Chapter – 4
While writing a program it is
necessary to repeat a part of a program for a number of times. The best
solution for this is to use a loop it is a method which executes a part of the
program repeatedly as specified by the programmer.
TYPES OF LOOPS
a) Pre-Test Loop
b) Post-Test Loop
a) PRE –
TEST LOOP
As its name indicates, checks the
looping condition at the start of the loop if the condition is true the loop
execution starts otherwise stops. This loop is used in a situation when part of
program is to be repeated for a fixed number of time.
e.g., FOR …… NEXT.
b) POST –
TEST LOOP
This loop checks the looping
condition at the end of loop and decides whether or not to continue the
execution. This loop is used when a part of the program needs to be repeated
until a specified condition is met.
e.g., WHILE …… WEND.
FLOW CHART
Flow chart is a symbolic
representation of flow of programme. It can graphically represent data
processing procedure Flow chart serves two purposes, i.e., the planning of the
program structure as an aid in writing the computer program. Secondly the
documentation of the program logic and work flow for the purpose of the
communication with other person and to recall a program at a later time. If the
program needs to be changed. The second purpose is very important because it is
very difficult to check the logic of the program.
SYMBOLS
OF FLOW CHART
TERMINAL
The terminal symbol is used to
indicate the beginning, ending of the program logic flow. It is the first and
last symbol of the flow chart.
INPUT /
OUTPUT BOX
The input/output box is used to
denote any function of an input and output in the program. If there is a
program instruction to input or output the data from any I/O device that step
will be indicated on this symbol.
PROCESS
BOX
A processing symbol is used in a
flow chart to represent arithmetical and data movement instructions.
FLOW
LINES
Flow lines are used to indicate the
flow of operations. The exact sequence in which the instructions are to be
executed. The flow of flow chart is normally from top to bottom and left to
right.
DECISION
BOX
The decision box is used in a flow
chart to indicate a point at which a decision has to be made and to branch to
one or more alternative points that is possible during execution. The
appropriate path followed, depends on the result of the decision.
CONNECTOR
A flow chart becomes complex, when
the number and direction of flow lines is confusing or it spread over more than
one page. It is useful to utilize the connector symbol as a substitute for flow
lines.
SYSTEM
FLOW CHART
System Flow Chart is used to
describe complete data processing cycle including the hardware devices and
media used. A System Flow Chart shows flow of data in a system where and in
what form it is received by the system, how it is input to the computer and
what storage media are used to hold it. The symbol used in system flow chart
indicates the peripherals to be used in data processing procedure, where output
will be produced. System flow chart do not explain the logic of the programs.
ERRORS
Errors are the mistakes made by the
programmer during program writing. A program relays run perfectly for the first
time and each program needs to be tested to ensure that it is correct and
contains no errors. There are three types of errors generally encountered.
a) Syntax Error
b) Logical Error
c) Run Time Error
a. SYNTAX
ERROR
The syntax of a programming language
is the set of rules to be followed when writing a program in that language.
These rules are similar to the grammar rules of Urdu or English language. When
a program violates these rules, computer generates a syntax error. Syntax must
be correct before program execution.
b.
LOGICAL ERROR
These errors are the most difficult
ones to find, if a programmer enters a sign of multiply instead of addition at
the time of making the program, the answer will not be correct because
interpreter will not be able the detect these types of errors. A logical error
will not stop the execution of the program but the result will not be accurate.
c. RUN
TIME ERROR
These types of errors stop the
execution of the program. It may be due to entering the invalid data at the
time of executing a program. For example, if a program is expecting the user to
enter the numeric data and the user enters a string data, computer will display
an error message.
BITS & BYTES
The smallest piece of data that can
be recognized and used by a computer, is the bit, a binary digit. A bit is a
single binary value, either a 1 or 0. A group of eight bits is called a byte.
The byte is the basic unit for measure of the size of the memory, with today’s
computer memory sizes. It is more common to hear the term kilobyte (KB) or
megabyte (MB).
COMPUTER WORD
A computer word is the number of
adjacent bits that can be stored and manipulated as a unit. Just an English
vocabulary words are of varying lengths so are computer words. Many
microcomputers have the ability to manipulate a 32 bit word, while some models
have words length of 8 and 16 bits.
LIBRARY FUNCTIONS
GW-BASIC provides many built in
functions which are called binary functions. Each function is composed of a
format or a short program, that requires at least one parameter to be passed. A
parameter is a value which is in that formula or program and this formula or
program returns a result. This result can be stored in any appropriate variable
or directly displayed on monitor. A library function which has “$” sign is
called string library function while others are called numeric library
function. A string function returns string value while a numeric function
returns numeric value. Following are some library functions.
ABS, MID$, LEN, LEFT$, ASC, CHR$, TAB
EXPRESSIONS
An expression is a combination of
constants and variables linked by arithmetic operators like (+, -, *, /, \).
Expressions are used to perform different operations. The expressions are
evaluated from left to right but some operators have priority over the others.
Parenthesis are evaluated first then multiplication and division have the same
priority from left to right. Similarly the addition and subtraction are
evaluated with equal priorities. If all the operators are used in an expression
have the same priority then the expression be executed from left to right.
The expressions can be of three types.
- Arithmetic expressions
- Logical expressions
- Relational expressions.
ARITHMETICAL
EXPRESSIONS
In an arithmetic expression the
following operators are used in conjunction with the operands.
SymbolMeaning
+ Addition
- Subtraction
- Multiplication
/ Division
^ Exponentiation
( Left parenthesis
) Right parenthesis
RELATIONAL
EXPRESSION
A Relational expression is composed
of operands linked by the relational operators. The relational operators used
in the relational expression are given.
SymbolMeaning
= equal to
> greater than
< less than
<> not equal to
>= greater than or equal to
<= less than or equal to Example A > B
B <> C
LOGICAL
EXPRESSIONS
When a selections is based upon one
or more condition being true. It is possible to combine the condition together
using logical operators and the resulting condition would either be true or
false the most commonly used logical operators are AND, OR and NOT.
COMPUTER LANGUAGE
Language is a system for
representation and communication of information or data. Like human beings, a
language or signal is required to communicate between two persons. Similarly,
we cannot obtain any result by computer without langtage. Computer does not
understand directly what we are communicating with computer as English or
Arabic, it understands only machine language (binary codes 0-1). Computer
translates English language into machine codes through interpreter then process
instructions and give us the results.
The computer languages can be divided into two main levels.
- Machine language (0-1)
- Symbolic language (A-Z)
Symbolic languages are further
divide into two main levels
- High-level language
- Low-level language
MACHINE LANGUAGE
Although computers can be programmed
to understand many different computer language. There is only one language
understood by the computer without using a translation program. This language
is called the machine language or the machine codes. Machine codes are the
fundamental language of the computer and is normally written as strings of
binary 0-1.
ADVANTAGES AND LIMITATIONS OF MACHINE LANGUAGE
Programs written in machine language can be executed very fast by the computer.
This is mainly because machine instructions are directly understood by the CPU
and no translation of program is required.
However, writing a program in machine language has several disadvantage.
MACHINE
DEPENDENT
Because the internal design of every
type of computer is different from every other type of computer and needs
different electrical signals to operate. The machine language also is different
from computer to computer.
DIFFICULT
TO PROGRAM
Although easily used by the
computer, machine language is difficult to program. It is necessary for the
programmer either to memorize the dozens of code number for the commands in the
machine’s instruction set or to constantly refer to a reference card.
DIFFICULT
TO MODIFY
It is difficult to correct or modify
machine language programs. Checking machine instructions to locate errors is
difficult as writing them initially.
In short, writing a program in machine language is so difficult and time
consuming.
SYMBOLIC LANGUAGES
In symbolic languages, alphabets are
used (a-z). symbolic languages are further divide into two main levels.
- High level languages
- Low level languages
LOW LEVEL
LANGUAGE
A language which is one step higher
than machine language in human readability is called Assembly Language or a
low-level language. In an assembly language binary numbers are replaced by
human readable symbols called mnemonics. Thus a low-level language is better in
understanding than a machine language for humans and almost has the same
efficiency as machine language for computer operation. An assembly language is
a combination of mnemonic, operation codes and symbolic codes for addresses.
Each computer uses and has a mnemonic code for each instruction, which may vary
from computer to computer. Some of the commonly used codes are given in the
following table.
COMMAND NAMEMNEMONIC
Add – ADD
Subtract – SUB
Multiply – MUL
Compare Registry – CR
Compare – COMP
Branch Condition -BC
Code Register -LR
Move Characters -MVE
Store Characters -STC
Store Accumulator – STA
An assembly language is very
efficient but it is difficult to work with and it requires good skills for
programming. A program written in an assembly language is translated into a
machine language before execution. A computer program which translates any
assembly language into its equivalent machine code is known as an assembler.
HIGH –
LEVEL LANGUAGE
A language is one step higher than
low-level languages in human readability is called high-level language. High –
level languages are easy to understand. They are also called English oriented
languages in which instruction are given using words. Such as add, subtract,
input, print, etc. high level language are very easy for programming,
programmer prefer them for software designing that’s why these languages are
also called user’s friendly languages. Every high level language must be
converted into machine language before execution, therefore every high level
language has its own separate translating program called compiler or
interpreter. That’s why some time these languages are called compiler
langauges. COBOL, BASIC, PASCAL, RPG, FORTRAN are some high level languages.
INTERPRETER
An interpreter is a set of programs
which translates the high-level language into machine acceptable form. The
interpreters are slow in speed as compared to compilers. The interpreter takes
a single line of the source code, translates that line into object code and
carries it out immediately. The process is repeated line by line until the
whole program has been translated and run. If the program loops back to earlier
statements, they will be translated afresh each time round. This means that
both the source program and the interpreter must remain in the main memory
together which may limit the space available for data. Perhaps the biggest
drawback of an interpreter is the time it takes to translate and run a program
including all the repetition which can be involved.
PROGRAM DEVELOPMENT PROCESS
In order to develop a computer
program, a programmer has to go through the following stages:
1.
DEFINING AND ANALYSING THE PROBLEM
In this step a programmer studies
the problem and decides how the problem will be best solved. Studying a problem
is necessary because it helps a programmer to decide about:
- The facts and figures to be collected.
- The way in which the program will be designed.
- The language in which the program will be most
suitable.
- What is the desired output and in which form it is
needed, etc.
2.
DESIGNING THE ALGORITHM
An algorithm is a set of
instructions or sequence of steps that must be carried out before a programmer
starts preparing his program. The programmer designs an algorithm to help
visualize possible alternatives in a program.
3.
FLOWCHARTING
A flow chart is a graphical
representation of a program which helps a programmer to decide on various data
processing procedures with the help of labeled geometrical diagrams. A flow
chart is mainly used to describe the complete data processing system including
the hardware devices and media used. It is very necessary for a programmer to
know about the available devices before developing a program.
4. CODING
OR WRITING THE PROGRAM
The next job after analysing the
problem is to write the program in a high-level language, usually called
coding. This is achieved by translating the flow chart in an appropriate
high-level language, of course according to the syntax rules of the language.
5. TEST
EXECUTION
The process of execution of any program
to find out for errors or bugs (mistakes) is called test execution. This is
very important because it helps a programmer to check the logic of the program
and to ensure that the program is error-free and workable.
6.
DEBUGGING
It is a term which is used
extensively in programming. Debugging is the process of detecting, locating and
correcting the bugs by running the programs again and again.
7. FINAL
DOCUMENTATION
It is written information about any
computer software. Final document guides the user about how to use the program
in the most efficient way.
MODES OF OPERATION
There are two modes of operation for
BASIC. The mode that you are in determines what BASIC will do with the
instruction you give it. When you start BASIC you receive the OK prompt. You
then have two modes available to you immediately.
DIRECT
MODE
In the direct mode BASIC acts like a
calculator. No line numbers are required. Direct mode is not of course the main
purpose of BASIC, but it is useful at times particularly when you are debugging
program or short problems in which you want to perform quick calculation e.g.,
PRINT 3+4.
INDIRECT
MODE
In this mode you first put a line
number on each statement. Once you have a program you can run it and get your
results. The indirect mode saves your instructions in the computer along with
their line number, you can execute the program as many times as you wish simply
by typing RUN.
STEPS OF PROGRAMMING
There are five steps in preparing a
computer programme which are also called ABCDE of Programming.
ANALYSIS
In this step the system analyst
tries to become familiar with the problem. He has to study the problem and
prepare some notes upon that problem. He also notes that what is given, what is
required and what will computer can do.
BLOCKING
In this step the programmer converts
the analyst report to a series of steps through which the computer will give
the required result. The steps are commonly known as Algorithm. There are
different ways to write those detailed sequential steps. The most common method
used is flow charting. A flow chart is symbolic representation of flow of a
programme.
CODING
In this step the programmer writes
the programme in any computer language. This step is known as coding. After
this programme is fed into the computer and is compiled with the help of a
given compiler.
DEBUGGING
Debugging is a step in which a
programmer corrects a syntax error which may come after the compilation.
TESTING
Testing is a step where the
programmer is finally testing the programme for execution (there may be any
logical mistake which compiler cannot trace).
EXECUTION
In this step we send the programme
for execution where company’s data will be fed and process.
VARIABLES
AND CONSTANTS
VARIABLES
Variables are names used to
represent values used in BASIC programme. The values of a variable may be
assigned by the programmer or it may be assigned as the result of calculation
in the programme. Before a variable is assigned a value is assumed to be zero
(or null for string variable).
VARIABLES
NAMES & DECLARATION CHARACTERS
Variable names may be of any length
up 40 characters are significant. Variable name can contain letters, numbers
and decimal point. However, the first character must be a letter. A variable
should not be a reserve word.
Variable may represent either a numeric value or a string.
NUMERIC
VARIABLE
Numeric variable names may be
declared as integer, single precision, double precision values. The type of
declaration characters for these variable names are as follows.
STRING
VARIABLE
In programming we often use to
handle alphanumeric or string variables. String variables consist of names,
addresses or in general any string or series of letters, number or special
characters.
Without the use of string variable whenever any differentiation needs to be
made e.g., from employee to employee or from application no. to application
no., a number has to be assign to each employee or each application.
A string variable name starts with an alphabet and terminate with a dollar sign
($). In a string the value must be in quotation marks. No arithmetic operations
except addition and in addition the values do not add they are just written
together. The maximum number of character allowed in a string varies from
system to system but a typical value is 256.
CONSTANTS
Constants are those that do not
change while executing the programme. BASIC deals with two types of constant.
1. Numeric constant
2. String constant
1.
NUMERIC CONSTANT
All numbers belong to this category
because their values remain constant during execution.
They may take any of the following forms:
1. Commas are not allowed in a numeric constant.
2. The use of the positive sign is optional but negative sign should be used if
the number is negative.
2. STRING CONSTANT
A string constant is any set of valid basic characters enclosed in quotation
marks. The quotation marks do not form part of string. Strings are used to
represent non-numeric information such:
1. Name of persons and places
2. Addresses
3. Days, Months, Years.
INTRODUCTION TO BASIC
BASIC is a high level language used
for purpose of writing a program on a computer. It stands for Beginners
All-purpose Symbolic Instruction Code. BASIC is an easy to use “friendly”
language where instruction resembles elementary, algebraic formulas and certain
English keywords such Let, Read, Print, Goto etc.
HISTORY
OF BASIC
The language was developed at Dartmouth College
In 1978, the American National Standard Institute standardized an essential
subset of BASIC in order to promote uniformity from one version of BASIC into
another.
In recent years some new version of BASIC have been developed which included a
variety of features that are not included in more traditional versions.
STRUCTURE
OF BASIC PROGRAM
Each instruction in a BASIC program
is written as a separate statement. Thus a program will be composed of a
sequence of statements.
The following rules apply to all BASIC statements:
1. Every statement must appear on a separate line number.
2. Each statement number must be followed by a BASIC keyword which indicates
the type of instruction.
3. On one single line number we cannot write two statements, if we need we must
use “colon”.
4. We can write 255 characters on one line.
0 comments :
Post a Comment