Simple Control Structures booleans the if statement and the while loop What are control structures Our programs so far consist of just a list of commands to be done in order The program cannot choose whether or not to perform a command The program cannot perform the same command more than once Such programs are extremely limited Control structures allow a program to base its behavior on the values of variables For C and C programmers only Statement types are almost identical to those in C and C Main difference true false conditions must be boolean not numeric Some unusual uses of the comma in for statements are not permitted in Java There is one new statement type try which we won t talk about today boolean boolean is one of the eight primitive types booleans are used to make yes no decisions All control structures use booleans There are exactly two boolean values true yes and false no boolean true and false are all lowercase booleans are named after George Boole the founder of Boolean logic Declaring boolean variables boolean variables are declared like any other kind of variable boolean hungry boolean passingGrade boolean taskCompleted false boolean values can be assigned to boolean variables taskCompleted true Numeric comparisons The following numeric comparisons each give a boolean result x x x x x x y y y y y y is x less than y is x less than or equal to y is x equal to y do not use is x unequal to y is x greater than or equal to y is x greater than y Don t use or for floating point numbers The if statement The if statement has the form if boolean expression statement Examples if passingGrade System out println Whew if x largest largest x if citBook price 40 00 citBook purchase The if statement controls one other statement Often this isn t enough we want to control a group of statements Compound statements We can use braces to group together several statements into one compound statement statement statement statement Braces can group any number of statements OK this is an empty statement x 0 OK braces don t hurt temp x x y y temp typical use The compound statement is the only kind of statement that does not end with a semicolon The if statement again The if statement controls one other statement but it can be a compound statement Example if cost amountInPocket System out println Spending cost amountInPocket amountInPocket cost Flowchart for the if statement condition fals e true statement The if else statement The if else statement chooses which of two statements to execute The if else statement has the form if condition statement to execute if true else statement to execute if false Either statement or both may be a compound statement Notice the semicolon after each statement Example if else statements if x 0 absX x else absX x if itemCost bankBalance writeCheck itemCost bankBalance bankBalance itemCost else callHome askForMoreMoney 2 itemCost Flowchart for the if else statement true statement 1 condition fals e statement 2 Aside the mod operator The modulo or mod operator returns the remainder of an integer division The symbol for this operation is Examples 57 10 gives 7 20 6 gives 2 x is divisible by y if x y 0 Nesting if or if then statements A year is a leap year if it is divisible by 4 but not by 100 unless it is also divisible by 400 if year 4 0 if year 100 0 if year 400 0 leapYear true else leapYear false else leapYear true else leapYear false Operations on booleans Assume p and q are booleans There are four basic operations on booleans Negation not p is true if p is false and false otherwise Conjunction and p q is true if both p and q are true Disjunction or p q is true if either of p and q is true Exclusive or xor p q is true if just one of p and q is true Simpler tests A simpler leap year test if year 4 0 year 100 0 year 400 0 leapYear true else leapYear false An even simpler leap year test leapYear year 4 0 year 100 0 year 400 0 The while loop This is the form of the while loop while condition statement If the condition is true the statement is executed then the whole thing is done again The statement is executed repeatedly until the condition becomes false If the condition starts out false the statement is never executed at all Flowchart for the while loop condition fals e true statement Countdown example seconds 5 while seconds 0 System out print seconds seconds seconds 1 System out println Blast off Result 5 4 3 2 1 Blast off The End
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