Programming and Basics
(Chapter-4)
LOOP
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
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.
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.
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
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
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.
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
SymbolMeaning
+ Addition
- Subtraction
* Multiplication
/ Division
^ Exponentiation
( Left parenthesis
) Right parenthesis
^ 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
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.
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.
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.
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.
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.
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
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.
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.
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 ($). In a string the value must be in quotation marks. No arithmetic operations except addition and in addition the values do not add they are just written together. The maximum number of character allowed in a string varies from system to system but a typical value is 256.
In programming we often use to handle alphanumeric or string variables. String variables consist of names, addresses or in general any string or series of letters, number or special characters.
Without the use of string variable whenever any differentiation needs to be made e.g., from employee to employee or from application no. to application no., a number has to be assign to each employee or each application.
A string variable name starts with an alphabet and terminate with a dollar sign ($). 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
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.
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.
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 1967, under the direction of John Kemeny
and Thomas Kurtz. It was quickly discovered and adopted. All the major computer
manufacturers offered their own version of BASIC for their particular
computers.
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.
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.
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