Introduction to C programing for problem

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CS11001:
Programming & Data Structures
Dept. of Computer Science & Engineering
Prof. Bivas Mitra

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Course Materials
Books:
1. Programming with C (Second Edition)
Byron Gottfried, Third Edition, Schaum’s Outlines
Series,
2. The C Programming Language
Brian W Kernighan, Dennis M Ritchie
3. Data structures
S. Lipschutz, Schaum’s Outline Series

3
About the Course
 Section 8, 9, 10
 Teacher: Prof. S Dey (SD)
 Section 11, 12
 Teacher: Prof. A Das (AD)
 Section 13, 14
 Mon (9:30-10:25), Wed (8:30-9:25), Thu (9:30-10:25)
 In Classroom F-142 (in Main Building)
 Teacher: Prof. B. Mitra (BM)

 Teachers’ office hours (meeting time outside class):
 BM: Friday (5:30-6:30), Room 315, CSE Dept.
 Homepage: http://cse.iitkgp.ac.in/~bivasm/
 Tutorial classes (one hour per week) will be conducted
on a “per section” basis before Lab hours
 Evaluation in the theory course:
 Mid-semester 30%
 End-semester 50%
 Two class tests and attendance 20%

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Attendance
REALLY
matters
 Important for understanding the course
 Any student with low attendance may be
deregistered from the course
 Leave due to medical reasons must be certified
by the B.C. Roy Technology Hospital

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Important Dates
 Class Test 1: Sept. 2, 2014 (18:30 – 19:30)
 Class Test 2: Nov. 4, 2014 (18:30 – 19:30)
(Class test dates are tentative and may change. The
exact dates will be announced in the class)
 Mid-semester: Sept 15 – Sep 23, 2014
 End-semester: Nov 20 – 28, 2014

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Computer is a machine that can perform computation
1.Input : User gives a set of Input
2.Processing : Input data is processed by a well defined
and finite sequence of steps
3.Output: Some data available from processing step and
output to the user

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A Computer (Level 0 Version)
Central
Processing
Unit (CPU)
Storage
Peripherals
Output
Peripherals
Input
Peripherals
Main
Memory

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I/O and Peripherals: Examples
 Input Devices
 Keyboard, Mouse, Digital Camera
 Output Devices
 Monitor, Printer, Speaker
 Storage Peripherals
 Magnetic Disks: hard disk
 Optical Disks: CDROM, CD-RW, DVD
 Flash Memory: pen drives

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CPU: A first cut
PC
IR
MAR
MDR
ALU
R1
R2
R3
R4 FLAGS
Control Unit

Register
13
Temporary storage inside CPU
1.General purpose Register
2.Special purpose Register
R1
5
PC: Program Counter
IR: Instruction Register
MDR: Memory Data Register
MAR: Memory Address Register

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CPU
MAR
MDR
Memory
Control
Unit
Address Bus
Data Bus
Read Write
ALU
Von Newman Architecture

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C=A+B
Step 1: Fetch Operand A from memory
Step 2: Store in Register R1
Step 3: Fetch Operand B from memory
Step 5: Add content of R1 and R2
Step 6: Store the result in register R3
Step 7: Store the content of R3 in memory C
CPU
MAR
MDR
Memory
Control
Unit
Address Bus
Data Bus
Read
ALU
A
(2)
B
(5)

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C=A+B
CPU
MAR
MDR
Memory
Control
Unit
Address Bus
Data Bus
Read
ALU
A
(2)
B
(5)
A
2

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C=A+B
CPU
MAR
MDR
Memory
Control
Unit
Address Bus
Data Bus
Read
ALU
A
(2)
B
(5)
B
5

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C=A+B
CPU
MAR
MDR
Memory
Control
Unit
Address Bus
Data Bus
Read
ALU
A
(2)
B
(5)
ALU
R1 R2
R3

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C=A+B
CPU
MAR
MDR Memory
Control
Unit
Address Bus
Data Bus
Read
ALU
A
(2)
B
(5)
C
7 C
(7)

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What can a computer do
Solve Types of Problem
1.Functional Problem (Matrix Inverse)
2.Decision Problem (Primality test)
3.Search Problem
4.Optimization problem

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What can a computer do
 Determining if a given integer is a prime number
 Root finding
 Read in airline route information as a matrix and determine the
shortest time journey between two airports
 Telephone pole placement problem
 Matrix multiplication
 Finger-print recognition
 Chess Player
 Speech Recognition
 Language Recognition
 Discovering New Laws in Mathematics
 Automatic drug discovery
 …..

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Programming and Software
Computer needs to be programmed to do such
tasks
Programming is the process of writing
instructions in a language that can be
understood by the computer so that a desired
task can be performed by it
Program: sequence of instructions to do a task,
computer processes the instructions
sequentially one after the other
Software: programs for doing tasks on computers

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Challenges
 CPU understands machine language
Different strings of 0’s and 1’s only!!
Hard to remember and use
 Instruction set of a CPU
Mnemonic names for this strings
CPU
MAR
MDR
Control
Unit
ALU

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Instruction Set
 Start
 Read M
 Write M
 Load Data, M
 Copy M1, M2
 Add M1, M2, M3
 Sub M1, M2, M3
 Compare M1, M2, M3
 Jump L
 J_Zero M, L
 Halt

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Instruction Set
 Start
 Read M
 Write M
 Load Data, M
 Copy M1, M2
 Add M1, M2, M3
 Sub M1, M2, M3
 Compare M1, M2, M3
 Jump L
 J_Zero M, L
 Halt
0: Start
1: Read 10
2: Read 11
3: Add 10, 11, 12
4: Write 12
5: Halt
Program

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Problems with programming using
instruction sets directly
 Instruction sets of different types of CPUs
different
 Need to write different programs for computers with
different types of CPUs even to do the same thing
 Solution: High level languages (C, C++, Java,…)
 CPU neutral, one program for many
 Compiler to convert from high-level program to low
level program that CPU understands

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High-Level Program
Variables x, y;
Begin
Read (x);
Read (y);
If (x >y) then Write (x)
else Write (y);
End.

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High-Level Program
0: Start
1: Read 20
2: Read 21
3: Compare 20, 21, 22
4: J_Zero 22, 7
5: Write 20
6: Jump 8
7: Write 21
8: Halt
Variables x, y;
Begin
Read (x);
Read (y);
If (x >y) then Write (x)
else Write (y);
End.
Low-Level Program

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Three steps in writing programs
Step 1: Write the program in a high-level
language (in your case, C)
Step 2: Compile the program using a C
compiler
Step 3: Run the program (as the computer
to execute it)

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A Computer (Level 0 Version)
Central
Processing
Unit (CPU)
Storage
Peripherals
Output
Peripherals
Input
Peripherals
Main
Memory

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 CPU register and main memory are volatile
 Data gets erased after power switched off
 Secondary storage is non-volatile
 Disk
 Keep the program on the disk
 During execution
 Load the program from the disk to memory Disk
Memory
CPU
Program
How does program run in a
computer?

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computer?
Disk

computer?
 Data area
 Instruction area
 Fetch-decode-execute
33

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computer?
Disk

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PC
MAR
MDR
Memory
Control
Unit
Address Bus
Data Bus
Read Write
ALU
Von Newman Architecture
IR

The fetch-decode-execute
cycle
 Sequence of machine instructions is copied to the instruction area of the memory.
 A particular control register, called the program counter (PC), is loaded with the
address of the first instruction of the program.
 The CPU fetches the instruction from that location in the memory that is currently
stored in the PC register.
 The instruction (in IR) is decoded in the control unit of the CPU.
 The instruction may require one or more operands.
 An operand may be either a data or a memory address. A data may be either a constant (also
called an immediate operand) or a value stored in the data area of the memory or a value
stored in a register. Similarly, an address may be either immediate or a resident of the main
memory or available in a register.
 Finally, data is fetched from the data part of the main memory.
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PC MAR IR
MDR
Read
Instruction fetch

 If the instruction is a data movement operation, the corresponding movement is
performed. For example, a "load" instruction copies the data fetched from memory
to a register, whereas a "store" instruction sends a value from a register to the
data area of the memory.
 If the instruction is an arithmetic or logical instruction, it is executed in the ALU
after all the operands are available in the CPU (in its registers). The output from
the ALU is stored back in a register.
 If the instruction is a jump instruction, the instruction must contain a memory
address to jump to. The program counter (PC) is loaded with this address. A jump
may be conditional, i.e., the PC is loaded with the new address if and only if some
condition(s) is/are true.
 If the instruction is not a jump instruction, the address stored in the PC is
incremented by one.
 If the end of the program is not reached, the CPU continues its fetch-decode-
execute cycle.
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The fetch-decode-execute
cycle

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Binary Representation
 Numbers are represented inside computers in
the base-2 system (Binary Numbers)
 Only two symbols/digits 0 and 1
 Positional weights of digits: 20
, 21
, 22
,…from right to
left for integers
 Decimal number system we use is base-10
 10 digits, from 0 to 9, Positional weights 100
, 101
,
102
,…from right to left for integers
 Example: 723 = 3x100
+ 2x101
+ 7x102

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Binary Numbers
Dec Binary
0 0
1 1
2 10
3 11
4 100
5 101
6 110
7 111
8 1000

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Binary Numbers
Binary to Decimal Conversion
101011  1x25
+ 0x24
+ 1x23
+ 0x22
+ 1x21
+ 1x20
=
43
(101011)2 = (43)10
111001  1x25
+ 1x24
+ 1x23
+ 0x22
+ 0x21
+ 1x20
= 57
(111001)2 = (57)10
10100  1x24
+ 0x23
+ 1x22
+ 0x21
+ 0x20
= 20
(10100)2 = (20)10
Dec Binary
0 0
1 1
2 10
3 11
4 100
5 101
6 110
7 111
8 1000

Decimal to binary conversion
(15)10=>(1111)2
Step 1: Divide the number by the “Base” until the quotient becomes 0
Step 2: Collect the remainder after each step
Finally: Trace the remainders in the reverse direction
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Bits and Bytes
 Bit – a single 1 or 0
 Byte – 8 consecutive bits
 2 bytes = 16 bits
 4 bytes = 32 bits
 Max. integer that can represented
 in 1 byte = 255 (=11111111)
 In 4 bytes = 4294967295 (= 32 1’s)
 No. of integers that can be represented in 1 byte
= 256 (the integers 0, 1, 2, 3,….255)

Bits and Bytes
 Addressable unit
 Length of the word
 Size of memory
 Size of register
CPU
MAR
MDR
Control
Unit
Address Bus
Data Bus
Read
ALU
10
54
90
07
18
0
1
2
3
8 bits
16 bits
word
Size of the memory (16 × 8)
1.16 addressable units
2.Each unit can store 8 bits (word)

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Structure of a C program
 A collection of functions (we will see what they
are later)
 Exactly one special function named main must
be present. Program always starts from there
 Each function has statements (instructions) for
declaration, assignment, condition check,
looping etc.
 Statements are executed one by one

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The C Character Set
 The C language alphabet
 Uppercase letters ‘A’ to ‘Z’
 Lowercase letters ‘a’ to ‘z’
 Digits ‘0’ to ‘9’
 Certain special characters:
A C program should not contain anything else
! # % ^ & * ( )
- _ + = ~ [ ]
| ; : ‘ “ { } ,
. < > / ? blank
Keywords (reserved
words)
Identifies (user
defined words)

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First C program – print on screen
#include <stdio.h>
void main()
{
printf ("Hello, World! n") ;
}
functions
Library
(standard)
User defined

48
Variables
 Very important concept for programming
 An entity that has a value and is known to the
program by a name
 Can store any temporary result while executing a
program
 Can have only one value assigned to it at any given
time during the execution of the program
 The value of a variable can be changed during the
execution of the program

49
Contd.
 Variables stored in memory
 Remember that memory is a list of storage
locations, each having a unique address
 A variable is like a bin
 The contents of the bin is the value of the variable
 The variable name is used to refer to the value of
the variable
 A variable is mapped to a location of the memory,
called its address

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Example
#include <stdio.h>
void main( )
{
int x;
int y;
x=1;
y=3;
printf("x = %d, y= %dn", x, y);
}

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Variables in Memory
Instruction executed Memory location allocated
to a variable X
T
i
m
e
X = 10
10
X = 20
X = X +1
X = X*5

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Variables in Memory
to a variable X
T
i
m
e
X = 10
20
X = 20
X = X +1
X = X*5

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Variables in Memory
to a variable X
T
i
m
e
X = 10
21
X = 20
X = X +1
X = X*5

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Variables in Memory
Instruction executed
Memory location allocated
to a variable X
T
i
m
e
X = 10
105
X = 20
X = X +1
X = X*5

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Variables (contd.)
20
?
X
Y
X = 20
Y=15
X = Y+3
Y=X/6
Main memory

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Variables (contd.)
20
15
X
Y
X = 20
Y=15
X = Y+3
Y=X/6
Main memory

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Variables (contd.)
18
15
X
Y
X = 20
Y=15
X = Y+3
Y=X/6
Main memory

58
Variables (contd.)
18
3
X
Y
X = 20
Y=15
X = Y+3
Y=X/6
Main memory

59
Data Types
 Each variable has a type, indicates what type of
values the variable can hold
 Four common data types in C
int - can store integers (usually 4 bytes)
float - can store single-precision floating point numbers
(usually 4 bytes)
double - can store double-precision floating point numbers
(usually 8 bytes)
char - can store a character (1 byte)

60
Contd.
 Must declare a variable (specify its type and
name) before using it anywhere in your program
 All variable declarations should be at the
beginning of the main() or other functions
 A value can also be assigned to a variable at the
time the variable is declared.
int speed = 30;
char flag = ‘y’;

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Variable Names
 Sequence of letters and digits
 First character must be a letter or ‘_’
 No special characters other than ‘_’
 No blank in between
 Names are case-sensitive (max and Max are two
different names)
 Examples of valid names:
 i rank1 MAX max Min class_rank
 Examples of invalid names:
 a’s fact rec 2sqroot class,rank

More Valid and Invalid Identifiers
 Valid identifiers
X
abc
simple_interest
a123
LIST
stud_name
Empl_1
Empl_2
avg_empl_salary
 Invalid identifiers
10abc
my-name
“hello”
simple interest
(area)
%rate

C Keywords
 Used by the C language, cannot be used
as variable names
 Examples:
int, float, char, double, main, if else, for, while.
do, struct, union, typedef, enum, void, return,
signed, unsigned, case, break, sizeof,….
There are others, see textbook…

64
Output: printf function
 Performs output to the standard output device
(typically defined to be the screen)
 It requires a format string in which we can specify:
 The text to be printed out
 Specifications on how to print the values
printf ("The number is %dn", num);
 The format specification %d causes the value listed
after the format string to be embedded in the output
as a decimal number in place of %d
 Output will appear as: The number is 125

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Contd.
 General syntax:
printf (format string, arg1, arg2, …, argn);
format string refers to a string containing formatting
information and data types of the arguments to be
output
the arguments arg1, arg2, … represent list of
variables/expressions whose values are to be printed
 The conversion characters are the same as in
scanf

66
Input: scanf function
 Takes input from keyboard
 It requires a format string and a list of variables into which
the value received from the keyboard will be stored
 format string = individual groups of characters (usually ‘%’
sign, followed by a conversion character), with one
character group for each variable in the list
int a, b;
float c;
scanf(“%d %d %f”, &a, &b, &c);
Format string
Variable list (note the &
before a variable name)

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 Commonly used conversion characters
c for char type variable
d for int type variable
f for float type variable
lf for double type variable
Examples
scanf ("%d", &size) ;
scanf ("%c", &nextchar) ;
scanf ("%f", &length) ;
scanf (“%d%d”, &a, &b);

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#include <stdio.h>
void main()
{
int num ;
scanf ("%d", &num) ;
printf (“No. of students is %dn”, num) ;
}
Reading values from keyboard

69
Write a C program that reads a temperature in
Celsius, for example t C, and outputs the
equivalent temperature in Fahrenheit, f F.

70
Centigrade to Fahrenheit
#include <stdio.h>
int main()
{
float cent, fahr;
scanf(“%f”,&cent);
fahr = cent*(9.0/5.0) + 32;
printf( “%f C equals %f Fn”, cent, fahr);
}

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Example 1
#include <stdio.h>
void main()
{
int x, y, sum;
scanf(“%d%d”,&x,&y);
sum = x + y;
printf( “%d plus %d is %dn”, x, y, sum );
}
Three int type variables declared
Values assigned

72
Example - 2
#include <stdio.h>
void main()
{
float x, y;
int d1, d2 = 10;
scanf(“%f%f%d”,&x, &y, &d1);
printf( “%f plus %f is %fn”, x, y, x+y);
printf( “%d minus %d is %dn”, d1, d2, d1-d2);
}
Assigns an initial value to d2,
can be changed later

73
Read-only variables
 Variables whose values can be initialized during
declaration, but cannot be changed after that
 Declared by putting the const keyword in front of
the declaration
 Storage allocated just like any variable
 Used for variables whose values need not be
changed
 Prevents accidental change of the value

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void main() {
const int LIMIT = 10;
int n;
scanf(“%d”, &n);
if (n > LIMIT)
printf(“Out of limit”);
}
void main() {
const int Limit = 10;
int n;
scanf(“%d”, &n);
Limit = Limit + n;
printf(“New limit is %d”, Limit);
}
Correct
Incorrect: Limit changed

75
Contd.
 Character constants
 Contains a single character enclosed within a pair of
single quote marks.
Examples :: ‘2’, ‘+’, ‘Z’
 Some special backslash characters
‘n’ new line
‘t’ horizontal tab
‘’’ single quote
‘”’ double quote
‘’ backslash

76
Reading a single character
 A single character can be read using scanf with
%c
 It can also be read using the getchar() function
char c;
c = getchar();
 Program waits at the getchar() line until a
character is typed, and then reads it and stores it
in c

Program and Algorithm
 Swap two elements
 Search an element from the list
 …
77
Problem or
Task
Design the
Algorithm (Logic)
Write the
program (say
in C)

Introduction to C programing for problem

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