Sequence & Data Abstraction

Sequences & Data abstraction

 

Sequences

: a sequence is an ordered collection of values

-   strings : sequence of characters

-   lists : sequence of values of any data type

Sequence Abstraction

: All sequences have finite length

: Each element in a sequence has a discrete integer index

: sequence share common behaviors based on the shared trait of having a finite length and indexed elements

-   Retrieve an element at a particular position

-   Create a copy of a subsequence

-   Check for membership

-   Concatenate two sequences together

What can you do with sequences?

Get item : get the ith element

slice a subsequence : create a copy of the sequence from i to j

check membership : check if the value of <expr> is in <seq>

concatenate : combine two sequences into a single sequence

 

Sequence Processing

Iterating through sequences

: you can use a for statement to iterate through the elements of a sequence

Range

: the range function creates a sequence containing the values within a specified range

range( <start>, <end>, <skip> )

: creates a range object from <start> (inclusive) to <end> (exclusive), skipping every <skip> element

List comprehensions

: you can create out a list out of a sequence using a list comprehension

[ <expr> for <name> in <seq> if <cond> ]

-   rules for execution

1. create an empty result list that will be the value of the list comprehension

2. for each element in <seq>:

 A. bind to that element to <name>

 B. If <cond> evaluates to a true value, then add the value of <expr> to the result list

 

Data Abstraction

-   Compound values combine other values together

: a date _ a year, a month, and a day

: a geographic position _ latitude and longitude

-   Data abstraction lets us manipulate compound values as units

-   Isolate two parts of any program that uses daya

: how data are represented ( as parts )

: how data are manipulated ( as units )

-   data abstraction : a methodology by which functions enforce an abstraction barrier between representation and use

Rational Numbers

: Exacy representation as fraction

: a pair of integers

: as soon as division occurs, the exact representation my be lost ! (demo)

: assume we can compose and decompose rational numbers

Rational Numbers Arithmetic

Rational Numbers Arithmetic Implementation

Representing Rational Numbers

Return to Lowest Terms

Abstraction Barriers

-   use rational numbers to perform computation -> whole data values

: add_rational, mul_rational, rationals_are_equal, print_rational

-   create rationals or implement rational operations -> numerators and denominators

: rational, numer, denom

-   implement selectors and constructor for rationals -> two-element lists

: list literals and element selection

Violating Abstration Barriers