introduction to computer & c computers computers are programmable machines capable of performing...
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Introduction toComputer & C
ComputersComputers
Computers are programmable machines capable of performing calculations
Examples of special-purpose computers are calculators and game-playing machines
Examples of general-purpose computers are personal computers and notebooks
Hardware/SoftwareHardware/Software
A computer consists of hardware and softwareThe hardware consists of various physical devices that performs wired and basic operationsThe software consists of various programs that coordinates basic operations to accomplish flexible and complex tasks
ProgramsPrograms
A Program is a sequence of instructions (basic operations) to control the operation of the computer
Programming is the task of designing programs
Programming languages are notations used to represent the instructions of computers
HardwareHardware
storage unit
input unit
primary storage (memory) unit
output unit
secondary storage unit
control unit
arithmetic and logic unit (ALU)
central processin
g unit (CPU)
I/O unit
control bus
data bus
Control UnitControl Unit
Control unit repeatedly fetches instructions from memory unit to control unit via data bus
Control unit then interprets instructions, and coordinates the operations of other units via control bus
Arithmetic and Logic UnitArithmetic and Logic Unit
ALU is responsible for performing calculations based on arithmetic operations such as addition, subtraction, multiplication, and division
ALU is also responsible for making decisions based on logical operations such as equality (=, ≠) and relation (<, ≦, >, ≧)
Arithmetic and Logic UnitArithmetic and Logic Unit
Arithmetic or logical operations performed by ALU are controlled by control unit via control bus
Data on which operations are performed are transferred from memory unit via data bus
Data resulted from operations are transferred to memory unit via data bus
Primary Storage UnitPrimary Storage Unit
Memory unit stores both instructions and data
It retains information that is actively being used by the computer
It is the short-term, rapid-access, low-capacity warehouse of a computer
Secondary Storage UnitSecondary Storage Unit
Secondary storage unit retains information that is not actively being used by the computer
It is the long-term, slow-access, high-capacity warehouse of the computer
Common secondary storage devices are disks and tapes
Input UnitInput Unit
Input unit transfers information from various input devices to memory unit
It also transforms information in human-readable form to information in machine-readable form
Common input devices are keyboards and mouse devices
Output UnitOutput Unit
Output unit transfers information from memory unit to various output devices
It also transforms information in machine-readable form to information in human-readable form
Common output devices are screens and printers
Users
SoftwareSoftware
Application Programs
Operating System
Hardware
Operating SystemOperating System
The operating system provides efficient management of the hardware so that application programmers can easily use the computer without knowing the detailed operations of the hardware
Common operating systems are DOS, Windows 2000, Windows NT, Unix, Linux
Application ProgramsApplication Programs
An application program allows users to use a computer to solve problems in a specific domain without knowing the details of the computer
Common application programs are MS Word, MS Excel, MS Access, MS Internet Explorer, Netscape Communicator
Biological Applications:
BLAST, FastA, DOCK, DSSP, …
High-level languages
Machine language: computer’s native language.
Add two numbers: 00100111 1010 0101
Assembly language: uses mnemonics.
Add two numbers: ADD R1 R2
An assembly code needs to be translated to machine code.
High-level languages
High-level programming languages: bridging the gap between machine language and natural languages.
Examples: COBOL, Fortran, Algol, C
Add two numbers (in C): Z = A + B;
Compilation: translation to machine code.
CompilersCompilers
A compiler translates high-level language
programs into assembly language programs
or machine language programs
High-level language
C, C++, Pascal
Interpreter
Basic, PHP, …
Each high-level instruction is usually
translated into several assembly instructions
AlgorithmsAlgorithms
An algorithm is an abstract strategy for
solving a problem
Solving a problem by computer consists of
designing an algorithm and expressing the
algorithm as a program
The Programming Language CThe Programming Language C
C is a general-purpose programming language
C is developed by Dennis Ritchie at Bell Laboratories
C has become one of the most widely used languages in the world
The C Programming SystemThe C Programming System
C programming language
A set of notations for representing
programs
C standard library
A set of well-developed programs
C programming environment
A set of tools to aid program development
ProgramsPrograms
A C program consists of functions and variables
A function contains instructions that specify the operations to be done during the computation
Variables denote memory locations that store data used during the computation
FunctionsFunctions
Functions can be either user-defined functions or library functions
A C program begins the execution at the beginning of the function named main
A function may call other functions to help it perform one of its subtasks
The First C ProgramThe First C Program
#include <stdio.h>/* include library
information */main( ) /* name of starting function */{ /* beginning of instructions */ printf(“hello, world\n”);
/* call library function */} /* end of instructions */
C Programming EnvironmentC Programming Environment
Edit
Compile
Link
Execute
prog1.c, prog1.h, prog2.c, prog2.h
prog1.obj, prog2.obj
prog.exe
lib.h
lib.obj
input outputDebug
Compilation in C
Use the cc or gcc compiler:cc prog.c
Produces executable file a.out if no errors
To specify name of executable file, use -o option:
cc -o prog prog.c
Name of executable file becomes ‘prog’.
Workstation
Errors
Compilation errors: occur during compilation.
Reason: syntax errors.
Easy to rectify.
Run-time errors: occur during execution.
Reasons: logic errors, data errors, computation errors.
Harder to rectify.
Program development
EDIT
Understanding the problem
Writing an algorithm
Converting to code
COMPILE
RUN
errors
Result
Verifying the algorithm
Program = Algorithm + Data Structures
Algorithm
Algorithm: a well defined computational procedure consisting of a set of instructions, that takes some value(s) as input, and produces some value(s), as output.
Al-Khowarizmi Algorismus Algorithm
Algorithm
Input OutputAlgorithm
Algorithm
Understanding the problem
Writing an algorithm
Converting to code
Verifying the algorithm
Algorithm embeds logic of solution.
Algorithm is converted to program.
Algorithmic problem solving:
writing an algorithm -- tough
translate algorithm to code -- easy
Euclidean algorithm
First documented algorithm by Euclid (300 B.C.) to compute greatest common divisor (gcd). Examples: gcd(3,21) = 3; gcd(15,40) = 5.
1. Let A and B be integers with A > B 0.
2. If B = 0, then the gcd is A and the algorithm ends.
3. Otherwise, find q and r such thatA = qB + r where 0 r < B
Note that we have 0 r < B < A and gcd(A,B) = gcd(B,r).Replace A by B, B by r. Go to step 2.
Euclidean algorithm
Walk through the algorithm with examples: A = 40, B =15.
A = 2B + 10 A = 15 ; B = 10
A = 1B + 5 A = 10 ; B = 5
A = 2B + 0 A = 5 ; B = 0
gcd is 5
1. Let A and B be integers with A > B 0.
2. If B = 0, then the gcd is A and the algorithm ends.
3. Otherwise, find q and r such thatA = qB + r where 0 r < B
Note that we have 0 r < B < A and gcd(A,B) = gcd(B,r).Replace A by B, B by r. Go to step 2.
Data types and structures
Data types: integer, real number, character, Boolean, pointer, etc. Examples: 32, 0.0264, 'a', true.
Data structures: arrays, records, files, etc.
Program = Algorithm + Data Structures
Data are stored in variables in a program. Variables take up memory space in the computer.
Euclid’s algorithm in C
int gcd(int m, int n){ int r; while ( (r = m % n) != 0) { m = n; n = r; } return n;}
“New Style” function declaration lists number and type of arguments
Originally only listed return type. Generated code did not care how many arguments were actually passed.
Arguments are call-by-value
m n
1 15 4
2 4 3
3 3 1
m n
1 30 15
m n
1 48 15
2 15 3
3 3
Euclid’s algorithm in C
int gcd(int m, int n){ int r; while ( (r = m % n) != 0) { m = n; n = r; } return n;}
Automatic variable
Storage allocated on stack when function entered, released when it returns.
All parameters, automatic variables accessed w.r.t. frame pointer.
Extra storage needed while evaluating large expressions also placed on the stack
nm
ret. addr.r
Frame pointer Stack
pointer
Excess arguments simply ignored
Euclid’s algorithm in C
int gcd(int m, int n){ int r; while ( (r = m % n) != 0) { m = n; n = r; } return n;}
Expression: C’s basic type of statement.
Arithmetic and logical
Assignment (=) returns a value, so can be used in expressions
% is remainder
!= is not equal
Euclid’s algorithm in C
int gcd(int m, int n){ int r; while ( (r = m % n) != 0) { m = n; n = r; } return n;}
High-level control-flow statement. Ultimately becomes a conditional branch.
Supports “structured programming”
Each function returns a single value, usually an integer. Returned through a specific register by convention.
Characteristics of an algorithm
Each step must be exact.
Must terminate.
Must be effective.
Must be general.
Pseudo-code
How to represent an algorithm?
Pseudo-code
Flowchart
Pseudo-code: a combination of English text, mathematical notations, and keywords from the programming language.
Pseudo-code
To find average, min and max among a list.First, you initialise sum to zero, min to a very big number, and max to a
very small number.
Then, you enter the numbers, one by one.
For each number that you have entered, assign it to num and add it to the sum.
At the same time, you compare num with min, if num is smaller than min, let min be num instead.
Similarly, you compare num with max, if num is larger than max, let max be num instead.
After all the numbers have been entered, you divide sum by the numbers of items entered, and let ave be this result.
End of algorithm.
Pseudo-code
To find average, min and max among a list.sum count 0 { sum = sum of numbers;
count = how many numbers are entered? }min ? { min to hold the smallest value eventually }max ? { max to hold the largest value eventually }
for each num entered,
increment count
sum sum + num
if num < min then min num
if num > max then max numave sum/count
Flowchart
Diagrammatic form:
terminator
connector
process box
decision box
Flowchart
To find minimum and maximum among a list.
start
sum count 0min ?max ?
end of input?
increment countsum sum + num
num<min?
num>max?
min num
Yes
No
ave num/count
A
A
end
Yes
max numYes
No
No
Exercise
Dedign a flowchart for gcd function
Write a program for gcd function