CMPT 125: Lecture 4 Conditionals and Loops

Transcription

1 CMPT 125: Lecture 4 Conditionals and Loops Tamara Smyth, tamaras@cs.sfu.ca School of Computing Science, Simon Fraser University January 17,

2 Flow of Control The order in which statements are executed is called the flow of control. Unless otherwise specified, the flow of control is linear, that is, statements are executed in the order in which they occur. Java begins a program on the first statement of the main method. A change in the flow of control can be accomplished by: invoking a method conditional statements loops or some combination. CMPT 125: Conditionals and Loops, Lecture 4 2

3 Conditional Statements A conditional statement allow us to choose which statement will be executed next. Java has three conditional statements: 1. if-else 2. else if 3. switch These statements allow us to decide which statement to execute next based on a condition, that is, the outcome of a boolean expression. CMPT 125: Conditionals and Loops, Lecture 4 3

4 Boolean Expressions Recall that a boolean is a data type that evaluates to either true or false. A boolean statement also evaluates to true or false, and may consist solely of a single variable, or a larger statement involving a logical operator. boolean hungry = false; //hungry is a boolean if (hungry) System.out.println( Let s go eat! ); else system.out.println( Let s go for a drink instead. ); The if statement allows a program to choose whether to execute a particular statement. if (num > 20) System.out.println( Count is over 20. ); else system.out.println( Count is less than or equal to 20. ); CMPT 125: Conditionals and Loops, Lecture 4 4

5 Evaluating Boolean Expressions Boolean expressions are evaluated using: equality operators: == test whether two values ARE equal!= test whether two values ARE NOT equal relational operators: > test whether one value is greater than another < test whether one value is less than another >= greater than or equal <= less than or equal logical operators:! logical NOT && logical AND logical OR Logical operators take boolean operands CMPT 125: Conditionals and Loops, Lecture 4 5

6 Logical NOT The logical NOT is unary, and has only two possible values: true or false. boolean hungry = false; if (!hungry) system.out.println( Let s go for a drink instead. ); OUTPUT: Let s go for a drink instead. The! is used to perform the logical NOT, or logical complement, that is, it returns the opposite value. boolean hungry = true; if (!hungry) System.out.println( Let s go for a drink instead. ); else System.out.println( Let s go eat. ); OUTPUT: Let s go eat. CMPT 125: Conditionals and Loops, Lecture 4 6

7 Logical AND, OR Logical AND, OR are binary operators that use two operands, both of which are boolean. The result of a logical AND operation is true if and only if both operands are true, but false otherwise. The result of a logical OR operation is true if one, or both, of the operands are true, but false otherwise. There are four possible combinations of two logical operands a and b: a b a && b a b false false false false false true false true true false false true true true true true CMPT 125: Conditionals and Loops, Lecture 4 7

8 Precedence of Operators Operators in order of precedence: 1. logical NOT (highest) 2. logical AND 3. logical OR if (!done && (count > MAX)) System.out.println( Completed ); To print or not to print. Let s look at the truth table which tells us all the possible combinations of values for the variables involved. done count > MAX!done!done && (count > MAX) false false true false false true true true true false false false true true false false CMPT 125: Conditionals and Loops, Lecture 4 8

9 Example code using if-else, else if public class testnum public static void main(string[] args) int num = Integer.parseInt(args[0]); } } if (num > 5) System.out.println(num + " > 5"); else if (num > 4) System.out.println(num + " > 4"); else if (num > 2) System.out.println(num + " > 2"); else System.out.println(num + " is 2 or less"); CMPT 125: Conditionals and Loops, Lecture 4 9

10 Short Circuiting Operations The AND and OR operators may be short-circuited meaning that is the left operand is sufficient to decide the boolean result, the right operand is not evaluated. You can use this to your advantage by ensuring variables will not result in an expression that is undefined, e.g. preventing a division by zero. if (count!= 0 && total/count > MAX) System.out.println( Testing ); An equivalent way of achieving the same result would involve nesting the if statements: if (count!= 0) if (total/count > MAX) System.out.println( Testing ); CMPT 125: Conditionals and Loops, Lecture 4 10

11 Block Statement An if statement consists of the reserved word if followed by a boolean expression, followed by one or more statements. That is, a single statement can be replaced with a block statement, a collection of statements enclosed in braces. if (guess == answer) System.out.println( You re right! ); else System.out.println( I m sorry. ); System.out.println( The correct answer is + answer); } It is common practice, and very important to the human reader, to indent the statement(s) following the boolean expression to show they are part of the if statement. CMPT 125: Conditionals and Loops, Lecture 4 11

12 A Loop, or Repetitive Statement A loop, or repetitive statement, allows us to execute a programming statement more than once. Like a conditional, a loop is based on the evaluation of a boolean expression Java has three loop statements: 1. while 2. do 3. for CMPT 125: Conditionals and Loops, Lecture 4 12

13 The while Statement A while statement is followed by a boolean expression, and evaluates a statement, or a block of code, repeatedly, while the condition remains true. public static void main(string[] args) int sum = 0, value, count = 0; Scanner scan = new Scanner(System.in); System.out.println("Enter an integer (0 to quit): "); value = scan.nextint(); while(value!= 0) count++; sum += value; System.out.println("The sum so far is " + sum); System.out.println("Enter an integer (0 to quit): "); value = scan.nextint(); } if (count==0) System.out.println("No values were entered."); else DecimalFormat fmt = new DecimalFormat("0.###"); System.out.println("The average is " + fmt.format((double)sum/count)); } } Avoid infinite loops! CMPT 125: Conditionals and Loops, Lecture 4 13

14 The Conditional Operator When using the conditional operator, a boolean condition is followed by the? operator which is followed by two expressions separated by the : operator. boolean condition? 1st expression : 2nd expression Figure 1: The conditional operator. The first of the two expression is evaluated if the boolean condition is true, and if false, the second expression is evaluated. The following is an example use of the conditional operator A ternary operator because it requires three operands. (total > MAX)? total + 1 : total * 2; The Conditional operator returns a value, and therefore might be used as follows: total = (total > MAX)? total + 1 : total * 2; CMPT 125: Conditionals and Loops, Lecture 4 14

15 Comparing Floats Two floating point values are equal, according to the == operator, only if all the binary digits of their underlying representations match. Since this is unlikely to be the case (perhaps due to rounding errors), it is best not to use this operator for comparing floating point values. A better way computes the absolute value of the difference between the two, and then compare the result to some tolerance level. if (Math.abs(f1-f2) < TOLERANCE) System.out.println( Effectively equal. ); CMPT 125: Conditionals and Loops, Lecture 4 15

16 Comparing Characters Equality and relational operators may also be used on character data. Because of the Unicode character set, it is possible to determine if one is less than another. if (ch1 > ch2) System.out.println(ch1 + is greater than + ch2); else System.out.println(ch1 + is NOT greater than + ch2); CMPT 125: Conditionals and Loops, Lecture 4 16

17 Comparing Objects The String class contains an equals method that returns true if two strings have exactly the same characters, and false otherwise. if (name1.equals(name2)) System.out.println( The names are the same. ); else System.out.println( The names are NOT the same. ); It is valid to test two String objects using using the equality operator, but it will test whether they reference the same object: String name1 = new String( string ); String name2 = new String( string ); if (name1 == name2) System.out.println( These variables contain + the same address. ); else System.out.println( These variables contain + different addresses. ); OUTPUT: These variables contain different addresses. CMPT 125: Conditionals and Loops, Lecture 4 17

18 The switch Statement The switch statement evaluates an expression to determine a value. It then matches that value with one of several possible cases. switch(grade) case A : System.out.println( Excellent Work! ); break; case B : System.out.println( Good Work. ); break; case C : System.out.println( Satisfactory Work. ); break; default: System.out.println( Error in identifying your grade! ); } If no case matches the evaluated expression, execution continues with the default case. When a break statement is encountered, the program exits the switch statement, and continues with the next statement. CMPT 125: Conditionals and Loops, Lecture 4 18

19 Nested Loops The body of a loop can contain another loop. public class Palindrome public static void main(string[] args) String str, another = "y"; int left, right; Scanner scan = new Scanner(System.in); while(another.equalsignorecase("y")) System.out.println("Enter a palindrome"); str = scan.nextline(); left = 0; right = str.length() - 1; while(str.charat(left) == str.charat(right) && left < right) left++; right--; } System.out.println(); if (left<right) System.out.println("The string is NOT a palindrome."); else System.out.println("The string IS a palindrome."); System.out.print("Test another palindrome (y/n)?"); another = scan.nextline(); } } CMPT 125: Conditionals and Loops, Lecture 4 19

20 } CMPT 125: Conditionals and Loops, Lecture 4 20

21 Iterators An iterator is an object that has methods that allow you to process a collection of items one at a time. Since it is a repetitive process, it is related to loops. An iterator in Java is defined using the Iterator interface. The Scanner class has methods such as hasnext, hasnextint, hasnextdouble, to determine if the next input token is of a particular type. The Scanner class as an iterator is useful when processing a data file or processing parts of a character string. A delimiter is used to separate tokens in a Scanner object. filescan.usedelimeter( ; ); CMPT 125: Conditionals and Loops, Lecture 4 21

22 The do statement The do loop executes the statement in the loop until the condition, written at the end of of the loop, becomes false. int count = 0; do count++; System.out.println(count); } while (count < 5); A do statement executes its loop body at least once. This may give you reason to choose a do, rather than a while loop. CMPT 125: Conditionals and Loops, Lecture 4 22

23 The for statement The do and while loops are usually used when you don t know exactly how many times you want to execute the loop body. The for loop is suited to situations where you know precisely how many times the loop should be executed. for (int count = 0; count < 10; count++) System.out.println(count); The header of a for loop contains three parts separated by semicolons: 1. initialization: executed before the loop begins. 2. boolean condition: evaluated before the loop body; if true the loop is executed. 3. increment: executed after each loop iteration. It is also possible to decrement a value: for (int count = 10; count > 0; count--) System.out.println(count); CMPT 125: Conditionals and Loops, Lecture 4 23

24 Iterators and for loops A variation of the for loop lets us process the items in an iterator with a different syntax. Consider the iterator Booklist that manages Book objects: for(book mybook : BookList) System.out.println(myBook); is equivalent to Book mybook; while (BookList.hasNext()) mybook = BookList.next(); System.out.println(myBook); } This version can also be used on arrays. CMPT 125: Conditionals and Loops, Lecture 4 24