DIG1108 Lesson 5

INTRO TO SERVER SIDE PROGRAMMING
Lesson Five
Thursday, September 26, 13

Lists and Dictionaries
In computer science, a list (or sequence) is an abstract
data type that implements an ordered collection of
values. Each instance of a value on a list is usually called
an item, entry or element of the list, and if the same value
appears multiple times in the list, each is considered a
distinct item of the list.
An associative array (or dictionary) is an abstract data
type composed of a collection of key/value pairs, such
that each key appears at most once in the collection.
Pairs can be added or removed, and values can be
modiﬁed or retrieved based on the associated key.

What's The Difference?
A list is always ordered, can usually return elements or
items by numeric index(starting at zero), and might not
allow modiﬁcation of values. A decent implementation of
a list should provide speciﬁc operations for adding and
removing elements to enable them to be used:
push - append to the end(or tail) of the list
pop - remove from the tail of the list
queue(unshift) - append to the beginning(or head) of the list
dequeue(shift) - remove from the head of the list
A hash is usually not immutable and may be
indeterminately ordered. Values can usually be altered.

Arrays
An array type is a data type that is meant to describe a
collection of values or variables, each selected by one or
more indices(keys) tha can be computed at run-time of the
program.
By default, the array type in PHP generates computed,
numerical indices, starting with zero, to make a list:
var_dump(array( 'one', 'two', 3, 4, 4.1, 4.2 ));
$list = array(); $list[] = 'one'; $list[] = 'two';
The value for the key can also be speciﬁed as any scalar
literal (neither array, object nor resource), often a string
var_dump(array( 'one' => 1, 2 => 'two' )):
$list[4] = 'four'; $list['ﬁve dot one'] = 5.1;

Array Tricks
PHP relies heavily on arrays, so there are a lot of array-
speciﬁc functions, like to use an array as a queue or stack
array_unshift(), array_shift() - append to and remove from head
array_push(), array_pop() - append to and remove from tail
PHP arrays have an "internal position" pointer that can be
used as an iterator:
current(), key() - return the current element or key
next(), prev(), end(), reset() - advance forward or back
each() - return the current key and value and advance the
pointer

Even More Array Tricks
array_merge() - merge two or more arrays
array_slice() - remove an array section
array_splice() - insert or replace a section
count(), sizeof() - calculate length
array_ﬁll(), array_pad(), range() - generate values
str_split(), explode(), implode(), join() - array to string

ASSIGNMENT 5.1
Find Some Lists and Arrays

Lists of Lists
Open Github and Cloud 9
Look through your forked projects for examples of lists
and dictionary deﬁnitions and methods
Copy adn paste your examples into a ﬁle named
"homework-5.1.md" with code delimiters as needed
Write comments identifying the lists, keys, indexes and
other list and dictionary pieces discussed
Save, add and commit, then push to Github when done

Recursion

Control Flow Statements
Control Flow - refers to the order in which the individual
statements, instructions or function calls of an
imperative or declarative program are executed or
evaluated. Execution results in a choice being made as to
which of two or more paths should be followed.
Types of Control Flow statements:
continuation at a different statement (unconditional branch or jump)
execute statements only if some condition is met (conditional branch)
execute statements until some conditional is met(loop, conditional branch)
execute deﬁned statements and return (sub/co-routines, continuations)s
stop executing statements (unconditional halt)

Loops
A loop is a sequence of statements which is specified once
but which may be carried out several times in succession, a
specified number of times or indefinitely
Specific number of times:
for ( $count = 0; $count < $max; $count++ ) do_something();
Once per each item in a collection(array):
foreach ( $collection as $item ) do_something();
foreach ($collection as $key => $value ) do_something();
Until some condition is met:
while ( $condition == true ) do_something();
do something(); while ( $condition );
Indefinitely(infinitely):
while ( true ) do_something();
do something(); while ( true);

Infinite Loops
An unconditional(or
infinite) loop returns to a
fixed point in the
diagram, usually the top
of the workflow. Without a
breaking statement or
escape clause, it executes
forever.

Conditional Loops
A conditional loop (while,
do-while, for, foreach)
returns to a condition
check ($count < $max).
Until the condition
evaluates FALSE, the loop
will continue to execute

Diagramming Part Two
In your pair, find a section of a project that has some
significant looping and conditional logic (at least three
branches)
Individually, sketch a simple workflow diagram of the
logic; assemble a truth table if needed
Discuss differences in your diagrams and make a new
diagram and truth table for the logic to demonstrate

ASSIGNMENT 5.2
Loops and Conditionals

Create a file called "homework-5.2.md" in your "assignments"
workspace on Cloud 9
Find a section in your projects that has some decent looping and
branching code: at least five branches that you can diagram.
Copy and paste your example into your file and attempt to identify the
loop conditions with comments:
while ( $count < $max ) {
// while $count is less than $max
foreach ( $collection as $item ) {
// until there are no more $items in the $collection
Save your file, then git add, git commit -m "explain why" and git push

DIG1108 Lesson 5

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