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ITM 352 Expressions, Precedence, Working with Strings

Learn about PHP file structure, statements, expressions, operators, and precedence rules. Understand how PHP processes code and returns results. Dive into PHP programming basics.

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ITM 352 Expressions, Precedence, Working with Strings

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  1. ITM 352 Expressions, Precedence, Working with Strings

  2. Review: What is a PHP File? • PHP files have extension ".php” • A PHP file can contain text, CSS, HTML, JavaScript and, of course, PHP code. • PHP code is executed on the server, and the result is returned to the browser as plain text. If that text has HTML, CSS, or JavaScript it will be interpreted as configured by the browser (client). • PHP code will not be present in the “source” on the browser because it was already executed on the server. That that is, only the output from executing the PHP file on the server is given to the browser. • Let’s see this for ourselves (again). Try this in a junk.php file then “view page source” after accessing it through a browser: Did you know the sqrt(2) is about <?php print sqrt(2) ?> Did you see any PHP code? Can you see what was executed and outputted versus what was just “passed through” the PHP interpreter?

  3. Review: What is a PHP Program? • A PHP program starts with <?php • and ends with ?> • In between is a series of statements in the PHP language. This is a PHP block • You can have as many PHP block’s as you want. • Anything else, before or after, is NOT interpreted by PHP and is passed though unprocessed as plain text.

  4. Review: What is a PHP Statement? • A PHP programs normally contain comments and code (statements) • Comments: • start with // or # (single line) • or begin with /* and end with */ (multiple lines) • Statements MUST end with a ; • The most common error is forgetting the ; • Let’s take a look at the most common types of statements…

  5. Expressions • Expressions are PHP statements that return values and use operators • the value produced by an expression when evaluated is "returned", i.e. it is the statement's "return value." $numberOfBaskets = 5; // assignment returns 5 $eggsPerBasket = 8; $totalEggs = $numberOfBaskets * $eggsPerBasket; • In the first two lines when the assignment operation is evaluated the expression returns the value 5 for the first line and 8 for the second • $numberOfBaskets * $eggsPerBasket is an expression that returns the integer value 40 • $check = ($totalEggs = $numberOfBaskets * $eggsPerBasket) / 40; • Similarly, functions return values (more on this later…) How many Expressions does this statement have??

  6. Operators • Operators take operands and perform some operation • There are unary, binary, and tertiary operators that use 1, 2, and 3 operands respectively. Examples: • Unary: $endOfFile++, !is_string() • Binary: 2 * 3, $a % 3, $a = 40, $a > 3, $a == 5, $a || $b • Usually arithmetic, but can be other operations • See next slide for example…

  7. Simple Expressions • PHP will interpret operations between different data types according to certain casting rules and precedence • $whatIsThis = 1.2 + " Fred" . 0777 * true; • Our goal is to understand why the value of the above expression is 1.2511 (after the next bunch of slides you will!). • The same variable can appear on both sides of the assignment operator • $a = $a + 1; // add 1 to value in $a and set in $a • This is unlike algebra and actually very useful! Can you see why this is useful?

  8. Operators and Precedence (Partial) Precedence

  9. Operator Precedence and Parentheses • Expressions follow certain rules that determine the order an expression is evaluated • e.g. should 2 + 3 * 4 be 5*4 or 2+12? • PHP operators have precedence similar to algebra to deal with this • Operators with higher preference are evaluated first • Ex. In the expression 2 + 3*4, the * operator has higher precedence than + so 3*4 is evaluated first to return the value 12, then the expression 2 + 12 is evaluated to return 14 • Use parentheses '( )' to force evaluation for lower precedence • Ex. (2 + 3)*4 • Do not clutter expressions with parentheses when the precedence is correct and obvious!

  10. Precedence An example of precedence rules to illustrate which operators in the following expression should be evaluated first: $score < $min/2 – 10 || $score > 90 • Division operator has highest precedence of all operators used here so we show it as if it were parenthesized: $score < ($min/2) – 10 || $score > 90 • Subtraction operator has next highest precedence : $score < (($min/2) – 10) || $score > 90 • The < and > operators have equal precedence and are evaluated left-to-right: ($score < (($min/2) – 10)) || ($score > 90) This is a fully parenthesized expression that is equivalent to the original. It shows the order in which the operators in the original will be evaluated.

  11. Operator Associativity • Some operators have the same precedence order. • Ex. Is 4 / 2 *3 first evaluated 4 / 6 or 2 * 3 ? • In such cases, the associativity determines the order of evaluation • The / and * operators are left-associative and so the operations are performed from left to right • 4 / 2 is first, returning 2, then 2 * 3 returning 6

  12. Examples of Arithmetic Precedence Do Lab Exercise #1

  13. Casting: Changing the Data Type of the Returned Value • Casting is the process of converting from one data type to another data type. This can be useful. • Casting only changes the type of the returnedvalue (the single instance where the cast is done), not the type of the variable • For example: $n = 5.0; $x = (int) $n; • Since $n is a float, (int) $n converts the value to an int, now $x is an integer type.

  14. Implicit Type Casting • Casting is done implicitly (automatically) when a "lower" type is assigned to a "higher" type, depending on the operator • The data type hierarchy (from lowest to highest):bool  int double string array object • For example: $x = 0; $n = 5 + 10.45; $x = $n; • the value 5 is cast to a double, then assigned to $n • $x now is a double • This is called implicit casting because it is done automatically.

  15. Data Types in an Expression: More Implicit Casting • Some expressions have a mix of data types • All values are automatically elevated (implicitly cast) to the highest level before the calculation • For example: $n = 2; $x = 5.1; $y = 1.33; $a = (n * x)/y; • $n is automatically cast to type double before performing the multiplication and division

  16. Casting: Changing the Data Type of the Returned Value • Casting is the process of converting from one data type to another data type. This is sometimes useful. • Casting only changes the type of the returnedvalue (the single instance where the cast is done), not the type of the variable • For example: $n = 5.0; $x = (int) $n; • Since $n is a float, (int) $n converts the value to an int, now $x is an integer type. • ** WARNING ** Odd things can happen when casting from a higher to a lower type: $low = (int) 5.2345; // returns 5 $high = (int) 5.999; // returns 5 So why does 1.2 + " Fred" . 0777 * true evaluate to 1.2511?

  17. Useful Tools: print_r(), var_dump() • You can easily output variable types and their values with print_r(), var_dump() • These functions are very useful for debugging your programs • print_r() displays just the value, var_dump() displays the type and value Do Lab Exercise #2

  18. Increment and Decrement Operators • Shorthand notation for common arithmetic operations used for counting (up or down) • The counter can be incremented (or decremented) before or after using its current value ++$count; // preincrement: $count = $count + 1 before using it $count++; // postincrement: $count = $count + 1 after using it --$count; // predecrement: $count = $count - 1 before using it $count--; // postdecrement: $count = $count - 1 after using it • You can also increment and decrement non-integers • $var = 'a'; $var++; // $var now 'b' • What if $var = 'az' ?

  19. Increment/Decrement Operator Examples Common code: $n = 3; $m = 4; What will be the value of m and result after each of these executes? (a) $result = $n * ++$m;//preincrement $m (b) $result = $n * $m++;//postincrement $m (c) $result = $n * --$m;//predecrement $m (d) $result = $n * $m--;//postdecrement $m

  20. Specialized Assignment Operators • A shorthand notation for performing an operation on and assigning a new value to a variable • General form: $var <op>= expression; • equivalent to: $var = $var <op> (expression); • Where <op> is +, -, *, /, %, . , |, ^, ~, &, <<, >> • Examples: $amount += 5; //same as $amount = $amount + 5; $amount *= 1 + $interestRate; //$amount = $amount * (1 + $interestRate); • Note that the right side is treated as a unit Do Lab Exercises #3, #4

  21. Single vs. Double Quotes vs. Heredoc • Three ways to declare a string value • Single quotes • 'So this is Christmas?'; • Double quotes • "How about Kwanza?"; • Here Documents <<< When_Will_It_End It was a dark and stormy night, my son said, "Dad, tell me a story" so I said, "It was a dark and stormy night, my son said, "Dad, tell me a story" so I said, "It was a dark and stormy night, my son said, "Dad, tell me a story" so I said, When_Will_It_End

  22. Strings • Any sequence of characters e.g. abc123$%! • In PHP these are referred to as string • Designated as a literal by single or double quotes • 'string', "string" • Note: Any variables inside a double quoted string are expanded into that string, single quoted strings are not expanded $a = 10; echo "I have $a more slides"; I have 10 more slides $a = 10; echo 'I have $a more slides'; I have $a more slides

  23. Escape Sequences • To use characters in strings that can't be typed or seen or are interpreted specially by php (such as $, {,},\, etc.) there are escape sequences • \n newline (note: not the same as HTML <br>) • \t tab • \$ the $ character • \\ the \ character • Example: • echo "I need more \$ but they \\'ed the budget\n"; I need more $ but they \'ed the budget

  24. What's the Difference? • The way you declare a string will affect how • Escape codes (e.g. \n ) are recognized; variables are interpreted • Single quotes are literal • $variable = 10;echo 'This prints $variable\n'; This prints $variable\n • All characters print as written, No escape codes are recognized except \' and \\ • Double quotes expand variable values and escape characters • $variable = 10;echo "This prints $variable\n"; This prints 10

  25. Heredoc • Multi-line string constants are easily created with a heredoc expression $variable = 'Here Here!'; $wind = <<< QUOTE Line 1. \n Line 2. " " Line 3. ' ' Line 4. $variable QUOTE; echo $wind; • Variables are expanded, escape characters are recognized Do Lab Exercise #5

  26. Concatenating (Appending) Strings The '.' operator is used for string concatenation (merging two strings): $name = "Mondo"; $greeting = "Hi, there!"; echo $greeting . $name . "Welcome"; causes the following to display on the screen: Hi, there!MondoWelcome • Note that you have to remember to include spaces if you want it to look right: $greeting . " " . $name . " Welcome"; • causes the following to display on the screen: Hi, there! Mondo Welcome Do Lab Exercise #6

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