <= 10) int square = i * i; IO.print(square); IO.print(" "); ++i; Note: Can be re-worked into a for statement. Example16.java 35 Syntax... switch Statement switch (switch-expression) case a: body for switch-expression = a case b: body for switch-expression = b case c: body for switch-expression = c default: body for other condtions 36

19 switch Statement Example IO.print("Enter your grade (A, B, C, D, or F)... "); char grade = IO.readChar(); switch (grade) case 'A' : IO.println( A: Excellent work"); break; case 'B' : IO.println( B: Above average. Congratulations."); break;... Example17.java 37? : Construct if (condition) x = value-1; else x = value-2; is the same as... x = condition? value-1 : value-2; 38

20 continue Statement Provides an immediate transfer of control out of the current iteration of a loop and into the next iteration 39 continue Statement Example IO.println("Elevator going up to the 25th floor... "); for (int i = 1; i < 25; ++i) // No 13th floor in this building! if (i == 13) continue; // kill some time (cheap and dirty way) for (int j = 1; j < 4; ++j) for (int k = 0; k < ; ++k) ; IO.print("."); IO.print(" "); IO.print(" Floor "); IO.println(i); IO.println("Here we are!"); Example18.java 40

21 break Statement Provides an immediate transfer of control out of the enclosing loop scope Whenever you feel temped to do this, rethink your for, while, or do loop design Do not use it Note: The statements above are verbatim from the course text. I use the break statement all the time. 41 break Statement Example int x; do IO.print("Enter a positive integer (or -1 to quit)... "); x = IO.readInt(); if (x == -1) break; IO.print("You entered "); IO.println(x); while (true); IO.println("Thanks!"); Example19.java 42

22 Thank You 43