Flow Control I

Transcript

1. ITM 352 Flow-Control: if and switch
2. ITM 352 - © Port, Kazman Flow-Control - 2 What is "Flow of Control"? • Flow of Control is the execution order of instructions in a program • All programs can be written with three control flow elements: 1. Sequence - just go to the next instruction 2. Selection - a choice of at least two either go to the next instruction or jump to some other instruction 3. Repetition - a loop (repeat a block of code) at the end of the loop either go back and repeat the block of code or continue with the next instruction after the block • Selection and Repetition are called Branching since these are branch points in the flow of control
3. ITM 352 - © Port, Kazman Flow-Control - 3 The Type boolean • A primitive type • Can have expressions, values, constants, and variables just as with any other primitive type • Only two values: true and false • Every expression is boolean in some way • 0, 0.0, '0', '' are all false • Anything other than the above is true • Comparison operators always return boolean $is_desired_grade = ($grade == 'A'); $is_drinking_age = ($age >= 21); $not_graduating = ($year != 'senior');
4. ITM 352 - © Port, Kazman Flow-Control - 4 Boolean Expressions • Boolean expressions can be thought of as test conditions (questions) that are either true or false • Often two values (numbers, strings) are compared, return value is a boolean (i.e. true or false) • For example: Is A greater than B?, Is A equal to B?, Is A less than or equal to B? • Comparison operators are used for boolean expressions (<, >, <=, >=, ==, ===, !=. !==, …) • A and B can be any data type (or class), but they generally are a "compatible" data type (or class) • Comparisons are either numeric or lexicographic but can be user-defined via objects and functions. • Comparing non-compatible types is legal but may have unexpected results.
5. ITM 352 - © Port, Kazman Flow-Control - 5 Basic PHP Comparison Operators Math Notation Name PHP Notation PHP Examples = equal to == $balance == 0 $answer == 'y' ≠ not equal to != $income != tax $answer != 'y' > greater than > $income > $outgo ≥ greater than or equal to >= $points >= 60 < less than < $pressure < $max ≤ less than or equal to <= $income <= $outgo
6. ITM 352 - © Port, Kazman Flow-Control - 6 A Note on Printing Boolean Values • The echo (and print) command will convert values to strings for printing • true is converted to '1' • false is converted to “” (the empty string) • echo true 1 • echo false <no output>
7. ITM 352 - © Port, Kazman Flow-Control - 7 "Identical" Comparison Operators • Sometimes you really want to be sure two values are exactly the same value and type • Is 0.0 equal to 0? • Use the '===' and '!==' to test equality and non-equality for both value and type • 0.0 == 0 returns true • 0.0 === 0 returns false Do lab exercise #1
8. ITM 352 - © Port, Kazman Flow-Control - 8 Compound Boolean Expressions • Use && or and to AND (intersect) two or more conditions • Use || to OR (union) two or more conditions • See text for definitions of AND and OR • For example, write a test to see if B is either 0 or between the values of A and C : (B == 0) || (A <= B && B < C) (B == 0) or (A <= B) and B < C) • In this example the parentheses are not required but are added for clarity • Subject to Precedence rules • Note the short-circuit, or lazy, evaluation rules (later in slides) • Use a single & for AND and a single | for OR to avoid short- circuit evaluation and force complete evaluation of an expression
9. ITM 352 - © Port, Kazman Flow-Control - 9 Truth Tables for boolean Operators Value of A Value of B A && B true true true true false false false true false false false false Value of A Value of B A || B true true true true false true false true true false false false Value of A !A true false false true && (and) || (or) ! (not)
10. ITM 352 - © Port, Kazman Flow-Control - 10 Precedence Rules for Common Operators Highest Precedence • the unary operators: ++, --, and ! • the binary arithmetic operators: *, /, % • the binary arithmetic operators: +, - • the boolean operators: <, >, =<, >= • the boolean operators: ==, != • the boolean operator & • the boolean operator | • the boolean operator && • the boolean operator || Lowest Precedence Do lab exercise #2
11. ITM 352 - © Port, Kazman Flow-Control - 11 PHP Flow Control Statements Sequential • the default • PHP automatically executes the next instruction unless you use a branching statement Branching: Selection • if • if-else • if-else if-else if- … - else • switch Branching: Repetition • while • do-while • for • foreach
12. ITM 352 - © Port, Kazman Flow-Control - 12 PHP if statement • Simple decisions • Do the next statement if test is true or skip it if false • Syntax: if (Boolean_Expression) Action if true; //execute if true next action; //always executed • Note the indentation for readability (not compiler or execution correctness)
13. ITM 352 - © Port, Kazman Flow-Control - 13 if Example • The body of the if statement is conditionally executed • Statements after the body of the if statement always execute (not conditional unless grouped inside {}'s) if ($eggsPerBasket < 12) //begin body of the if statement echo "Less than a dozen eggs per basket"; //end body of the if statement $totalEggs = $numberOfEggs * $eggsPerBasket; echo "You have a total of $totalEggs eggs.";
14. ITM 352 - © Port, Kazman Flow-Control - 14 PHP Statement Blocks: Compound Statements • Action if(true) can be either a single statement or a set of statements enclosed in curly brackets (a compound statement, or block). For example: if ($eggsPerBasket < 12) { //begin body of the if statement echo "Less than a dozen ..."; $costPerBasket = 1.1 * $costPerBasket; } //end body of the if statement $totalEggs = $numberOfEggs * $eggsPerBasket; echo "You have a total of $totalEggs eggs."); All statements between braces are controlled by if
15. ITM 352 - © Port, Kazman Flow-Control - 15 PHP Statement Blocks: Compound Statements Alternatively, "if(<exp>): … endif; " also works: if ($eggsPerBasket < 12) : //begin body of the if statement echo "Less than a dozen ..."; $costPerBasket = 1.1 * $costPerBasket; //end body of the if statement endif; $totalEggs = $numberOfEggs * $eggsPerBasket; echo "You have a total of $totalEggs eggs."); All statements between : and endif; are controlled by if This style is useful when using PHP in large blocks of HTML (see example in textbook) Do lab exercise #3
16. ITM 352 - © Port, Kazman Flow-Control - 16 Two-way Selection: if-else • Select either one of two options • Either do Action1 or Action2, depending on test value • Syntax: if (Boolean_Expression) { Action1 //execute only if Boolean_Expression true } else { Action2 //execute only if Boolean_Expression false } Action3 //anything here always executed
17. ITM 352 - © Port, Kazman Flow-Control - 17 if-else Examples • Example with single-statement blocks: if ($time < $limit) echo "You made it."; else echo "You missed the deadline."; • Example with compound statements: if ($time < $limit) { echo "You made it."; $bonus = 100; } else { echo "You missed the deadline."; $bonus = 0; } Do lab exercise #4
18. ITM 352 - © Port, Kazman Flow-Control - 18 Multibranch selection: if-else if-elseif-…-else • One way to handle situations with more than two possibilities • Syntax: if(Boolean_Expression_1) Action_1 elseif(Boolean_Expression_2) Action_2 . . . elseif(Boolean_Expression_n) Action_n else Default_Action
19. ITM 352 - © Port, Kazman Flow-Control - 19 if-elseif-elseif-…-else Example if($score >= 90 && $score <= 100) $grade= 'A'; elseif ($score >= 80) $grade= 'B'; elseif ($score >= 70) $grade= 'C'; elseif ($score >= 60) $grade= 'D'; else $grade= 'E'; • Note how the sequence is important here and must use elseif rather than just if (why?)
20. ITM 352 - © Port, Kazman Flow-Control - 20 if-elseif-elseif-…-else Non-Example?? if ($profRel == "colleague" ) $greeting = "Thomas"; elseif ($profRel == "friend" ) $greeting = "Tom"; elseif ($profRel == "grad student" ) $greeting = "TC"; elseif ($profRel == "undergrad student" ) $greeting = "professor"; else $greeting = "Dr. Collins";
21. ITM 352 - © Port, Kazman Flow-Control - 21 Use switch for single-variable if switch($profRel) { case "colleague" : $greeting = "Thomas"; break; case "friend" : $greeting = "Tom"; break; case "grad student" : $greeting = "TC"; break; case "undergrad student" : $greeting = "professor"; break; default : $greeting = "Dr. Collins"; break; } Do lab exercise #5