Procedural Abstraction and Functions · 8 Abstraction and Functions 13 Procedural Abstraction in...

1

8 Abstraction and Functions 1

Procedural Abstraction and!

Functions"1E3 !

Topic 8!


Objectives"

n  This topic should allow students to!n  Understand the importance of abstraction in

programming!n  Recognise when a function would be useful.!

2


The plan"

n  Today we’ll introduce the idea of procedural abstraction and look at some examples.!

n  Next topic will cover the detail and get you to start using functions.!


Abstraction"n  In computer science abstraction means

identifying and naming something complex and then using that name to refer to the complex thing.!n  Data abstraction : !

n  design a data structure to hold details about a bank account; call it bank_account!

n  Procedural abstraction:!n  Give a name to a piece of code that does something; use the

name to get that something done.!

3


Abstraction contd./"n  Imagine this scenario: every time we start a lecture I request the

following sequence of actions:!n  come in please.!n  turn on the lights.!n  turn on the computer.!n  turn on the projector.!n  everyone have a seat.!n  get out your copybooks.!n  get out your pens.!


Abstraction contd./"n  Imagine this scenario: every time we start a lecture I request the

following sequence of actions:!n  come in please.!n  turn on the lights.!n  turn on the computer.!n  turn on the projector.!n  everyone have a seat.!n  get out your copybooks.!n  get out your pens.!n  Wouldn’t it be easier if we just define this procedure once

and then give it a simple name by which we invoke it? !n  “Let’sStart ( )”"

4


Abstraction contd./"n  What are the benefits of using Let’sStart ()?!

n  Abstraction"n  short name, less details in main stream of lecture.!




n  Single point of editing"n  imagine if I’ve written this action sequence at the beginning of every lecture’s

powerpoint, then one day we came to the point where you all work on iPads and I no more want to tell you: get out your copybooks and pens;!

n  in how many slides will I have to change those lines?!n  if we had agreed on the procedure and put it on the wall here, I could just

edit it in one place and get over with it.!

5







edit it in one place and get over with it.!n  Single point of error-checking and debugging!

n  if you sense an error, you only need to review this code in one place.!







edit it in one place and get over with it.!n  Single point of error-checking and debugging!

n  if you sense an error, you only need to review this code in one place.!n  Reusability"

n  We could package this procedure and give it to others to use… just like we use some functions in C++ (e.g. getline, pow, sqrt, etc.)!

6


Procedural Abstraction"n  Give a name to an algorithm !n  Use the name to execute that algorithm within

another one.!n  Example!

n  procedure sqrt (x)"!{algorithm for computing sq. root}!

n  sqrt (789.54)!n  Means APPLY the sqrt procedure to the number 789.54.!


Procedural Abstraction"n  Example 2!

n  procedure sort (list) "!{algorithm for sorting a list}!

n  sort (list-of-names)!n Means APPLY the sort procedure to the list of

names.!n  Abstraction allows you to separate details

of how to carry out the procedure from the algorithm that uses the procedure. !

7


Procedural Abstraction in C++"n  In C++ procedural abstractions are called

functions.!n  A function encapsulates and names an algorithm

to solve a particular problem.!n  That algorithm can be used in a program simply by

calling the function by name, generally supplying information and receiving a result.!

n  sqrt, pow, setprecision are built-in functions that we have used.!

n  We can design and implement our own functions as we need them.!


Some built-in functions"n  By including <cmath> you can use !n  pow(i,n)!

n  A built-in function to raise i to the power of n. We used it in the investment table program.!

n  sqrt(x), fabs(x), sin(x), cos(x)!n  sqrt returns the square root.!n  fabs returns the absolute value of a double!

n  Note that you don’t have to worry about how these functions are coded.!

8


The role of functions"n  Hide detail

n  We don’t need to care how sqrt(x) is implemented.

n  While we are writing the main payroll program we can ignore how gross_pay (hrs, rate) operates.


The role of functions"n  Hide detail

n  We don’t need to care how sqrt(x) is implemented.

n  While we are writing the main payroll program we can ignore how gross_pay (hrs, rate) operates.

n  Increase readability n  is_a_factor_of(n,d) should be more meaningful

than (n%d == 0)

9


Role of functions cont./"n  Allow reuse of code!

n  sqrt function is useful in all sorts of mathematical programs.!


Role of functions cont./"n  Allow reuse of code!

n  sqrt function is useful in all sorts of mathematical programs.!

n  Modular software development:!n  Aids program design !

n  Use a function to implement subtasks of an algorithm.!n  Allows incremental development and testing!n  Allows division of labour.!

10


Modular System Development"n  Build software like you would build a car.!n  Identify the components you need.!n  Specify what each needs to do!n  Buy one that meets the spec or get someone to

build one.!n  Test each component before putting it in the

system.!n  To improve the car, replace components with

better versions.!


Modular Software Development"

n  Large programs must be broken down into components.!

n  Each component is implemented by a function.!n  Specify the behaviour of each component and

the interface between the system and the component.!

n  Develop and test components separately before integrating into the whole.!

Procedural Abstraction and Functions · 8 Abstraction and Functions 13 Procedural Abstraction in...

Documents

Transcript of Procedural Abstraction and Functions · 8 Abstraction and Functions 13 Procedural Abstraction in...