Lecture 5 Relational and Equivalence Operators Relational SYS-1S22 / MTH-1A66 Operator Meaning < Less than > Greater than Logical Operators <= Less than or equal to and switch >= Greater than or equal to Equivalence Stuart Gibson Operator Meaning [email protected] == equal to != not equal to S01.09A • A variable may be tested against a constant 1 or another variable. 2 Examples of Simple Logical Expressions Evaluation of logical expressions int a=10, b=20, c=5; final int b=21; evaluates to evaluates to final char no = ‘n’; if( a < b ) 10 < 20 true { int a = 20; evaluates to evaluates to if ( a > c ) 10 > 5 true int c = 20; { char choice = ‘y’; System.out.println(“Second if”); } Expression Value System.out.println(“End of first if”); } a < b true else a == c true { c > b false System.out.println(“In else”); } choice == no false System.out.println(“After”); c != a false (Incorrect in Lecture 4 L) Output • In the last lecture we saw how these logical expressions are Second if Flow of program evaluated at run time as either true or false. End of first if execution. 3 After 4 1 Evaluation of logical expressions Evaluation of logical expressions int a=10, b=20, c=25; int a=25, b=20, c=25; evaluates to evaluates to evaluates to evaluates to if( a < b ) 10 < 20 true if( a < b ) 25 < 20 false { { evaluates to evaluates to if ( a > c ) 10 > 25 false if ( a > c ) Does NOT get { { evaluated System.out.println(“Second if”); System.out.println(“Second if”); as the first if } } statement System.out.println(“End of first if”); System.out.println(“End of first if”); evaluates to } } false. else else { { System.out.println(“In else”); System.out.println(“In else”); } } System.out.println(“After”); System.out.println(“After”); Output Output Flow of program Flow of program End of first if In else execution. execution. After After 5 6 Logical Operators The AND Operator ( && ) • Example: • The logical operators AND, OR and NOT can be used in Java to form more complex logical expressions by if ( x >= 0 && x <= 10) combining simple logical expressions. System.out.print(“x in range 0 to 10”); • An AND expression evaluates as true only if BOTH && AND Two Arguments arguments are true. || OR Two Arguments • Truth Table: ! NOT One Argument Argument 1 Argument 2 Argument 1 && Argument 2 true true true true false false false true false false false false 7 8 2 The AND Operator ( && ) - Examples The OR Operator ( || ) int x = 10, y = 11, z = -3; • Example: if (x > z && x > y) if ( x < 0 || x > 100) è(10 > -3 && 10 > 11) System.out.print(“x is invalid”); è(true && false) è false • An OR expression evaluates as true if EITHER arguments are evaluated as true. if (z < x && y > x) è(-3 < 10 && 11 > 10) • Truth Table: è(true && true) Argument 1 Argument 2 è true Argument 1 Argument 2 || true true true if (z > x && x > y) true false true è(-3 > 10 && 10 > 11) false true true è(false && false) è false false false false 9 10 The OR Operator ( || ) - Examples Evaluation of && and || Operator int x = 10, y = 11, z = -3; • The computer is quite clever, so when evaluating && and || operators in some cases it only evaluates the first argument. if( x > z || x > y ) For example: è(10 > -3 || 10 > 11) è(true || false) int a = 0; è true if (a > 0 && b / a > 1) if( z < x || y > x ) • In an AND (&&) statement, if the first argument is false, then è(-3 < 10 || 11 > 10) whatever the second argument is, doesn’t matter! This is è(true || true) because according to the truth table, any combination starting è true with false evaluates to false. if( z > x || x > y ) • In an OR (||) statement, if the first argument evaluates to è(-3 > 10 || 10 > 11) true, then whatever the second argument is, doesn’t matter! è(false || false) This is because and combination starting with true, evaluates è false to true. 11 12 3 The NOT Operator ( ! ) The NOT Operator ( ! ) - Examples • Example: int x = 10, y = 11, z = -3; if ( ! ( x == y ) ) { if( !( x > z )) System.out.print(“x is not equal to y”); è!(10 > -3) System.out.print(“easier to use x!=y in”); è!(true) System.out.print(“ this case.”); è false } • A NOT expression is true when its argument is false. if( !( z > y )) è!(-3 > 11) • Truth Table: è!(false) Argument ! Argument è true true false false true 13 14 Compound Logical Expressions Compound Logical Expressions • We can build more complex expressions containing more int x = 1, y = 2, z =3; than one logical operator. ( x == 1 || y == 2 ) && z == 4 int x = 1, y = 2, z =3; x == 1 || y == 2 && z == 4 • Now the expression in the parentheses is evaluated first: • Unless parentheses (brackets) are used AND is always ( x == 1 || y == 2 ) && z == 4 evaluated before OR so: ( true || true ) && z == 4 true && z == 4 x == 1 || y == 2 && z == 4 true && false x == 1 || true && false false x == 1 || false true || false • However the expression to evaluate can be even more true complicated such as: if ( !( x > y && y < z) && z > 0 || (y>=12)) But we could rewrite it using parentheses as: 15 16 4 Compound Logical Expressions Precedence Table if ( !( x > y && y < z) && z > 0 || (y>=12)) Operator Description Highest • Q: How does the computer know what to evaluate first ? ! + - Logical not, unary plus and minus A: Precedence ! * / % Multiplication, division and mod • Brackets have the highest precedence. Always solve the Addition and subtraction deepest nested set first (L:N where N is the level of nesting): + - < <= >= > Relational operators if ((x >5 && (y < z || y > 3)) && (a > 3)) == != Equivalence operators L:3 L:3 && Logical AND L:2 L:2 L:2 L:2 L:1 L:1 || Logical OR Assignment • So, solve level 3 (deepest nested) first, then level2, note ties = Lowest on the same level are solved left to right and finally level 1. 17 18 More Logical Expressions Character Comparisons • You can compare characters as well: • Logical expression can involve arithmetic operators: Expression Value if ((x = y + 5) > 10 && (z = z + y) == 4)) System.out.print(“Some condition met”); ‘A’ < ‘B’ true ‘A’ < ‘a’ true • But it is much better to write: ‘?’ > ‘@’ false x = y + 5; • Q: Where does those values come from ? z = z + y; A: The computer converts the characters to numbers and then compares those numbers. if (x > 10 && z == 4) System.out.print(“Some condition met”) • The numbers come from the ASCII / UNICODE values: A = 65, B = 66, a = 97, ? = 63, @ = 64. 19 20 5 Control Structures Selection: switch • In the last lecture we saw that there were three main types of • switch statements can be used to chose between one of Control Structures: several alternatives. • Sequence • The alternatives are dependant on the value of a selector • Selection • Repetition • The selector must be an expression that evaluates to an • Selection integer (int), character (char), or boolean. So far we have seen the following Java control sequences: • Note that with boolean selector you can only have two choices, true or false, like a two alternative, if else One Alternative: if statement statement. Two Alternatives: if else statement More than two Alternatives: nested if else statements 21 22 switch statement switch statement switch( selector ) • switch statements provide an alternative syntax to multiple { if statements. Consider the following nested if: case label1: statements1; break; char staffType; … … … if( staffType == ‘A’) case label2: System.out.print(“Academic Staff”); statements2; break; else if( staffType == ‘U’ ) . System.out.print(“Undergraduate Student”); . else if( staffType == ‘P’ ) System.out.print(“Postgraduate Student”); else if( staffType == ‘T’ ) case labeln: System.out.print(“Technical Staff”); statementsn; break; else System.out.print(“Unknown Staff Type”); default: statementsd; } 23 24 6 switch statement Execution of a switch statement char staffType; • The selector expression is evaluated and compared to each … … … case label in turn, for example: switch (staffType) { case ‘A’: staffType = ‘P’; System.out.print(“Academic Staff”); break; case ‘A’: à staffType == ‘A’ à ‘P’ == ‘A’ à false case ‘U’: case ‘U’: à staffType == ‘U’ à ‘P’ == ‘U’ à false System.out.print(“Undergraduate Student”); case ‘P’: à staffType == ‘P’ à ‘P’ == ‘P’ à true break; case ‘P’: System.out.print(“Postgraduate Student”); • If the selector is equal to the condition of one of the case break; labels, then the statements associated to this case are case ‘T’: executed, so in our example the output would be: System.out.print(“Technical Staff”); break; default : Postgraduate Student System.out.print(“Unknown Staff Type”); } 25 26 switch – the use of break switch – with break • All statements after the matched label are executed until char staffType; Output case staffType = ‘P’; Postgraduate Student a break statement is reached. switch (staffType) After Switch { • The purpose of the break statement is to break out of the case ‘A’: false System.out.print(“Academic Staff”); current control structure, in this case the switch statement. break; case ‘U’: false • In our example where the program System.out.print(“Undergraduate Student”); staffType = ‘P’; break; executes the one line of code associated with the case ‘P’: case ‘P’: and then breaks out of the switch. Program execution true System.out.print(“Postgraduate Student”); continues at the next statement after the last curly brace of the break; case ‘T’: switch statement. At the break System.out.print(“Technical Staff”); execution break; jumps to after default : the switch System.out.print(“Unknown Staff Type”); } System.out.println(“After Switch”); 27 Flow of execution 28 7 switch – missing break switch – default