Transcript Foundations of Algorithms

Programming Process
 The programmer’s job can be broken down into
six programming steps:
1. Understand the problem
2. Plan the logic
3. Code the program
4. Translate the program into machine language
5. Test the program
6. Put the program into production
Understand the Problem
 Programmers must first understand what it is the user
wants
 To understand the problem, you must analyze it
 Really understanding the problem may be one of the most
difficult aspects of programming
o The description of what the user needs may be vague
o The user may not even really know what he or she wants
o Users who think they know what they want frequently change their
minds after seeing sample output
Understand the Problem
 Analysis & Really Understanding
 Example
o To invite some people, I need the list of people who work during
five more years.
o Definite problem or ambiguous problem ?
Understand the Problem
 Analysis & Really Understanding
 Example
o To invite some people, I need the list of people who work during
five more years.
o Definite problem or ambiguous problem ?
 Ambiguous problem
o Full-time or part-time worker or both of them ?
o Regular employee or contract employee ?
o Which type of list ?
 A good programmer is often part counselor, part detective
Programming Process
 The programmer’s job can be broken down into
six programming steps:
1. Understand the problem
2. Plan the logic
3. Code the program
4. Translate the program into machine language
5. Test the program
6. Put the program into production
Plan the Logic
 Programmer plans the steps to the program, deciding what
steps to include and how to order them
 Example
o Planning tour
o Planning party
 The two most common tools
o flowcharts : pictorial representation
o Pseudocode : English-like representation
Plan the Logic
 Flowcharts
 A pictorial representation of the logical
steps it takes to solve a problem.
o Uses Standardized Symbols
start
get
InputNumber
o Utilize Formula Notation
o Typically Read from Top to Bottom or from
calculatedAnswer =
InputNumber * 2
o Left to Right on a Page
print
calculatedAnswer
stop
Plan the Logic
 Basic flowchart symbols
Start/Stop
(Terminator)
Process
(Rectangle)
Input/Output
(Parallelogram)
Decision
(Diamond)
Connector
(Circle)
Flowlines
(Arrows)
Predefined Process
(Rectangle)
Sequence, Selection, Repetition
sequence
selection
entrance
entrance
repetition
entrance
exit
exit
exit
Plan the Logic
 Pseudocode
 An English-like representation of the logical steps it takes to
solve a problem
o pseudo – a prefix that means false
o Short English-Like Statements
o Not Standardized
o Proper use of indentation
 Example
start
get InputNumber
compute calculatedAnswer as InputNumber times 2
print calculatedAnswer
stop
Programming Process
 The programmer’s job can be broken down into
six programming steps:
1. Understand the problem
2. Plan the logic
3. Code the program
4. Translate the program into machine language
5. Test the program
6. Put the program into production
Code the program
 Writing the program in one of more than 400 programming
languages
o Pascal, Fortran, C, C++, Java…..
 Concentrate on the syntax of the language
o Exact instruction, symbol, …. ?
 Some very experienced programmers
o successfully combining the logic planning and the actual instruction
writing, or coding of the program, in one step
o Writing a post card
o Writing a cinema scenario
Code the program
 Which is harder: Planning the Logic or Coding the Program?
 Example
o Planning the logic :Planning mystery novel
o Coding the program : Writing English or Spanish based on the
original scenario.
o Who gets more annual salary ?
Programming Process
 The programmer’s job can be broken down into
six programming steps:
1. Understand the problem
2. Plan the logic
3. Code the program
4. Translate the program into machine language
5. Test the program
6. Put the program into production
Translate the Program
 Objective
o Each computer knows only one language, Machine Language.
o High-level Languages must be translated into Machine Language
 Need to compiler or interpreter
o Compiler catches every syntax error.
o When writing a program, a programmer might need to recompile
the code several times
o An executable program is created only when the code is free of
syntax errors
Translate the Program
 Creating an executable program
Programming Process
 The programmer’s job can be broken down into six
programming steps:
1. Understand the problem
2. Plan the logic
3. Code the program
4. Translate the program into machine language
5. Test the program
6. Put the program into production
Test the Program
 Why does it need to be tested ?
o Syntax Errors : by compile
o Logical Errors : by test
 Test
o Executing the program with some sample data
o Seeing whether or not the results are logically correct.
o being tested with many sets of data carefully
 Example
Logically incorrect
start
get InputNumber
compute calculatedAnswer as InputNumber times 20
print calculatedAnswer
stop
Programming Process
 The programmer’s job can be broken down into six
programming steps:
1. Understand the problem
2. Plan the logic
3. Code the program
4. Translate the program into machine language
5. Test the program
6. Put the program into production
Put the program into the production
 Once the program is tested adequately, it is ready for the
organization to use.
 Putting the program into production might mean simply
running the program once if it was written to satisfy a
user’s request for a special list.
Flowchart & Pseudocode
 Flowcharts (pictorial representations) and pseudocode
(English-like representations) are used by programmers to
plan the logical steps for solving a programming problem
 Some professional programmers prefer writing pseudocode
to drawing flowcharts, because using pseudocode is more
similar to writing final statements in programming language
Flowchart & Pseudocode
 Almost every program involves the steps of input,
processing, and output, necessitating some graphical way
to separate them
 Arithmetic operation statements are examples of processing
in a flowchart, where you use a rectangle as the processing
symbol containing a processing statement
Flowchart & Pseudocode
 To represent an output statement, you use the
parallelogram, which is also the same symbol used for input
statements
Flowchart & Pseudocode
 In flowcharts:
o Arrows, or flowlines, connect and show the appropriate
sequence of steps
o A terminal symbol, or start/stop symbol, should be included at
each end
o Often, “start” or “begin” is used as the first terminal symbol
and “end” or “stop” is used in the other
o The standard terminal symbol is shaped like a racetrack; often
called a lozenge, because it resembles the shape of a
medicated candy lozenge you might use to soothe a sore
throat
Flowlines
(Arrows)
Start/Stop
(Terminator)
Flowchart & Pseudocode
 Complete flowchart for the program that doubles a number,
and the pseudocode for the same problem
start
get
InputNumber
calculatedAnswer =
InputNumber * 2
print
calculatedAnswer
stop
start
get InputNumber
compute calculatedAnswer
as InputNumber times 2
print calculatedAnswer
stop
Naming Variables
 Variables
o memory locations, whose contents can vary or differ over time.
o reasonable and descriptive variable names
 Example
o InputNumber
o caluculatedAnswer
start
get InputNumber
compute calculatedAnswer as InputNumber times 2
print calculatedAnswer
stop
Naming Variables
 Naming Rules
o Every programming language has its own set of rules for naming
variables.
o
o
o
o
o
o
o
most languages allow both letters and digits : a,b,c, 1,2,3
some languages allow hyphens and/or underscores : - _
some allow dollar signs or other special characters : $, #, @
some allow for foreign characters : Japanese, Spanish
different languages put different limits on lengths
some languages are case sensitive, others are not : name, Name
in general, variable names may not begin with a digit : name1
Naming Variables
 Textbook Conventions – two rules:
(1) Variable names must be one word
o can contain letters, digits, hyphens, underscores, with the exception
of spaces.
(2) Variable names should have some appropriate meaning
o Undesirable : G, u84, fred, mike, richard, pink,
o Desirable
: rate, name, age, count, score, index, lastName
employeeLastName, 5employeeLastName, employeeLast, empLast,
emlstnam, lastNameofTheEmployeeInQuestion, last name (x),
employeelastname,
Naming Variables
 Assignment values to variables
o Whatever operation is performed to the right of the equal sign
results in a value that is placed in the memory location to the left of
the equal sign.
compute calculatedAnswer
as InputNumber times 2
is the same as
calculatedAnswer = InputNumber * 2
Naming Variables
 Constant
o A memory location, similar to a variable, except its value never
changes during a program.
o taxRate = .0825
o Pi = 3.141592
 Variables
o memory locations, whose contents can vary or differ over time.
Naming Variables
 Data Types – two basic types
 Character
o Character : ‘a’ ‘b’ ‘c’ ‘d’ ‘e’
o Character string : “Richard” “Michale”
 Numeric
o Integer : …, -3, -2, -1, 0, 1, 2, 3, ….
o Floating-Point : …, -2.0, -1.5, -1, -0.5, 0, 0.5, 1.0, ….
Ending a Program
 Infinite loop
start
get
InputNumber
calculatedAnswer=In
putNumber*2
Print
calculatedAnswer
start
get InputNumber
compute calculatedAnswer
as InputNumber times 2
print calculatedAnswer
get InputNumber
compute calculatedAnswer
as InputNumber times 2
print calculatedAnswer
get InputNumber
compute calculatedAnswer
as InputNumber times 2
print calculatedAnswer
………
stop
Ending a Program
 An infinite loop is a repeating flow of logic with no end
 To end the program,
o set a predetermined value for inputNumber that means “Stop the
program!”
o The program can then test any incoming value for inputNumber and,
if it is a 0, stop the program
 Testing a value is also called making a decision
o Represented in flowchart by diamond shape called a decision
symbol
Ending a Program
 Decision symbol
start
start
get
InputNumber
get
InputNumber
calculatedAnswer=In
putNumber*2
Print
calculatedAnswer
inputNumber
=0?
No
calculatedAnswer=In
putNumber*2
Print
calculatedAnswer
Yes
stop
Ending a Program
 Dummy value
o A pre-selected value that stops the execution of a program is often
called a dummy value since it does not represent real data, but just
a signal to stop
o Sometimes, such a value is called a sentinel value because it
represents an entry or exit point, like a sentinel who guards a
fortress
Using a Connector
 If a flowchart has six processing steps and a page provides
room for only three, you might represent the logic as
shown below:
o On-page connector symbol
o Off-page connector symbol
Evolution of Programming Technique
 Currently, there are two major techniques used to
develop programs and their procedures
o Procedural programming focuses on the procedures that
programmers create
o Object-oriented programming, focuses on objects, or “things”,
and describes their features, or attributes, and their behaviors
Summary
 A programmer’s job involves six steps:
o Understanding the problem
o
o
o
o
o
Planning the logic
Coding the problem
Translating the program into machine language
Testing the program
Putting the program into production
 When programmers plan the logic for a solution to a
programming problem, they often use flowcharts or
pseudocode
Summary
 Testing a value involves making a decision
 Most programming languages use the equal sign to assign
values to variables
 Procedural and object-oriented programmers approach
program problems differently