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Lecture 4 – PHP Flow Control

SFDV3011 – Advanced Web Development

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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 instruction2. Selection - a choice of at least two

either go to the next instructionor 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 codeor continue with the next instruction after the block

Selection and Repetition are called Branching since these are branch points in the flow of control

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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');$is_driving_age = ($age >= 17);$not_graduating = ($year != 'senior');

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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.

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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

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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 true1

echo false<no output>

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"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 true0.0 === 0 returns false

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Compound Boolean Expressions

Use && or and to AND (intersect) two or more conditions

Use || to OR (union) two or more conditions

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• Use a single & for AND and a single | for OR to avoid short-

circuit evaluation and force complete evaluation of an expression

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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)

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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

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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• switchBranching: Repetition• while• do-while• for• foreach

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PHP if statementSyntax:

if (Boolean_Expression) Action if true; //execute if truenext action; //always executed

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.";

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)

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PHP Statement Blocks:Compound Statements

Action if(true) is a set of statements enclosed in curly brackets (a compound statement, or block).

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."); This style is useful when using PHP in large blocks of

HTMLif ($eggsPerBasket < 12) ://begin body of the if statement echo "Less than a dozen ..."; $costPerBasket = 1.1 * $costPerBasket;//end body of the if statementendif;$totalEggs = $numberOfEggs * $eggsPerBasket;echo "You have a total of $totalEggs eggs.");

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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

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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;}

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Multibranch selection:if-else if-elseif-…-else

One way to handle situations with more than two possibilities

Syntax:if(Boolean_Expression_1) Action_1elseif(Boolean_Expression_2) Action_2 . . .elseif(Boolean_Expression_n) Action_nelse Default_Action

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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?)

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Use switch for single-variable

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;

}

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Iteration

Three different forms of iteration:• while• for• do-while

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While loops

Syntax: while (cond) statement

What it means: Test cond.

If false, then while loop is finished; go on to next statementIf true, execute statement, then go back and start again

Continue like this until cond becomes false or a break is executed.

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While loop Examples

$x = 5;$s = 0;while ($x > 0) { $s = $s + $x; $x = $x-1;}

$s = 0; $x = 1;while ($x <= 10) { $s = $s + $x; $x = $x + 1;}

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Infinite and Empty Loops while (true); is an infinite loop and you will have to waita long time!

while (false){//stuff} and while ($n++ < 10); {//stuff} are empty loops and will never execute anything

You should take care to ensure your loop is good by

(1) Checking that the initial condition is true(2) Checking that the loop condition will eventually change

to false(3) Do NOT EVER use ; after while()(4) Make sure you initialize loop variables BEFORE the loop

Generally PHP has a maximum execution time to help avoid crashing the system (default 60 secs).

You can set this to a lower value such as 5 sec by using set_time_limit (5);

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While loop examples

while (true)echo "I will do the reading assignments";

while (false) echo "I will start my HW early";

Output?

Output?

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The exit Function If you have a situation where it is pointless to continue

execution you can terminate the program with the exit() or die() functions

A string is often used as an argument to identify if the program ended normally or abnormally

$x = 5;while ($x++>0) {// terminate program at 3 if($x == 3) die(“done!”); echo “$x<br>”;}echo “I will not execute”;

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break and continue If you have a situation where need to exit the loop but not

terminate the program use break

If want to stop executing the statements in the loop for a given cycle and immediately jump to the top of the loop to start the next cycle use continue

$x = 5;while ($x++>0) {// stop early at 3 if($x == 3) break; echo “$x<br>”;}echo “I will always execute”;

$x = 5;while ($x++>0) {// skip 3 if($x == 3) continue; echo “$x<br>”;}echo “I will always execute”;

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for loops

Convenient syntax for loops with loop variables that are incremented or decremented each time through the loop (as in previous examples).

Recall:

for (init; cond; incr) statement

init;while (cond) { statement incr;}

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for loop examples$s = 0; $x = 5;while ($x > 0) { $s = $s + $x; $x = $x-1;}

$s = 0; for ($x = 5; $x > 0; $x = $x-1) $s = $s + $x;

$s = 0; for ($x = 1; $x <= 10; $x = $x+1) $s = $s + $x;

$s = 0; $x = 1;while ($x <= 10) { $s = $s + $x; $x = $x + 1;}

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for loop usage

for ($i = 1; $i <= n; $i++) statement

One use of for loops is simply to execute a statement, or sequence of statements, a fixed number of times:

executes statement exactly n times.

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for loop usage

for ($i = 1; $i <= 100; $i++) echo "I will not turn HW in late");

Example: print "I will not turn HW in late" a hundred times.

for ($i = 0; $i < 20; $i++) { if ($i%2 == 1)

echo "My robot loves me\n"; else echo "My robot loves me not\n";}

Loop bodies can be as complicated as you like. This loop prints:

"My robot loves me" and "My robot loves me not" on alternate lines, ten times each:

for ($i = 1; $i <= 20; $i++) { echo "\nMy robot loves me"; if ($i%2 == 0) echo " not"; }

OR

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Practical ConsiderationsWhen Using Loops

The most common loop errors are unintended infinite loops and off-by-one errors in counting loops

An off-by-one error is an error that occurs when a loop is executed one too many or one too few times.

Sooner or later everyone writes an unintentional infinite loop (To get out of an unintended infinite loop enter ^C (control-C) or stop button on browser)

Loops should tested thoroughly, especially at the boundaries of the loop test, to check for off-by-one and other possible errors