lecture 3: developing procedural thinking (how to think like a programmer) b burlingame 16 feb 2016

Lecture 3: Developing Procedural Thinking(How to think like a programmer)

B Burlingame16 Feb 2016

Ref: xkcd: www.xkcd.com/1195

Announcements Homework 1 due up front

Trouble with homework 1, hang out after class

Homework 2 posted, due two weeks Read chapters 1 – 3 in text Office hours 5 – 7, Eng 213

The Plan for Today Program design process Algorithms, decomposition, and step-wise refinement

Example Program design example For flowcharts, use a proper graphics program.

Visio is the standard and is in labs Gliffy is online, free, and good enough (http://www.gliffy.com) Don’t use Powerpoint or Word. They are time wasters and

SUPER ugly.

http://www.gliffy.com/

Learning Objectives List and describe the steps in designing a

computational solution (computer program) to a problem

Articulate what is meant by an algorithm Apply the steps to a particular problem

Program Design ProcessDefine the problem

List the inputs and outputs

Design the solution algorithm

Check the algorithm by hand

Write the program code

Test the program code

Program Design Process – step 1Define the problem






State the problem in aclear and concise manner

ExampleWrite a program to find the

distance between two points

P1 P2or

P1 P2 P1

P2or

BetterWrite a program to find the straight line distance between two points

Is the problem statement okay?







Write a program to find the straight line distance between two points

Inputs• •

Outputs• •







Decompose Refine


Definition of an Algorithm An algorithm is a well-ordered collection of

unambiguous and effectively computable operations, that when executed, produces a result and halts in a finite amount of time. Well-ordered means the steps are in a clear order Unambiguous means the operations described are understood

by a computing agent without further simplification A computing agent is the thing that is supposed to carry out the

algorithm Effectively computable means the computing agent can actually

carry out the operation

This definition comes from, An Invitation to Computer Science (Gersting/Schneider) viahttp://www.cs.xu.edu/csci170/08f/sect01/Overheads/WhatIsAnAlgorithm.html (visited 19JUN2009)

http://www.cs.xu.edu/csci170/08f/sect01/Overheads/WhatIsAnAlgorithm.html

Program Design Process – step 3,cont.

Define the problem






Two approaches are often used to help think through the steps to be carried out by the program code:1. Pseudocode2. Flow ChartsWe’ll use the pseudocode method first.

Pseudocode Pseudocode (also called Program Design Language,

PDL) is English-like statements that precisely describe specific operations1: Action statements Focuses on the logic of the program Avoids language-specific elements Written at a level so that code can be

generated almost automatically. Will likely need to refine in more and more detail

1This definition comes from, McConnel, S. (1993). Code Complete, Microsoft Press, Redmond, WA, p. 54.

Pseudocode – First Pass

1. Prompt user to enter points2. Get points from user3. Calculate the straight line distance4. Return distance


Comments1. High level – just the major steps2. Focus on the logic

Pseudocode - Refinement

1. Start2. Declare variables: X1, Y1, X2, Y2, Distance3. Prompt user to enter X1 and Y14. Prompt user to enter X2 and Y25. Calculate the straight line distance6. Return Distance7. Stop


Comments1. Refine high level ideas down to computable actionsWhat could still be refined?

Flowcharts A graphical tool that diagrammatically depicts

the steps and structure of an algorithm or program Symbol Name/Meaning Symbol Meaning

Process – Any type of internal operation: data transformation, data movement, logic operation, etc.

On-page Connector – connects sections of the flowchart, so that the diagram can maintain a smooth, linear flow

Input/Output – input or output of data

Off-Page Connector – same as above, but directs flow to a given page number

Decision – evaluates a condition or statement and branches depending on whether the evaluation is true or false

Flow lines – arrows that indicate the direction of the progression of the program

Sub-process – Any type of internal operation documented in another flowchart

Terminal – indicates start or end of the program or algorithm

start

Initialzation

Prompt for

X1, Y1

Prompt for

X2, Y2

Calculate distance

end

function dist()

p1 = [x1, y1]P2 = [x2, y2]distance = 0

distance = я( (p1x – p2x)² + (p1y – p2y)² )

Calculating the Distance, DDefine the problem







P1

P2

X1 X2

Y1

Y2 D

How do you find D?

P1

P2D

X

Y22 YXD

1212

YYYXXX

22 1212 YYXXD








What values of Xi, Yi (where i=1, 2) would be good to test the algorithm with?







If you have refined your algorithm sufficiently, writing the code should proceed straightforwardly from it.If not, continue refining the algorithm until you can write the code directly from it.Your pseudocode can be turned into the comments for your code.

We’ll start writing code, next week







Test your code with cases that you know the answer to.Try the ‘boundary’ cases to make sure your code works for them too.

Algorithm Practice Find the maximum of n values

Algorithm – Items to Consider Is the problem statement clear and

concise? Could be better:

Return the maximum numerical value amongst n floating point numbers

What are the inputs? n floating point numbers

What are the outputs? The largest value

Algorithm – Possible SolutionPseudocode

1. Start2. Initialize number = 0, counter = 0, n = 0, max = -∞3. Get the quantity of numbers from user4. Store that quantity in n 5. Is counter > n ?6. add 1 to counter7. get number8. is (number > max) ?9. max = number10. go to line 511. otherwise12. print max13.end

Flowchart

Notice the loops!

start

Init

Get the qty of number from the user

Store qty in n

counter > n

Add 1 to counter

Number> max?

max = number

Get number

Print max

End

number = 0counter = 0n = 0max = -ʺ

False True

YesNo

function find_max

return max

Find average start

Init

Get the qty of number from the user

Store qty in n

counter > n

Add 1 to counter

Get number

End

number = 0counter = 0n = 0avg = 0sum = 0

False True

function find_average

return avg

sum += number

Handle stats

(n, sum)

A

B

AB

return avg

Start handle stats

Avg = sum / n

end handle stats

return avg

function handle_stats(n, sum)

lecture 3: developing procedural thinking (how to think like a programmer) b burlingame 16 feb 2016

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