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• Why C?• History of C• Generic C Program Structure• Variables

– Type and Name– Assignment Operator

• Simple I/O– scanf/printf

Today’s Material

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Why C?• Simple and Simple and StructuralStructural

– Minimal syntaxMinimal syntax

• Availability and PortabilityAvailability and Portability– C compilers are available for a wide range of C compilers are available for a wide range of

platformsplatforms• Windows, UNIX, embedded systemsWindows, UNIX, embedded systems

• Language of choice for system programming • Most OS code are written in CMost OS code are written in C

• Widely knownWidely known– Almost every programmer knows CAlmost every programmer knows C– There are lots of public code written in CThere are lots of public code written in C

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History of C• C C evolvedevolved as follows:as follows:

– CPL Combined Programming Language (Barron, CPL Combined Programming Language (Barron, 1963)1963)

– BCPL Basic CPL (Richards, 1969)BCPL Basic CPL (Richards, 1969)– B (Thompson, 1970)B (Thompson, 1970)– C (C (Kernighan & Kernighan & Ritchie, 1973)Ritchie, 1973)– ANSI C American National Standards Institute C ANSI C American National Standards Institute C

(X3J11, 1989)(X3J11, 1989)!! The fact that a language originated in 1973 has required as little change as this one has in thirty years of heavy use is truly remarkable, and without parallels anywhere else in computer science or engineering.

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Generic C Program Structure#include <stdio.h>

/* main designates the starting place of our program */main() { /* Variables hold Data Items Needed in the Program */ Variable Declarations;

/* Steps of our program: I/O, computation (expressions) */

Statement1; Statement2; …

StatementN; } /* end-of-the program */

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First C Program#include <stdio.h>

/* Convert fahrenheit to celsius */main() { float fahrenheit; float celsius;

printf(“Enter a temp in fahrenheit: “);

scanf(“%f”, &fahrenheit); celsius = (fahrenheit-32)/1.8; printf(“%f degrees fahrenheit

equals %f degrees celsius\n”, fahrenheit, celsius);

}

Get the fahrenheit temperature

celsius = (fahrenheit-32)/1.8

Print the fahrenheit and celsius degrees

StartStart

EndEnd

Prompt the user for a temperature

PROBLEMConvert a fahrenheit temperature to celsius

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First C Program Dissected#include <stdio.h>

/* Convert fahrenheit to celsius */main() { float fahrenheit; float celsius;

printf(“Enter a temp in fahrenheit: “);

scanf(“%f”, &fahrenheit); celsius = (fahrenheit-32)/1.8; printf(“%f degrees fahrenheit equals

%f degrees celsius\n”, fahrenheit, celsius);

}

Load the standard library to handle I/O

Comments: In between /* …. */

main(): Designates where the execution will start

Curly bracesDetermines the beginning and the end of a code block

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First C Program Dissected (cont)

#include <stdio.h>

/* Convert fahrenheit to celsius */main() { float fahrenheit; float celsius;

printf(“Enter a temp in fahrenheit: “);

scanf(“%f”, &fahrenheit); celsius = (fahrenheit-32)/1.8; printf(“%f degrees fahrenheit

equals %f degrees celsius\n”, fahrenheit, celsius);

}

Statements• Steps of your program. • Statements end with a semicolon (;)

Variables• A variable is a memory location whose contents can be filled and changed during program execution• float is the variable type• fahrenheit & celsius are variable names

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First C Program Dissected (cont)

#include <stdio.h>

/* Convert fahrenheit to celsius */main() { float fahrenheit; float celsius;

printf(“Enter a temp in fahrenheit: “);

scanf(“%f”, &fahrenheit); celsius = (fahrenheit-32)/1.8; printf(“%f degrees fahrenheit

equals %f degrees celsius\n”, fahrenheit, celsius);

}

OutputUse printf function to print something on the screen

InputUse scanf function to read something from the keyboard

ComputationUse mathematical operators to perform computation

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From C Code-2-Machine Code

• Now that we have our C program, how does it get translated to machine code, i.e., to 0s and 1s– Remember a computer

understands just 0s and 1s

• We use several system software components to translate out C program to machine code– Compiler, assembler,

linker, loader+OS

prog.c

COMPILER

ASSEMBLER

LINKER

LOADER + OS

prog.s

prog.o printf.o

scanf.o

prog.exe

program running

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First C Program Executed

#include <stdio.h>

/* Convert fahrenheit to celsius */main() { float fahrenheit, celsius;

printf(“Enter a temp in fahrenheit: “); scanf(“%f”, &fahrenheit); celsius = (fahrenheit-32)/1.8; printf(“%f degrees fahrenheit equals %f

degrees celsius\n”, fahrenheit, celsius);

} /* end-main */

?fahrenheit

?celsius

DATA

?75.4

24.11

PROGRAM

Enter a temp in fahrenheit: 75.4 degrees fahrenheit equals 24.11 degrees celsius

75.4

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What’s a Variable?• A memory location whose

contents can be filled and changed during program execution– Each variable has a type and name– Type defines the type of information

that can be stored in that memory location

– Name is a label that allows us to refer to that memory location

MemoryMemory

namenamenumber2number2number1number1

– int number1; /* stores a natural number (152) */– float number2; /* stores a real number (65.324) */

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Variable Declaration• The basic format for declaring variables is datatype varName, varName, ... ;

where datatype may beint /* Stores a natural number, e.g., 34532 */

float /* Stores a real number, e.g., 15.342 */

double /* Also stores a real number, but more precision */

/* e.g., 345.22359573943 */

char /* Stores an ASCII char, e.g., ‘A’ */

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Variable (Identifier) Names• C identifiers consist of letters and digits in

any order, except that:– The first character must be a letter

• fahrenheit, celsius, sum, a123, i1, i2, i3

– The identifier can be in lowercase or uppercase – The upper and lower cases may be mixed

• sum, Sum, SUM

– The underscore (_) can be included and it can also be the first char of the identifier

• total_sum, num_students, _localVar

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Variable (Identifier) Names (cont)

• C identifiers consist of letters and digits in any order, except that:– The identifiers should not contain a blank

space, hyphen or quotes • “sum” : quotes(“) is illegal• total-sum : illegal character ( - ) • total sum : blank space should not be there

– Identifier names are case-sensitive• sum, Sum and sUm are all different identifiers

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Variable Examples & Assignment

int x; /* Define 1 int */float fahrenheit, celsius; /* Define 2 floats */double d; /* Define 1 double */

x = -85; /* Assign -85 to x */d = 3.4545673; /* Assign 3.4545673 to d */fahrenheit = 75.0; /* Assign 75.0 to fahrenheit */celsius = 23.4; /* Assign 23.4 to celsius */

• = is the assignment operator– Syntax: identifier = value;– Changes the contents of the memory location

that the variable refers to

-85

x

3.4545673

d

75.0

fahrenheit

23.4

celsius

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Variable Declaration & Initialization

int sum = 32000; /* can be positive */int x = -23, y = 458; /* and negative */

float f1 = 34.5;float fahrenheit = 75.3, celsius = 0;

double d1, d2 = -4.567; /* Can be negative */double d3 = 5e+3; /* Scientific notation: 5x10^3 */

d1 = 675e-4; /* 675x10^-4 = 0.0675 */

• Variables can be initialized during declaration

32000

sum

-23

x

458

y

34.5

f1

75.3

fahrenheit

0.0

celsius

0.0675

d1

-4.567

d2

5000

d3

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Character Variables• Represents a single character

– Characters are letters of the alphabet (both upper and lower case)

– Ten digits 0 through 9– Special symbols such as + $ . , - !

• Characters need to be enclosed in single quotes– e.g. 'A'

char letter;

letter = 'A'; /* Letter A */ letter = ‘9’; /* Digit 9 */

char letter = ‘c’; /* initialization */

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Character Variables (more)• In fact, char corresponds to 1 byte natural

number– char type variables occupy 1 byte in memory

• What’s really stored in a char variable is the ASCII value of the character– ASCII value of ‘A’ is 65– ASCII value of ‘B’ is 66

– ASCII value of ‘0’ is 48– ASCII value of ‘1’ is 49

– http://www.asciitable.com/

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ASCII Table

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Special Characters• Characters are enclosed in single quotes

– How do I denote a single quote?

char letter;letter = ''';

char letter;letter = '\'';

• When a backslash (\) is used in front of a character, the combination is called an Escape Sequence

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Escape Sequences• Combinations of a backslash (\) and a

character that tells the compiler to escape from the ways these character would normally be interpreted

• Commonly used escape sequences – \n move to next line– \t move to next tab setting– \\ backslash character– \' single quote– \" double quote

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sizeof Operator

int a = sizeof(char); /* Returns 1 */int b = sizeof(short); /* Returns 2 */int c = sizeof(int); /* Returns 4 */int d = sizeof(long); /* Returns 4 or 8 */

int e = sizeof(float); /* Returns 4 */int f = sizeof(double); /* Returns 8 */

• You can learn the number of bytes that a certain type occupies in memory using the sizeof operator

1

a

2

b4

c

4/8

d

4

e

8

f

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Keyboard Input/ Display Output

#include <stdio.h>

int main() {scanf();printf();

} /* end-main */

• Achieved using scanf/printf functions– Definitions of these functions are in <stdio.h>– So we must include these definitions at the start

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printf/scanf Functions• Formatted input/output functions in C

– Definitions in standard I/O library <stdio.h>– If we are using these functions in our programs,

we must include these definitions by #include <stdio.h> at the beginning of our programs

printf(FormatString, expression1, expression2, …);scanf(FormatString, &variable1, &variable2, …);

• FormatString is enclosed in double quotes– “abcdxyz”– “number is %d\n”

• Format specification %d is used to print an int

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printf Examplesint x = 45;float y = 56.7;

printf(“What’s up?\n”); /* Print a msg only */

printf(“Number x: %d, 2 times y: %f\n”, x, y);

printf(“Sum of x and y is %f\n”, x+y);

Format sectionFormat section Expression Expression sectionsection

What’s up?Number x: 45, 2 times y: 56.70000Sum of x and y is 101.7000

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printf Format Specifiers

Character

Type Output Format

c char A single-byte character

d int Signed decimal integer

f float Signed value having the form [ – ]dddd.dddd, where dddd is one or more decimal digits

lf double Signed value having the form [ – ]dddd.dddd, where dddd is one or more decimal digits

e floatdouble

Signed real number having the form

[ – ]d.dddd e [sign]ddd

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printf Examples

printf(“First line.\nSecond line. Percent char %% XYZ\n”);printf(“3rd line. Slash \\, single quote \`\n”);printf(“4th line. Double quote \”. End\n”);printf(“Line 5. char A: %c, ASCII value: %d\n”, ‘A’, ‘A’);printf(“Line 6. Tab\tTab.\n”);printf(“Line 7. Carriage return:\rOK\n”);printf(“Line 8.\n”);

First line.Second line. Percent char % XYZ3rd line. Slash \\, single quote `4th line. Double quote “. EndLine 5. char A: A, ASCII value: 65Line 6. Tab Tab.OKne 7. Carriage return:Line 8.

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printf Examples

char c1 = ‘A’, c2 = ‘c’, c3 = ‘9’, c4 = ‘\\’;int x = 11, y = 16;float f = 56.7;double d = 456.789345;

printf(“c1 is <%c>, c2 is <%c>, c3: <%c>, c4: <%c>\n”, c1, c2, c3, c4); printf(“x is <%d>, y is <%d>, avg is <%d>\n”, x, y,

(x+y)/2);printf(“Sum of <%f> and <%lf> is <%lf>\n”, f, d, f+d);

c1 is <A>, c2 is <c>, c3: <9>, c4: <\>x is <11>, y is <16>, avg is <13>Sum of <56.70000> and <456.7893450000000> is <513.4893450000000>

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printf: Specifying Field Length &

Justificationchar c1 = ‘A’, c2 = ‘c’;int x = 11, y = 16;float f = 56.7;double d = 456.789345;

printf(“+-------+-------+\n”);printf(“|%7c|%-7c|\n”, c1, c2);printf(“|%-7d|%7d|\n”, x, y);printf(“|%7.2f|%-7.1lf|\n”, f, d);Printf(“+-------+-------+\n”);

+-------+-------+| A|c ||11 | 16|| 56.70|456.7 |+-------+-------+

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More printf Examples

printf("%.9f\n", 300.00145678901f);printf("%.19lf\n", 300.0014567890123456789);

300.001464844300.0014567890123700000

float has 7 digit precision here (3000014) double has 16 digits (3000014567890123)

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scanf Exampleschar c;int x;float y;double d;

scanf(“%d”, &x); /* Read 1 int */

scanf(“%c”, &c); /* Read 1 char */scanf(“%f”, &y); /* Read 1 float */scanf(“%lf”, &d); /* Read 1 double */

/* Read 1 int, 1 float, 1 double */scanf(“%d%f%lf”, &x, &y, &d);

Format Format sectionsection

Variable Variable AddressAddress

• “%c” char• “%d” int• “%f” float• “%lf”

double

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getchar and putchar Functions

• getchar reads a single character from the keyboard• putchar writes a single character on the screen• Example:

char c;

printf(“What-to-do Menu \n”);printf(“ (a) To write a C program \n”);printf(“ (b) To go swimming \n”);printf(“ (c) To watch TV \n”);printf(“Select one option: ”);

c = getchar(); /* Read the user choice */getchar(); /* Skip newline ‘\n’ char */

putchar(‘B’); /* Prints B on the screen */c = ‘Z’; putchar(c); /* prints Z on the screen */