Changes made

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Changes made

• Imported all the slides from the creating
  functions ppt
• Deleted lets create some functions slide with
  array_key_exists, etc. on it
• COUNT 18 slides not including titles

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ITM 352Functions
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Having Trouble???

• If you are having trouble then please seek help
  as soon as possible!
• Im not a mind reader!!!!
• If you are silent, I will assume that everything
  is going fine.
• The longer you wait, the more difficult it is to
  catch up.

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Functions

• A function is a named block of code (i.e. within
  s) that performs a specific set of statements
• Possibly act on a set of values given to it
  (parameters)
• Possibly returns a single value (return-value)
• Functions are very useful for
• Repeated tasks in multiple locations
• Saving execution time
• Modularization

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Functions

• Weve already made use of several functions!
• Can you think of some?
• Some functions are built in to PHP, some are
  added from code libraries, some you will define

Do lab exercise 1
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Calling a Function

• All PHP functions are designated by
  ltidentifiergt(ltparametersgt)
• Ex. phpinfo(), rand(1,10)
• Not case sensitive
• Functions are used by calling them
• lt? phpinfo() ?gt
• Full interface for a function
• ltreturn typegt ltidentifiergt ( ltparametersgt )

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Passing Values into a Function Parameters

• Some Functions can be more flexible (therefore
  useful) if we pass them input values to use in
  their operations
• Input values for functions are called passed
  values or parameters
• Parameters must be specified inside the
  parentheses () of the heading in the function, be
  of the expected data type, in the expected order
  as defined by the functions interface
• rand(int min, int max)
• rand(1,10)

Do lab exercise 2
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Passing Variables

• Any legal expression can be used as a parameter
  (recall that an expression returns a value)
• rand(443, 2maxmax)
• It is very common to use a variable for a
  parameter (remember that a variable by itself is
  a simple legal expression it returns the value
  of the variable)
• rand(minGuess,maxGuess)
• substr(name,1,i)

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Return Values of Functions

• Some Functions just perform an action (e.g. read
  in a value from the keyboard, switch to a web
  page) and do not return a value
• Most functions perform an action and return a
  single value
• Return types may be
• a primitive data type, such as int, float,
  boolean, etc.
• a compound type such as array, object, etc.
• void if no value is returned
• Return values are how a function passes
  information back after it is called and after it
  performs its operations.

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Return Values of Functions

• You can use a function with a return type
  anyplace where it is legal to use an expression,
• guess rand(1,10)
• 3.14159rand(1,10)
• echo phpinfo()
• yummy piece of . substr(piece,0,3)
• if( is_int(guess) )
• It is common for a function to return a boolean
  value
• true is the function operations were successful
  with no problems
• false if the function failed to perform its task
• if( !writeFile() ) echo file not written

Do lab exercise 3
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Function Documentation

• Functions are documented not only with a
  description of the function, but also with a
  interface that shows the return value type, the
  name of the function, and the required and
  optional arguments
• type function_name (type arg1, type arg2 ,type
  optional_arg)
• PHP functions often document functions this way
• printf
• (PHP 3, PHP 4 )
• printf -- Output a formatted string
• Description
• void printf ( string format , mixed
  args)Outputs a string by using format and the
  given arguments
• You can look up functions using the book index,
  Appendix A function categories, or PHPeds help

function name
function signature
function description
Do lab exercise 4
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ITM 352Creating Functions
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Defining Functions

• A very important aspect of PHP is the ability to
  create your own functions. You will find out how
  useful this is later.
• You declare a function with the function keyword
  in front of a identifier and function parameter
  set and a code-block
• function ltidentifiergt ( ltparametersgt )
• // function operations (code statements) here
• return ltexpressiongt
• The way you define a function defines the
  functions interface (or how it is expected to be
  used)

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Function Syntax and Example

• function square(num)
• answer num num
• return answer
• To call the function
• quantity 3
• myvar square (quantity)
• myvar will contain the value 9

• function funcname(var1, var 2...)
• statement 1
• statement 2
• statement 3
• ...
• return retval

NOTE that the variable names do not need to be
the same the order of variables passed is what
will matter inside the function!
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Global vs. Local Variables

• Variables defined inside a function (or passed
  in) are local variables and are only available
  within functions
• after exiting the function the variable ceases to
  exist!!!
• Variables defined outside of a function are
  generally not available inside a function (this
  avoids accidental conflicts so variables should
  be passed into a function as arguments)
• Global variables are accessible both in and out
  of functions
• Where the variable is active (alive) is known as
  its scope
• use global and static to modify this
• also passing by reference will affect scope

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Scope of Variables

• Inside a function is like an entirely new
  program.
• Variables that you define within the function
  have local scope
• They dont exist before the function starts
• They dont exist once the function has completed

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Scope of Variables 2

• lt?php
• function deposit(amount)
• balance amount
• echo New balance is balance ltBRgt"
• balance 600
• deposit(50)
• echo "Balance is balance"
• ?gt

• What will this program print?
• Why?

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Defining Functions

• function swapTwoValues(a, b)
• tmp a // save old value temporarily
• a b // copy second parameter into first
• b tmp // copy original first value into
  second
• var1 1 var2 2
• echo \var1 is var1 and \var2 is var2ltBRgt
• swapTwoValues(var1, var2)
• echo \var1 is var1 and \var2 is var2
• What would happen if we used a and b instead?

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Function Naming Conventions

• Good Programming Practice (we will look for
  this!)
• Use verbs to name functions that do not return
  values or operate directly on variables
• They usually perform an action e.g. sort(),
  print_table()
• Use nouns to name functions that return a value
• they create (return) a piece of data, a thing
  e.g. date()
• Start function names with a lower case letter
• phpinfo()
• Use _ to separate words
• is_bool()
• Use descriptive names
• get_html_translation_table()

Do lab exercise 5
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Pass-By-Value vs. Pass-By-Reference

• When a function is called, the value of each
  argument is copied (assigned) and used locally
  within that function
• Variables used as arguments cannot be changed by
  a function!!!!!!!!!
• One way to change a value of a passed in variable
  is to make use of return value
• function doubler(value)
• return 2value
• echo doubler(doubleUp)
• doubleUp doubler(doubleUp)

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Pass-By-Value vs. Pass-By-Reference

• Sometimes its more convenient to directly
  operate on a variable, so you pass in its
  reference (address)
• function swapTwoValues(a, b)
• tmp a // save old value temporarily
• a b // copy second parameter into first
• b tmp // copy original first value into
  second
• var1 1 var2 2
• echo \var1 is var1 and \var2 is var2ltBRgt
• swapTwoValues(var1, var2)
• echo \var1 is var1 and \var2 is var2