Page 193 - DCAP104_EXPOSURE_TO_COMPUTER_DISCPLINES

Exposure to Computer Disciplines

Notes • input: Get data from the keyboard, a file, or some other device.
• output: Display data on the screen or send data to a file or other device.

• arithmetic: Perform basic arithmetical operations like addition and multiplication.
• conditional execution: Check for certain conditions and execute the appropriate sequence
of statements.
• repetition: Perform some action repeatedly, usually with some variation.

Many computer languages provide a mechanism to call functions provided by libraries such as in
.dlls. Provided the functions in a library follow the appropriate run time conventions (e.g., method
of passing arguments), then these functions may be written in any other language.
9.1.8 Paradigms

Computer programs can be categorized by the programming language paradigm used to produce
them. Two of the main paradigms are imperative and declarative.
Programs written using an imperative language specify an algorithm using declarations,
expressions, and statements. A declaration couples a variable name to a datatype. For
example: var x: integer; . An expression yields a value. For example: 2 + 2 yields 4. Finally, a
statement might assign an expression to a variable or use the value of a variable to alter the
program’s control flow. For example: x := 2 + 2; if x = 4 then do_something( ); One criticism
of imperative languages is the side effect of an assignment statement on a class of variables
called non-local variables.

Programs written using a declarative language specify the properties that have to be met by the
output. They do not specify details expressed in terms of the control flow of the executing machine
but of the mathematical relations between the declared objects and their properties. Two broad
categories of declarative languages are functional languages and logical languages. The principle
behind functional languages (like Haskell) is to not allow side effects, which makes it easier to
reason about programs like mathematical functions. The principle behind logical languages (like
Prolog) is to define the problem to be solved, the goal and leave the detailed solution to the Prolog
system itself. The goal is defined by providing a list of subgoals. Then each subgoal is defined by
further providing a list of its subgoals, etc. If a path of subgoals fails to find a solution, then that
subgoal is backtracked and another path is systematically attempted.
The form in which a program is created may be textual or visual. In a visual language program,
elements are graphically manipulated rather than textually specified.
9.1.9 Compiling or Interpreting

A computer program in the form of a human-readable, computer programming language is called
source code. Source code may be converted into an executable image by a compiler or executed
immediately with the aid of an interpreter.

Either compiled or interpreted programs might be executed in a batch process without human
interaction, but interpreted programs allow a user to type commands in an interactive session.
In this case, the programs are the separate commands, whose execution occurs sequentially, and
thus together. When a language is used to give commands to a software application (such as a
shell) it is called a scripting language.

186 LOVELY PROFESSIONAL UNIVERSITY