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ITM 352 Expressions, Precedence, Working with Strings Class #5

Learn about PHP files, programs, statements, expressions, operators, and precedence rules. Explore how to work with strings and handle different data types in PHP programming.

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ITM 352 Expressions, Precedence, Working with Strings Class #5

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  1. ITM 352 Expressions, Precedence, Working with StringsClass #5

  2. Review: What is a PHP File? • PHP files have extension ".php” • A PHP file can contain text, CSS, HTML and, of course, PHP code. • PHP code is executed on the server, and the result is returned to the browser as plain HTML.

  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. • Anything else, before or after, is interpreted as HTML!!!!

  4. Review: What is a PHP Statement? • A PHP programs normally contain comments and code (statements) • Comments: • start with // or # • or begin with /* and end with */ • Statements end with a ; • 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…)

  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). • 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!

  8. Operators and Precedence (Partial)

  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

  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 • The data type hierarchy (from lowest to highest):bool --> int --> float --> string --> array --> object • For example: $x = 0; $n = 5 + 10.45; $x = $n; • the value 5 is cast to a float, then assigned to $n • $x now is a float • 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 float 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. 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++; // postincrement: $count = $count + 1 after 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' ?

  18. 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

  19. 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

  20. Strings • Any sequence of characters e.g. abcd*123$!19 • 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

  21. 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

  22. 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

  23. Heredoc • Multi-line string constants are easily created with a heredoc expression $variable = 'Here Here!'; $wind = <<< EDGE Line 1. \n Line 2. " " Line 3. ' ' Line 4. $variable EDGE; echo $wind; • Variables are expanded, escape characters are recognized • Note: the end of the heredoc (EDGE in this case) must be the first (and only) thing on the line.

  24. 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 thereMondoWelcome! • Note: you have to remember to include spaces (and other punctuation) if you want it to look right: echo $greeting . " " . $name . “, Welcome!"; • which causes the following to display: Hi there Mondo, Welcome!

  25. 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

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