1. ITM 352 Expressions, Precedence, Working with Strings Class #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 = " Fred" * true;
Our goal is to understand why the value of the above expression is (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)
Associativity
Operators
non-associative
++ --
(int) (float) (string) (array) (object) (bool)
right !
left
* / %
+ - .
< <= > >= <>
== != === !==
&& ||

9. Operator Precedence and Parentheses
Expressions follow certain rules that determine the order an expression is evaluated
e.g. should * 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 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 returned value (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 = ;
$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 returned value (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) ; // returns 5
$high = (int) 5.999; // returns 5
So why does " Fred" * true evaluate to ?

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; // 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' ?

18. 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
Do Lab Exercise #2

19. 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 Exercise #3

20. 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

21. 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

22. I need more $ but they \'ed the budget
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

23. 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

24. 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.
Do Lab Exercise #4

25. 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!
Do Lab Exercise #5

26. 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 #6