ITM 352 Data types, Variables

Output in PHP Use echo for simple output echo 'hello'; echo 'hello', ' goodbye'; echo ('hello'); print is virtually the same syntax print 'hello'; You can use () if you like echo('hello'); print('hello'); New line for console output (we don’t do much of this) echo "line1\nline2"; New line for HTML output echo 'line1<br>line2';

What is a Variable? A named location to store data a container for data (like a box or bucket) It can hold only one type of data at a time for example only integers, only floating point (real) numbers, or only characters A variable with a scalar type holds one scalar value A variable with a compound type holds multiple scalar values, BUT the variable still holds only a single (the compound type itself) value Syntax for a variable is $<identifier> PHP Example: $name, $age JS Example: var name, var age Case sensitive! $name 'Dan' $age 30.3

Assigning Values to Variables The assignment operator: "=" "sets" a value for a variable not the "is equal to" sign; not the same as in algebra It means - "Assign the value of the expression on the right side to the variable on the left side." Can have the variable on both sides of the equals sign: $count = 10;// initialize counter to ten $count = $count - 1;// decrement counter var count = 10;// initialize counter to ten var count = count - 1;// decrement counter new value of count = 10 - 1 = 9

Creating Variables A variable is declared the first time a value is set for it A variable declaration associates a name with a storage location in memory and specifies the type of data it will store: $a = 1.1 ; // declares and sets a real number $a = true ; // declares and sets a boolean $a = 'Zip Zap' ; // declares and sets a string var a = 1.1 ; // declares and sets a real number var a = true; // declares and sets a boolean var a = 'Zip Zap' ; // declares and sets a string

Variable Names: Identifiers Rules (these must be obeyed) all identifiers must follow the same rules must not start with a digit must contain only numbers, letters, underscore (_) and some other special characters names are case-sensitive (ThisName and thisName are two different variable names) No spaces! Good Programming Practice (these should be obeyed) always use meaningful names from the problem domain (for example, eggsPerBasket instead of n, which is meaningless, or count, which is not meaningful enough) start variable names with lower case capitalize interior words (use eggsPerBasket instead of eggsperbasket) use underscore (_) for spaces CAPITALIZE constants (i.e. variables that do not change values)

Variable Default Values Variables have default values $a = $a + 1; // $a=0 by default $s = $s."Fred"; // default $s='' var a = a + 1; // a=0 by default var s = s + "Fred"; // default s="" IMPORTANT: It is best to not assume the default value is what you want. Always explicitly set the initial value of a variable!!!! e.g. $a = 0; $s = ""; $b = false;

Two Main Kinds of Data Types Scalar the simplest types also called "primitive" or "basic" types cannot decompose into other types contain single values only Examples: Integer Floating point (real) String Boolean Compound also call class types more complex composed of other types (primitive or class types) can contain multiple values Examples: Arrays Objects (more about these in ITM353)

Which Ones to Know for Now - 1 integer just whole numbers may be positive or negative no decimal point may use Octal: 0755 // starts '0' Hex: 0xFF // starts '0x' In PHP these are referred to as int boolean only two values – true or false used for 'conditional' tests (e.g. if, when) In PHP these are referred to as bool floating point real numbers, both positive and negative has a decimal point (fractional part) two formats number with decimal point, e.g. 514.061 e (or scientific, or floating-point) notation, e.g. 5.14061E2, which means 5.14061 x 102 In PHP these are referred to as double null The 'nothing' type (more on this later)

Which Ones to Know for Now – 2. A string is a sequence of characters A very common data type Names, passwords, addresses, histories, etc. Often used to represent complex data Dates, phone numbers, SS numbers, formatted output A common data-interchange or data-sharing type key-value pairs, XML, comma delimited data, logs PHP has a vast and powerful set of functions for working with strings. JS not so much, but there are frameworks such as JQuery that do. Manipulation, searching, comparing, translation, etc. Check out php.net Examples: “Mr. Smith”, ‘808-956-6948’, ‘21.7’, “1202 King St.”

NULL Null is a special type that means "no value" It can be used to unset a variable It is used as a place holder within compound types (more on this later…) $a = NULL; // $a is “unset”, also can use unset($a) var a = null; // var a; would set a as “undefined” not null Do Exercise #1 in lab

Simple Expressions Data types can be operated on (e.g. arithmetic, string operations) echo 1+2; echo 3*2; echo "Big" . " " . "Dude"; printf("5/3 is about %3d", 5/3); Operators: +, -, ., *, /, %

Simple Expressions With Variables Variables can be operated on (e.g. arithmetic) // add 1 to value in $a and set in $add $add = $a + 1; // multiply value in $a by 2 and set in $mult $mult = $a * 2; // concatenate string in $s with 'Fred' and // set in $str $str = $s . " Fred";

Do Exercise #2,#3 (and bonus if you wish) in lab Printf() Use printf() for more complex formatted output printf('This prints 2 decimal places %.2f', 3.1415927); This prints 2 decimal places 3.14 Printf() is a function whose first argument is a string that describes the desired format and the remaining arguments are the values to substitute into the type specifications (anything that starts with %) Do Exercise #2,#3 (and bonus if you wish) in lab