Lecture 2 Introduction to C Programming

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Lecture 2Introduction to C Programming

• Arne Kutzner
• Hanyang University / Seoul Korea

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This week

• You will learn
• From the first Computers to the C Programming
  Languages
• Von Neumann Architecture
• The Idea of binary computing
• Origins of Programming Languages
• About Flowcharts and Algorithms

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Von Neumanns Computer Architecture

• Foundations of modern Computers and their
  Programming

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Computers until the second world war and before

• The earliest computing machines had fixed
  programs.
• The earliest computers were not so much
  "programmed" as they were "designed".
• "Reprogramming", when it was possible at all, was
  a laborious process, starting with flow charts
  and paper notes, followed by detailed engineering
  designs, and then the often arduous process of
  physically rewiring and rebuilding the machine.

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Examples of fixed program computers

• Colossus Mark I (Dec. 1943) / Colossus Mark II
  (June 1944)

Purpose Message Decoding during World War 2
(was used to help decipher teleprinter messages
which had been encrypted using the Lorenz SZ40/42
machine (Enigma))
INFO in the Web http//en.wikipedia.org/wiki/Colo
ssus_computer
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Von Neumann Architecture (proposed 1945)
Memory
Control Unit
Arithmetic Logic Unit
Accumulator
Input
Output
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Von Neumann Architecture (2)

• The Control Unit understands instructions like
  e.g. copy data from the memory to the accumulator
  gt Instruction Set Architecture
• The Arithmetic Logic Unit performs arithmetic
  operations and Boolean operations.
• The Memory stores data and Program (Series of
  control unit instructions
• The Input takes data in form of an data stream
  from some medium
• The Output receives data in form of data stream
  and delivers them to some output medium

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Von Neumann Architecture (3)

• Von Neumanns approach was revolutionary
  becauseHe created an instruction set
  architecture and detailed the computation as the
  execution of a series of instructions (the
  program) stored in the memory.
• Program and data share the memory !!
• Highly flexible computer design

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Structure of Memory
Addresses
Memory Cells

• Ordered sequence of storage locations called
  memory cells.
• Each memory cell is identified by a unique address

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0000
87
0001
06
0002
00
0003
12
0004
00
0005
Content of a memory cell
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Instruction representation

• Instruction are encoded using sequences of bits
• Example PDP-11(single operand instructions)

INFO in the Web http//en.wikipedia.org/wiki/PDP-
11
16 bit 2 byte
10 bit
3 bit
3 bit
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Computer nowadays?

• Almost all modern computer have still a Von
  Neumann Architecture

CPU
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How to program von Neumann computer ?
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Assembler Code

• Sequence of processor instructions in human
  readable text-form this is called Assembler
  Code
• Example for PDP-11 Assembler Code

.TITLE HELLO WORLD .MCALL
.TTYOUT,.EXIT HELLO MOV MSG,R1 STARTING
ADDRESS OF STRING1 MOVB (R1),R0 FETCH
NEXT CHARACTER BEQ DONE IF ZERO,
EXIT LOOP .TTYOUT OTHERWISE
PRINT IT BR 1 REPEAT
LOOPDONE .EXIT MSG .ASCIZ /Hello,
world!/ .END HELLO
This PDP-11 Assembler Code prints Hello World
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Assembler and Programming Languages

• Assembler was the first form of some primitive
  (low-level) programming language
• DefinitionA programming language is a set of
  rules that provides a way of telling a computer
  what operations to perform.

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Disadvantages of Assembler Code

• Low level of abstraction
• Code is processor (vendor)-specificTherefore
  Non portable
• The code is unstructured
• Difficult to read
• Risk of spaghetti-structure
• Time intensive to learn and to program

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High-level Programming Languages

• The wish for some programming medium that is more
  abstract than assembler lead to the development
  of High-level Programming Languages
• DefinitionA high-level programming language is
  a programming language that, in comparison to
  low-level programming languages, may be more
  abstract, easier to use, or more portable across
  platforms.

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Early high-level programming languages

• FortranFor programming in the scientific area
  (number crunching)
• CobolFor programming of business applications
  (e.g. banking applications)
• LispDevelopment originally motivated be
  research in the Artificial Intelligence area

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Fortran Code Example
c2345678901234567890123456789012345678901234567890
1234567890123456789012 PROGRAM spherec This
program determines the surface area and volume of
a sphere, given c its radius.c Variable
declarations REAL rad, area, volume, pic
Definition of variablesc rad radius, area
surface area, volume volume of the spherec
Assign a value to the variable pi. pi
3.141593c Input the value of the radius and
echo the inputted value. PRINT , 'Enter the
radius of the sphere. ' READ , rad PRINT ,
rad, ' is the value of the radius. 'c Compute
the surface area and volume of the sphere. area
4.0 pi rad2 volume (4.0/3.0) pi
rad3c Print the values of the radius (given
in cm), the surface area (sq cm),c and the
volume (cubic cm). PRINT ,'In a sphere of
radius', rad, ' , the surface area is',
area, ' and its volume is', volume, '.
' STOP END
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COBOL Code Example
SET SOURCEFORMAT"FREE"IDENTIFICATION
DIVISION.PROGRAM-ID. Multiplier.AUTHOR.
Michael Coughlan. Example program using ACCEPT,
DISPLAY and MULTIPLY to get two single digit
numbers from the user and multiply them
together DATA DIVISION. WORKING-STORAGE
SECTION.01 Num1
PIC 9 VALUE ZEROS.01 Num2
PIC 9 VALUE ZEROS.01 Result
PIC 99 VALUE ZEROS. PROCEDURE
DIVISION. DISPLAY "Enter first number (1
digit) " WITH NO ADVANCING. ACCEPT Num1.
DISPLAY "Enter second number (1 digit) " WITH
NO ADVANCING. ACCEPT Num2. MULTIPLY Num1
BY Num2 GIVING Result. DISPLAY "Result is
", Result. STOP RUN.
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Programming Languages
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Why was C created ?

• The origin of C is closely tied to the
  development of the Unix operating system
• Originally Unix was implemented in Assembler for
  the PDP-7 (1970)
• 1973 Unix was rewritten for the PDP-11
  architecture using the newly developed C
  Programming Language
• An older language B was unsuitable for this
  purpose due to some language design lacks

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Characteristics of C

• Primitive but fast
• Quite low-level compared to e.g. Java
• Some people say More or less like assembler but
  independent of any specific instruction set
• Strange and quite cryptic syntax
• Difficult to grasp

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The Idea of Binary Computing

• Binary Number, Hexadecimal Number and the
  encoding of characters

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Binary Numbers

• A sequence of 1s and 0s can be interpreted as a
  decimal numbere.g. 101 122 021 120 5
• These type of number (0 and 1 represented) is
  called Binary Number

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8 bits 1 Byte

• Todays computer architectures are based on a 8
  bits ( 1 Byte) approach.
• E.g. one memory cell has nowadays 1 Byte

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6
5
4
3
2
1
0
1
0
1
0
1
1
0
0
172
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Hexadecimal Numbers (1)

• Counting on the foundation of 16 digits (0 F)
• Reasonable in the context of the decomposition of
  binary number into blocks of 4 bits

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Hexadecimal Numbers (2)
16 bit

• Example

8 bit (1 Byte)
8 bit (1 Byte)
binary
hex
decimal
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Exercise

• What is the number 01000111 in decimal notation?
• What is the number 156 in hexadecimal notation?

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Characters and Numbers

• Using a Code Table numbers can be mapped to
  characters of some alphabet.Example (part of the
  ASCII table)

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Sequences of Characters (Strings)

• Text is represented as a sequence of characters
  (numbers)
• Such sequences are called Strings
• Example

The Text "Hello" as sequence of characters
(numbers)
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Algorithms and Flowcharts

• Binary Number, Hexadecimal Number and the
  encoding of characters

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Algorithms - Introduction

• In order to solve some problem, we need some
  description how to solve it.E.g. Cooking some
  eggsHow to cook eggs?
• Put some water into a pot
• Heat the water until it cooks
• Put the eggs into the water
• Wait 3 Minutes
• Take the eggs out of the pot and cool them under
  floating water
• Such a description of a problem solution is
  called an Algorithm

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Algorithms described as Flowcharts
START
Our Cook-eggs-Algorithm as Flowchart
No
Water in pot ?
put water into pot
Yes
Wait 3 Minutes
Heat water
Take eggs out of pot
Water cooking ?
No
Cool the eggs
Yes
END
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Flowchart Atoms
Statement

• Statement Some operation as e.g. put the eggs
  into water

put water into pot
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Flowchart Atoms (cont.)
No
Condition
Yes

• Condition Some yes/no question (condition).
  Depending on the answer Yes (True) or No (False)
  we go different ways.

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Combined Flowchart Constructs
Yes
No
Condition
if side
else side
Statement
Statement
Statement
Statement

• If/else alternative Construct where select on
  of two alternative sequences of statements,
  depending on the result of the condition

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Combined Flowchart Constructs (cont.)

• If/else example (Here we do nothing on one side,
  but this is ok)

No
Water in pot ?
put water into pot
Yes
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Combined Flowchart Constructs (cont.)
Statement 1
Statement n
Question

• Loop Special construct with a Condition where
  one alternative of the Condition creates a
  circular flow

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Combined Flowchart Constructs (cont.)

• Loop Example

Heat water
Water not cooking ?
YES
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Algorithmic Description

• Solutions for all realistic Problems can be
  algorithmically described by combing the
  introduced elements
• Statements
• If/else alternatives
• Loops

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Variables

• Variables are named containers that store some
  type of data, e.g. a number.Example

Variable x
Value of x
x
0
read x from user input
user input is 93
x
93
User input is typically something typed on the
keyboard
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Variables in Flowcharts

• Example Calculation of the maximum of two
  numbers

read x from user input
read y from user input
Yes
No
x gt y
set max to x
set max to y
print max on output
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Exercise

• Take a piece of paper and draw a flowchart that
• reads the value of three variables x and y and z
• calculates the minimum of the three variables
  read before