lesson 27 Creating Computer Programs This lesson includes

lesson 27 Creating Computer Programs

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

What is a Computer Program? • Files • Hardware/Software Interaction

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.

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 (1 s and 0 s). 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.

How Programs Solve Problems • Program Control Flow • Algorithms • Heuristics • Common Flow Patterns • Variables and Functions

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.

This flowchart shows that the sequence can vary depending on conditions.

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.

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.

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.

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

This type of programming has fallen into disfavor.

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.

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.

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.