Creating Computer Programs

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lesson 27
Creating Computer Programs
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• This lesson includes the following sections
• What is a Computer Program?
• How Programs Solve Problems
• Two Approaches Structured
• Object-Oriented Programming

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What is a Computer Program?

• Files
• Hardware/Software Interaction

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What is a Computer Program? - Files
Typically, a program is stored as a collection
of files. Some common file types used in
programs are

• Executable (.EXE) files actually send commands to
  the processor.
• Dynamic Link Library (.DLL) files are partial
  .EXE files.
• Initialization (.INI) files contain configuration
  information for a program.
• Help (.HLP) files contain information for the
  user.

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What is a Computer Program? - Hardware/Software
Interaction

• The program tells the CPU to process interrupts,
  or sets of steps the CPU must follow to perform a
  task.
• To control hardware, a program must be written in
  binary numbers (1s and 0s). This code is called
  machine code or machine language.
• Programmers use programming languages to write
  code in nearly human language. The resulting
  description is called source code.
• Compilers and interpreters translate a program
  into object code, the binary version of source
  code.

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How Programs Solve Problems

• Program Control Flow
• Algorithms
• Heuristics
• Common Flow Patterns
• Variables and Functions

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How Programs Solve Problems Program Control
Flow

• The order in which program statements are
  executed is called program control flow.
• To determine program control flow, programmers
  may use a flowchart to map the program's
  sequence.
• Programmers may also create a simple text version
  of a program's code called pseudocode to
  determine how the program will flow.

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This flowchart shows that the sequence can vary
depending on conditions.
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How Programs Solve Problems - Algorithms

• An algorithm is a set of steps that always lead
  to a solution. The steps are always the same,
  whether the problem is being solved manually or
  with a PC.
• A computer program may contain thousands of
  algorithms, each one devoted to a single task.
• An algorithm, for example, will find the highest
  point in a mountain range by comparing all the
  points until the highest one is found.

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How Programs Solve Problems - Heuristics

• If a problem is too complex to be solved by an
  algorithm, a programmer may try to solve it by
  using heuristics.
• Heuristics are like algorithms, and will always
  find a solution to a problem. But unlike
  algorithms, heuristics are not guaranteed to find
  the best possible solution.
• A heuristic, for example, may find the highest
  point in a mountain range by comparing random
  points, but this process may never find the
  highest one.

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How Programs Solve Problems - Common Flow
Patterns

• To determine when and where to pass program
  control, a developer may use conditional
  statements or loops.
• A conditional statement determines whether a
  condition is true. If so, control flows to the
  next part of the program.
• A loop repeats again and again until a condition
  is met. Control then passes to another part of
  the program.

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How Programs Solve Problems - Variables and
Functions

• A variable is a named placeholder for data that
  is being processed. Programs contain variables
  to hold inputs from users, for example.
• A function is a set of steps that are followed to
  perform a specific task. By assembling a
  collection of functions together, a developer can
  build a complete program.

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Two Approaches Structured Object-Oriented
Programming

• Early programmers allowed control to pass from
  one part of a program to another by using goto
  statements. Control would "go to" a different
  part of the program when conditions allowed.
• Goto statements cause programs to become very
  complex. To eliminate their use, programmers
  developed two approaches to development

• Structured programming
• Object-oriented programming

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This type of programming has fallen into disfavor.
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Two Approaches Structured Object-Oriented
Programming - Structured Programming
Structured programming uses three types of
control structures to make program control flow
more predictable

• Sequence structure defines the default control
  flow.
• Selection structures are built around conditional
  statements.
• Repetition (looping) structures use loops, which
  execute according to the results of conditional
  statements.

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Two Approaches Structured Object-Oriented
Programming - Object-Oriented Programming

• In object-oriented programming (OOP), programs
  are built from blocks of code, called objects.
  Each object has functions and characteristics
  (attributes), and can contain (encapsulate) other
  objects.
• Objects that share common attributes can be
  grouped into classes. Classes can be divided
  into subclasses.
• In OOP, objects communicate with one another by
  exchanging messages.

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lesson 27 Review

• Define the term computer program.
• Describe the use of flowcharts and pseudocode in
  programming.
• Identify two ways in which a program can work
  toward a solution.
• Differentiate the two main approaches to computer
  programming.
• List and describe three elements of
  object-oriented programming.