lecture 5: developing procedural thinking (how to think like a programmer) b burlingame 30 sept 2015

Lecture 5: Developing Procedural Thinking(How to think like a programmer)

B Burlingame

30 Sept 2015

Ref: xkcd: www.xkcd.com

Announcements

Homework 2 due up front Homework 3 posted, due two weeks Thursday lab was canceled last week

Thursday students have a choice: Catch up now or catch up the week of Nov 11

Still don’t have Canvas…

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

Example Program design example Basic decisions in MatLab 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 Process

Define 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 1

Define 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

Better

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

Is the problem statement okay?


Define the problem







Inputs• •

Outputs• •


Define the problem






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. Pseudocode

2. Flow Charts

We’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 points

2. Get points from user

3. Calculate the straight line distance

4. Return distance


Comments

1. High level – just the major steps

2. Focus on the logic

Pseudocode - Refinement

1. Start

2. Declare variables: X1, Y1, X2, Y2, Distance

3. Prompt user to enter X1 and Y1

4. Prompt user to enter X2 and Y2

5. Calculate the straight line distance, Distance

6. Return Distance

7. Stop


Comments

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

Flowcharts - 1

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.

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

Input/Output – input or output of data

Terminal – indicates start or end of the program or algorithm

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

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, D

Define the problem







P1

P2

X1 X2

Y1

Y2 D

How do you find D?

P1

P2D

X

Y22 YXD

12

12

YYY

XXX

22 1212 YYXXD


Define the problem







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


Define the problem






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.

Matlab Distance Functionfunction [distance]=dist()% This method matches our algorithm x = int8(1); y = int8(2); p1 = [0, 0]; p2 = [0, 0]; p1=input('Enter [x1, y1]: '); p2=input('Enter [x2, y2]: '); distance = sqrt( (p1(x)-p2(x))^2 + (p1(y)-p2(y))^2 );

function [distance]=dist(p1, p2)% A bit more MatLabish way to do things distance = sqrt( (p1(1)-p2(1))^2 + (p1(2)-p2(2))^2 );


Define the problem






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 two values

Algorithm – Items to Consider Is the problem statement clear and

concise? Could be better:

Return the maximum numerical value between two floating point numbers

What are the inputs? Two floating point numbers (a and b)

What are the outputs? The larger value

Algorithm – Possible Solution

start

Is A larger?

max = A max = B

end

function mymax( A, B )

return max

True False

Pseudocode1. Receive A and B from function call2. If A is larger

2.1. Set max equal to A3. Otherwise

3.1. Set max equal to B4. Return max

Matlab Function

function [max]=mymax(A, B) if(A < B) max = B; else max = A; end

Flowchart

lecture 5: developing procedural thinking (how to think like a programmer) b burlingame 30 sept 2015

Documents

program codetest

program codestate

program codewrite

program code6so

proper graphics program

solution algorithmcheck

inputs information

straight line distance