COMP1105

1
COMP1105

• The Structure Theorem
• Modularization
• Top-Down design and
• Stepwise Refinement
• Pseudocode and
• Flowcharts
• Algorithms
• Control structures

• 2. Problem Solving and programming

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Problem solving and software development

• Software Engineering
• Small programs (less than 5000 lines) can be
  reliably developed by one person
• Most software is developed in much larger
  environments - with teams of programmers or even
  collections of teams
• Managing large projects involves many more
  organisational difficulties than the small
  single-person projects
• Software engineering is the study of processes by
  which large projects can be developed

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Problem solving process

• We want to build good problem solving skills for
  software development
• This means skills which can successfully be used
  to tackle larger and more complex problems
• The design/documentation techniques may seem like
  overkill for small problems, THE PRACTICE IS
  IMPORTANT, AND VASTLY WORTHWHILE LATER ON
• Planning, organisation, and documentation prevent
  a great deal of grief in program development and
  maintenance

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A typical Software life cycle

• Analysis - specifying exactly what the problem is
  and what are the requirements for an acceptable
  solution
• Design - developing a detailed solution to the
  problem
• Coding - turning the design into a working
  program
• Testing/verification - ensuring the software is
  correct
• Maintenance - fixing, enhancing, adapting the
  software
• Obsolescence - abandoning a program which is no
  longer effectively maintainable

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Program development

• Analysing/defining the problem we're given
• Developing a solution technique (algorithm)
• Documenting the program/technique
• Translating (implementing) the technique into
  code
• Compiling and running the program
• Testing the results with appropriate data

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Design and analysis

• Analysis includes
• determining the input data for the program
• determining the output that must be produced
• identifying any information needed to get from
  the inputs to the output
• Design consists primarily of working out an
  algorithm
• that is, a step by step procedure for solving the
  problem (based on the analysis)

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Structured Programming

• Top-down development
• Modular design
• The Structure Theorem

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Top-down design

• Take a problem, divide it into smaller logical
  sub-problems, and remember how they fit together
• For each sub-problem, divide it into still
  smaller sub-problems
• Continue sub-dividing until each little problem
  is small enough to be easily solved
• Solving a collection of small problems thus
  allows us to solve one much larger problem
  Modular Design

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The Structure Theorem

• 3 control structures (Bohm and Jacopini)
• Sequence structure statements executed
  sequentially by default
• Transfer of Control
• Selection if-Then-Else
• Repetition do/while, Repeat/Until

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Algorithms

• Computing problems
• Solved by executing a series of actions in a
  specific order
• Algorithm is a procedure determining
• Actions to be executed
• Order to be executed
• Example recipe
• Program control
• Specifies the order in which statements are
  executed

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Algorithms

• Must be
• Precise and unambiguous
• Give the correct solution in all cases
• Eventually end
• Example an algorithm for making coffee
• Stumble to kitchen
• Get coffee beans
• Get Coffee Grinder
• Grind.
• Drink your coffee!

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Pseudocode

• Artificial, informal language used to develop
  algorithms that is similar to everyday English
• Not executed on computers but is used to think
  out program before coding and Easy to convert
  into programs

• 6 basic computer operations
• Receive information Read, Get, ..
• Put out information display, Print, ..
• Perform arithmetic
• Assign value to a variable or memory location
• Compare two variable and select one of two
  alternative actions If/Else
• Repeat a group pf actions Do/While

• 3 basic control structures
• Sequence
• Selection
• Repetition
• 3 steps
• Define problem
• Design solution algorithm
• Check solution

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Pseudocode

• Suppose a program is required to read 2 numbers,
  add them and print their total.

Defining Diagram
Algorithm
Algorithm to add 2 numbers and print their
sum Begin Input the first number Input the
second number Add the two numbers Output the
sum End
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Pseudocode

• Suppose you want to determine which is the larger
  of the two numbers (if any).

Begin Input Num1 from the keyboard Input the
Num2 from the keyboard If the Num1 is greater
than the Num2 print Num1 Else if the Num2 is
greater than the Num1 print Num2 Else print
a message stating that the numbers are equal
End
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Flowchart

• Graphical representation of an algorithm using
  special-purpose symbols connected by arrows
  (flowlines)
• Parallelogram symbol
• Input/Output operations
• Diamond symbol - Decision
• Rectangle symbol Action/process
• Oval symbol
• Beginning or end of a program, or a section of
  code (circles)

Read Num2
Num1 gt Num2
no
yes
Print Num1
Stop
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if Selection structure

• Choose among alternative courses of action
• Pseudocode statement to determine whether a
  student has passed the course
• If students grade is greater than or equal to 40
  
• Print Passed
• If the condition is true
• Passed is printed, control continues on to next
  statement
• If the condition is false
• Print statement ignored, control continues to
  statement after the if
• Indenting makes algorithm easier to read

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if Selection structure

• Flowchart of pseudocode statement

• If - Performs action if condition true
• if/else Performs Different actions if
  conditions true or false
• Example
• if students grade is greater than or equal to
  40 print Passed
• else
• print Failed

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while Repetition Structure

• Action repeated while some condition remains true
• Pseudocode
• while there are more items on my shopping list
• Purchase the next item and cross it off
  the list
• while loop repeated until condition becomes false

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Example

• A fabric store wants a program to convert
  measurements in square metres to square yards
• Define problem program data?
• Inputs the incoming information is the number of
  square metres of fabric
• Outputs the number of square yards which are
  equivalent to the input number of square metres
• How to get from inputs to outputs the extra
  information we need (and can work out) is that 1
  square metre 1.196 square yards. Thus the
  output number is 1.196 times the input number

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Write an algorithm to Find the tallest person in
the class!

• Read number of people into the variable people
• Input the first height directly into the variable
  tallest
• Increment counter to 1
• WHILE counter is less than people DO
• Begin
• input a new height into the variable height
• IF height is greater than tallest
• replace tallest with height
• ENDIF
• Increment counter
• End
• Print the value of the largest
• (NB1 BEGIN and END of the algorithm is not
  shown!
• 2 The while condition becomes false when
  counter is equal
• to people)

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flowchart
END
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Solve this problem
BEGIN ALGORITHM //Let num1, num2 and num3 store
integer values   READ num1, num2   INITIALISE
num3 0   WHILE num2 gt 0 DO   BEGIN     
num3 num3 num1      num2 num2 - 1  
END   PRINT num3END ALGORITHM

• The following algorithm reads in 2 numbers and
  performs an operation on them. What is the
  relationship between the 2 numbers read in and
  the result printed?
• Draw a flowchart.

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Skills needed in programming

• Attention to detail
• Stupidity
• Good memory
• Ability to abstract, think on different levels
• Further Reading http//www.eskimo.com/scs/cclass
  /progintro/sx1.html