Chapter 4 Control Structures

Transcription

1 Chapter 4 Control Structures Foundational Java Key Elements and Practical Programming 1

2 if - else switch Control Structures break and continue Ternary operator while and do-while loops for loops 2

3 Two forms if if-else Making Selections The if is always followed by parentheses the expression in the parentheses must evaluate to a boolean (true or false) if (booleanexpression) 3

4 Using Braces If only one line of code is conditional, braces are not required if (booleanexpression) statement; For more than one line, braces are essential if (booleanexpression) statement(s); Safer to always use braces 4

5 if else Statements if(condition) // do this else // do this instead The else part is optional If the condition is false and there is no else part then no code in the conditional statement will be executed 5

6 Nested Statements Use braces for each block if(condition) // statements else if(condition) // statements else // statements 6

7 Comparison Operators Used to create Boolean expressions (return true or false) The relational operators: < less than > greater than <= less than or equal to >= greater than or equal to Can only be used with numeric primitives The equality operators: == equal!= not equal can be used with all data types a common error in Java is to confuse the = and == operators 7

8 Examples Condition Operator Example equal to == if(temperature == 100) not equal to!= if(grade!= 'F') less than < if(sales < target) less than or equal to <= if(enginesize <= 2000) greater than > if(hoursworked > 40) greater than or equal to >= if(age >= 18) 8

9 Boolean Operators Boolean Operator Java Operator Meaning AND && Return true if both sides of the expression are true OR Return true if at least one side of the expression is true NOT! Return true if the expression is false Example if(age > 4 && age < 16) if(timeelapsed > 60 stopped == true) if(!drawingchanged) // assumes drawingchanged // is a boolean variable & AND operator (full evaluation) OR operator (full evaluation) 9

10 Using Selection: the CoinExample Class if statement based on using a randomly generated number to represent a coin being flipped The Java Math class random operation Returns a random double >= 0.0 and < 1.0 double randomnumber = Math.random(); Using a random number in an if statement if (randomnumber < 0.5) System.out.println("The coin shows heads"); else System.out.println("The coin shows tails"); 10

11 Exercise 4.1 Create a class with a main method In main, generate a random integer between 1 and 10 (inclusive) Using an if statement, work out if the random number is odd or even Write a message to the console that displays both the random integer and whether it is an odd or an even number 11

12 Selection and Test Code A test compares the result you expect with the result you actually get if(dievalue >= 1 && dievalue <= 6) System.out.println("Valid die value: " + dievalue); else System.out.println("Error. Expected value between 1 and 6 but was " + dievalue); Tests written in code can be run repeatedly as regression tests 12

13 switch Statement When multiple selections are based on a single (whole number) variable Switch variable and case literals must be byte, char, short or int (can also be Strings since Java 7) switch(variable) case literalvalue1 : // some code here break; case literalvalue2 : // some code here break; // etc. for as many cases as need to be handled default: // default code to handle cases not already dealt with 13

14 Using break (or not) break sends control to the end of the switch statement without evaluating any other cases Often (but not always) what you need Don t use break if you want logic shared by multiple cases switch (achar) case a : case A : // my code here break; case b // etc 14

15 Dice Man Example Switches on the state of a die No default needed in this example switch (dievalue) case 1: System.out.println("forget the whole affair"); break; case 2: System.out.println("wait until the party on Saturday"); break; case 3: System.out.println("do what Arlene says"); break; case 4: System.out.println("have a platonic relationship"); break; case 5: System.out.println("follow your emotions"); break; case 6: System.out.println("go to Arlene s apartment tonight"); break; 15

16 break and continue The use of break in a switch statement is not the only context in which this keyword can be used moves execution to the end of the current block (i.e. the next closing brace) continue has a similar function takes control back to the beginning of the current code block rather than the end Useful for searching and sorting large arrays act rather like structured goto statements 16

17 Exercise 4.2 In a main method, generate a random integer between 1 and 13 to represent the possible values in a suit of playing cards Use a switch statement that applies cases to your random integer if the random number is 1, print out Ace to the console if the random number is 11, print out Jack to the console if the random number is 12, print out Queen to the console if the random number is 13, print out King to the console The default case should simply print the random integer to the console 17

18 Ternary Operator Shorthand if - else statement expression? op1 : op2 If expression is true, return op1, otherwise return op2 String result = x % 2 == 0? "even" : "odd"; Used when intent is to focus on returned result, rather than a branch in program flow 18

19 Iteration Iteration can be achieved in three slightly different ways: while loops do...while loops for loops In each case, there will be a condition that allows the loop to terminate Which one to use depends on a number of factors and we often find that more than one will meet our requirements we have to be aware of their differences in order to use them correctly 19

20 while and do while These loops continue while the condition is true while tests a precondition (zero or more iterations) do while tests a post condition (one or more iterations) do // statements while(condition); while(condition) // statements Note the semicolon! 20

21 do while example This do while loop simulates the throwing of a die until it shows a six int dievalue = 0; // loop until throwing the Die gets a six do // generate a random number in the range 0.0 to 1.0 double randomnumber = Math.random(); // to get a number in the range 1 to 6, we need to multiply the // random number by 6 and add 1 randomnumber *= 6; randomnumber++; // to convert this value into an integer we cast it dievalue = (int)randomnumber; System.out.println("You have thrown a " + dievalue); while (dievalue!= 6); 21

22 for Loops Statement has three sections initialise - executed once before loop begins condition - executed before each loop If true then loop body executes update - executed at the end of each loop for(initialisation; 'while' condition; action) // some code here 22

23 Initialisation and Update Any or all of the three sections can be left empty This is an endless loop for(;;)... Only use a for loop when you need initialisation and update actions Otherwise code is more complex that it needs to be e.g. endless while loop is simpler while(true) 23

24 Iteration with a for Loop This example uses a for loop to display the printable ASCII characters of the Unicode table // the 'for' loop counts from 33 to 126, the range of the printable ASCII characters for (int i = 33; i < 127; i++) // convert the integer to a 'char' using a cast char character = (char)i; // display the ASCII number along with its character then add a tab System.out.print(i + ": " + character + '\t'); // if there are 9 characters on a row, add a line feed if ((i - 32) % 9 == 0) System.out.println('\n'); 24

25 for Loops and Arrays for loops are frequently used to loop through arrays Use the array s length field to control the loop String[] monthnames = "January","February","March","April","May","June","July","August", "September","October","November","December"; int[] monthlyrainfall = 74,81,86,93,100,116,126,111,93,80,84,91; System.out.println("Average monthly rainfall"); for(int i = 0; i < monthlyrainfall.length; i++) System.out.println(monthNames[i] + ": " + monthlyrainfall[i] + "mm."); Note the use of two arrays one of int and the other of String 25

26 The Array Passed to main The parameter we always use with main is an array of Strings, which may or may not be empty The size of this array depends entirely on how many Strings are passed to it at run time public static void main(string[] args) for (int i = 0; i < args.length; i++) System.out.println(args[i]); 26

27 Passing Strings to main From the command line java com.introjava.chapter4.mainarrayloopexample hello I am some strings In Eclipse Run -> Run Configuration 27

28 Multiple Initialisations and Actions The initialisation and action sections of the for statement can contain lists of values, separated by commas public class MultiValueForLoopExample public static void main(string[] args) for (int lettercount = 1, uppercount = 65, lowercount = 97; lettercount <= 26; lettercount++, uppercount++, lowercount++) System.out.println("Lower case: " + (char)lowercount + " Upper case: " + (char)uppercount); 28

29 Exercise 4.3 Create a new class with a main method In main, take the following steps: Create and initialise an array containing the integers 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10 Use a for loop to print out the contents of the array Add a while loop that prints out the contents of the array in reverse order Add a do..while loop that iterates forward through the array. Inside the loop, add an if statement so that only even numbered values from the array are printed 29

30 Summary Control structures for selection and iteration Selection syntax if and switch statements Relational and Boolean operators Different kinds of loop while, do while, for Iterating through arrays Math class for random number generation Passing an array of Strings to the main method of a class at run time 30