Department of CSE1

1.1 Introduction to C Language

Objectives

2 Department of CSE

• To understand the structure of a C-Language Program

• To write a minimal C program

• To introduce the include preprocessor command

• To be able to create good identifiers for quantities in a program

• To be able to list, describe and use the basic data types in C

• To be able to create and use variables and constants in a C

program

Agenda

• Background of C Language

• Structure of a C program

• C Comments

• Identifiers in C

• Data types in C

• Variables in C

• Constants in C

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Background of C

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• C is a middle level language it combines the best elements of high-level

languages with the control and flexibility of assembly language

• Like most modern languages, C is also derived from ALGOL 60 (1960)

• Developed by Dennis Ritchie in 1972 using many concepts from its

predecessors – ALGOL,BCPL and B and by adding the concept of data

types

• American National Standards Institute (ANSI) began the standardization of

C in 1983 which was approved in 1989

• In 1990 International Standards Organization (ISO) adopted the ANSI

standard version known as C89

• Minor changes were made to C89 in 1995 which came to be known as

C95

• Much more significant updates were made in 1999 and named as C99

Features of C

• C is a structured programming language which allows compartmentalization

of code and data

• A structured language offers a variety of programming possibilities. For

example, structured languages typically support several loop constructs,

such as while, do-while, and for.

• A structured language allows one to place statements anywhere on a line

and does not require a strict field concept

• In a structured language, the use of goto is either prohibited or

discouraged and is not the common form of program control

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Structure of a C Program

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Preprocessor Directives

Global Declarations

int main (parameter list)

{

}

Local Declarations

Statements

Other functions as required

Describes the data which is visible to all parts

of the program (optional)

• main function is the starting point of every C

program

• One and only one function must be named

main in a C program

•Describes the data used in the function

•Visible only inside the function (optional)

To include standard input/output library file in

the program

•Follows the local declaration section

•Contains the instructions for the computer to

do something (e.g., add two numbers)

Optional functions also called modules similar

to main to accomplish specific tasks

Dissecting a minimal C program

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#include <stdio.h>

int main(void)

{

printf(“Welcome to Computer Programming”);

return 0;

}

Preprocessor Directive to

include stdio.h file

main function

Instruction to print a message

Instruction to stop the program

The first C program has

•Preprocessor Commands

• which are placed at the beginning of the program starting with # sign

• can start at any column but traditionally start at column 1

• tells the compiler to include standard input/output library file (stdio.h) in the

program which is needed for printing the welcome message on the screen

• must be typed exactly as shown above without any space between # and include

and the file name is written between < and >.

• General Syntax (writing format) of a Preprocessor Directive is

#include<filename.extension>

Examples:

#include<stdio.h> - Required for using input/ output instructions

#include<string.h> - Required for using pre-defined string functions

#include<math.h> - Required for using pre-defined math functions

• Main function

• Executable part of the program begins with main function

General Syntax:

int main(void)

• The word int before the main indicates that the function will return an integer value to the operating system

• Since in this program main does not require any parameters, the parameter list is specified as void

Dissecting the minimal C program

Creating and Compiling the minimal C program

• In the linux environment, locate gedit and open the editor

• Type in the minimal C program exactly with all puncuations as follows

• After typing the preceding source code, save the program as welcome.c.

• Instead of welcome, any name can be chosen, but the extension must be .c.

• This extension is the common convention in C programs and identifies the contents of the file as C source code.

• Most compilers will expect the source file to have the extension .c, and if it doesn‟t, the compiler may refuse to process it.

• After saving the program, locate terminal and type cc welcome.c near the $ symbol

#include <stdio.h>

int main(void)

{

printf(“Welcome to Computer Programming”);

return 0;

}

Executing the minimal C program

• If the program is present in the correct path and is error free, the compilation results in a $ being displayed right below the previous command typed

• If there is a syntax error, open the file again using gedit, locate the line number indicated by the compiler and do the necessary corrections

• If the compiler says file not found, change into the folder which contains the program to be compiled and repeat the compilation step

• Upon successful compilation, type ./a.out at the $ prompt and the program must print Welcome to Computer Programming

Summary of writing and executing a C program

C Comments

• Placed by the programmer in the code to help the reader to

understand the meaning of sections of code which are not very clear

• Comments are merely internal program documentations

• Compiler ignores the comments when it translates the program into

executable code

• To identify a comment, C uses two different comment formats:

• Line Comment

• Block Comment

Line Comment

• The Second format, the line comment uses two slashes - // to

identify a comment

• This format does not require an end-of-comment token

• The end of line automatically ends the comment

• Programmers generally use this format for short comments

• The line comment can start anywhere on a line and ends with the

end of that line

• Examples of Line Comment:

//This is a whole line comment

A=5; //This is a partial line comment

Block Comment

• Block comment is used when a comment spans several lines

• It uses opening and closing comment tokens

• Each comment token is made up of two characters without spaces

in between them

• The opening comment token is represented as /*

• The closing comment token is represented as */

• Everything that is placed in between /* and */ is ignored by the

compiler

• The block comments can start in any column and they need not

both be on the same line

• The only restriction is that the opening token /* must precede the

closing token */

Block Comment - Examples

/* This is a block comment that covers one line*/

/* This is a block comment that

covers two lines*/

/*

**Block comments can also be designed with the opening

**comment on a separate line and closing comment on a

**separate line. Some programmers also put at the beginning of

**each line in between to clearly mark the documentation part.

*/

Caution with Comments

• Comments cannot be nested i.e., comments cannot be given inside comments

• Example:

• /* This is an /*inner comment*/outer comment*/

• Once the compiler sees an opening block comment token /*, it ignores

everything it sees until it finds a closing token */

• Therefore, the opening token of the nested comment is not recognized and the

closing token of the nested comment matches the outer opening comment

thereby leaving the closing token of the outer comment without a matching

opening token

ignored Matches the first /* Left without an

opening /*

Minimal C program revisited with comments

/* Welcome to the minimal C program. This program demonstrates a small C

program

Written by: name

Date: program written date

*/

#include <stdio.h> // this is a preprocessor directive

int main(void) // this is the main function

{ //main begins here

//No local declarations needed for this simple program

//Statements

printf(“Welcome to Computer Programming”); //This is a print statement

return 0; //returns the control back to the operating system

} //main ends here

The C Character Set

• Character set specifies the valid symbols that can be used in a C

program

• The C character set consists of the following symbols and escape

sequence characters

Escape Sequence characters

• Escape sequence characters are a

combination of two characters but

treated as a single character.

• Often used in printf statements,they

are not printed explicitly on the

screen but the effect of these

characters can be observed.

• The table beside shows some ASCII

characters and their corresponding

escape character format

ASCII Character Escape representation

null character „\0‟

alert (bell) „\a‟

backspace „\b‟

horizontal tab „\t‟

newline „\n‟

vertical tab „\v‟

form feed „\f‟

carriage return „\r‟

single quote „\‟‟

double quote „\”‟

backslach „\\‟

Exercises

• Write a program that will output your name and address using a

separate printf() statement for each line of output

• Write a program to print the pattern of asterisks as shown below

• Write a program to print the following figure using suitable

characters

Solutions to Exercises

#include<stdio.h>

int main()

{

printf("Raghesh Krishnan K, ");

printf("Asst.Professor, CSE Dept, ");

printf("Amrita School of Engineering, ");

printf("Amrita Vishwa Vidyapeetham, ");

printf("Amrita Nagar Post, ");

printf("Coimbatore 641 112 ");

return 0;

}

Solutions to Exercises

//Prints a triangular pattern - Author: Raghesh Krishnan K

#include<stdio.h>

int main()

{

printf("*\n"); //prints first line

printf("*\t*\n");

printf("*\t*\t*\n");

printf("*\t*\t*\t*\n"); //prints last line

return 0;

}

//Prints a pattern - Author: Raghesh Krishnan K

#include<stdio.h>

int main()

{

printf("---------- ----------\n"); //prints the top horizontal

bar

printf("| | >>---> | |\n"); /* prints vertical bars*/

printf("| | | |\n");

printf("---------- ----------\n"); //prints the top horizontal

bar

return 0;

}

Solutions to Exercises

Identifiers in C – (Recollect Name Bindings in

CTPS)• Identifiers allow us to name data and other user created components

in a program

• Each identified component is stored at a unique location in the computer‟s memory

• With no identifiers to symbolically represent data locations, we should know and use the component‟s memory address

• Using Identifiers, we simply name the data and let the computer keep track of where they are physically located in the computer‟s memory

Rules for Identifiers in C

• The only valid alphabets to be used for naming are upper case letters from A to Z and lower case letters from a to z

• _ (underscore) is the only special symbol that can be used in an identifier

• Numeric symbols from 0 to 9 can be used but the first symbol of an identifier cannot be a numeric symbol

• The first symbol of an identifier must be an alphabet or an underscore

• Typically application programs do not use underscore as the first symbol because many of the identifiers in C system libraries start with an underscore

• Duplication of system names for identifiers could be confusing

• Last but not the least, C has 32 keywords also known as reserved words, which have a predefined meaning and cannot be used as identifier names.

Some Keywords/Reserved words in C

Examples of Valid and Invalid Identifier Names

Valid Names Invalid Names

a //valid but poor style $sum //$ not allowed

student_name //ok 2names //first character should

not be a digit

_aSystemName //valid but similar to

system identifier pattern

Sum-salary //Hyphen not allowed

_Bool //Boolean System id Stdnt Nmbr //Spaces not allowed

INT_MIN //System Defined Value int //keyword not allowed

Exercises:

• Identify whether the following identifiers are valid. If not give reason and correct them

(a) record1 (b) #tax (c) name_and_address (d) 1record (e) name

(f) name-and-address (g)file_3 (h) name and address (i) return

(j) 123-45-6789

• Assume a version of C that can recognize only the first 8 characters of an identifier name, though the identifier names can be arbitrarily long. Which of the following pairs of identifier names are considered to be identical and which are distinct?

(a) name, names (b) address, Address (c) list1, Iist2

(d) char1,char_1 (e) identifier_1, identifier_2 (f) answer, ANSWER

Solution to Exercises

(a) record1 - valid (b) #tax - invalid (#character not allowed)

(c) name_and_address - valid (d) 1record – invalid – first character

cannot be a number (e) name – valid (f) name-and-address –

invalid (hyphen not allowed) (g)file_3 - valid (h) name and

address - invalid (spaces not allowed) (i) return – invalid

(keyword) (j) 123-45-6789 – invalid (contains all numbers)

(a) name, names - distinct (b) address, Address - distinct (c) list1,

Iist2 – distinct (d) char1,char_1 - distinct (e) identifier_1,

identifier_2 - identical (f) answer, ANSWER - distinct

Types in C

• A type defines a set of values and a set of operations that can be

applied on those values

• For example a light switch can be compared to a type

• It can have two values on and off

• Only two operations can be applied to the light switch: turn on

and turn off

• The defined set of types in C can be divided into four general

categories: void, integral, floating-point and derived

Types in C

C Types

DerivedIntegral Floating Pointvoid

Boolean Character Integer Real Imaginary Complex

Types in C

• Void type:

• Is designated by the key word void

• Has no values and no operations

• Very useful type

• Used in cases where a function has no parameters or return values

and for many other purposes which will be covered in the

forthcoming topics

• Integral type:

• Integral types cannot contain fractional parts i.e., they can contain

only whole number quantities

• C Language has three kinds of integral types: Boolean, Character

and Integer

Types in C

• Boolean:

• Can represent only two values: true or false

• Is referred to by the key word bool

• Boolean type is stored in the memory as 0 or 1

• Character:

• A character data type holds any single symbol from the C character set

• Is referred to by the key word char

• The size of a character is 1 byte or 8 bits

• Integer:

• Is a number without a fraction part (whole numbers)

• C supports four different sizes of integer data types: short int, int, long int and

long long int

• C defines these data types so that they can be organized from smallest to

largest

Types in C

Integer

Type

No of

Bytes

Minimum value

that can be stored

Maximum Value

that can be stored

short int 2 -32,768 32,767

int 4 -2,147,483,648 2,147,483,647

long int 4 -2,147,483,648 2,147,483,647

long long int 8 -9,223,372,036,854,775,807 9,223,372,036,854,775,806

• A short int can also be referred to as short

• A long int can also be referred to as long

• A long long int can also be referred to as long long

• Although the size of a data type is machine dependent, c requires the following relation ship to be always true:

size of short <= size of int <=size of long <= size of long long

Types in C

• Depending on the usage of sign, each integer size can be of two forms – signed and unsigned

• If the integer is a signed number, one bit must be used for sign

• Positive sign is indicated by a sign bit with 0 value

• For example, assuming 16 bit representation, the integer number 5 is represented as 0000 0000 0000 0101

• Negative sign is indicated by a sign bit with 1 value

• For example, assuming 16 bit representation, the integer number -5 is represented as 1000 0000 0000 0101

• Unsigned integer can store a positive number that is twice as large as the signed integer of same size

• However the actual sizes are dependent on the physical hardware

Types in C

• Floating – Point:

• C standard recognizes 3 floating point types: real, imaginary and complex

• Real:

• Real type holds values that consists of an integral and a fractional part e.g., 43.32

• Real type can further be classified into three different types based on size as float, double and long double

• Similar to integer data type, C defines the real types so that they can be organized from smallest to largest

• Regardless of machine type, C requires the following relationship to be true:

size of float <= size of double<=size of long double

Types in C

Integer

Type

No of

Bytes

Minimum value

that can be stored

Maximum Value

that can be stored

float 4 3.4 e -38 3.4 e 38

double 8 1.7 e -308 1.7 e 308

long double 12 3.4 e - 4932 1e+493

• Imaginary type:

• Imaginary types is one of the components of complex type

• Similar to real type can be of three different sizes: float imaginary,

double imaginary and long imaginary

• Most C implementations do not support the imaginary type yet

and the functions to handle them are not a part of the standard

• Complex Type:

• Implemented by most compilers

• Similar to real type can be of three different sizes: float complex,

double complex and long long complex

• The size needs to be the same in both real and imaginary part

Types in C

Summary of Types in C

Category Type C Implementation

Void Void void

Integral Boolean bool

Character char

Integer short int, int, long int, long long int

Floating-Point Real float, double, long double

Imaginar

y

float imaginary, double imaginary, long double imaginary

Complex float complex, double complex, long double complex

Variables in C (Recall Name bindings in CTPS)• Variables are named memory locations which have a type such as

integer or character

• The type specified for a variable determines the values that a variable may contain and the operations that may be used with its values

• Variable Declaration:

• Each variable in a program must be declared and defined

• A C declaration is giving name to a variable

• While creating variables, declaration gives them a symbolic name

• Generally speaking where a variable is located in memory is not a programmer‟s concern, it is a concern of a compiler

• A Programmer must be able to access the data through their symbolic names, i.e., identifiers or variables

Variables in C

• Variable Definition:

• Definitions are used to create the variable

• Definition of a C variable assumes that declaration has been done

already or is being done at the same time as definition

• Once defined, variables are used to hold the data required by the

program for its operation

• Definition reserves memory for the variables

Variables in C

• Some Examples of Variable declaration and definition are illustrated below:

char option;int i;

long long natl_debt;

float payRate;double pi;

• A variable can be of type except void.

• To create a variable, first specify the type, which automatically specifies its size (precision) and then its identifier

• For example

float price;

denotes the definition of a real variable named price of type float

Variable’s type

Variable’s identifier

Variables in C

• Some examples of different styles of readable variable declarations and definitions:

bool fact;

short maxItems; //Usage of a capital letter (I) as a word separator

long long pi_value; // usage of underscore as word separator

float payRate; //Usage of a capital letter (I) as a word separator

double tax;

float complex voltage;

int a,b; //Multiple variables of same type declared on same line

char option, kind

Variables in C• Variable initialization

• When variables are defined they usually contain meaningless values left over in the memory space from previous use

• Hence there is a need to store data in them by initialization before accessing the values to avoid unwanted bugs in the program

• With a few exceptions (seen later) variables are not initialized automatically• A variable is initialized with the prescribed data required when the function

starts• A variable can be initialized at the time of declaration using an initializer• An initializer, when present establishes the first value that the variable will

contain• To initialize a variable when it is defined, the identifier is followed by an = sign

and then the initializer , which is the value the variable is to contain when the function starts

• An example initialization format is shown below:int count = 0;

• Every time the function containing count is entered, count is set to zero

Variables in C

• What will be the result of the following initialization?

int count, sum=0;

• Are both count and sum initialized or only sum is initialized?

• Answer:

• The initializer applies only to the variable defined immediately

before it

• Therefore only sum is initialized!

• If both variables need to be initialized, then provide two initializers

as shown below:

int count = 0, sum =0;

Variables in C• To avoid confusion and error, only one declaration could be specified

per line as follows:

int count = 0;

int sum =0;

• Let us revisit the example of variable declaration and definition along

with initialization and the memory representation

char option = „y‟;

int i = 25;

long long natl_debt = 1000000000000;

float payRate = 25.50;

double pi = 3.1415926536;

B

14

option

i

1000000000000 natl_debt

25.50 payRate

3.1415926536 pi

Program Memory Representation

Constants in C• Constants are data values that cannot be changed during the

execution of a program

• Like variables, constants also have a type

• Boolean, character, integer, real, complex and string constants are

some types of constants

• Boolean Constants

• A boolean value can have only one of the two values 0 (false) or 1

(true)

• The usage of a boolean constant requires the use of the boolean

library stdbool.h

Constants in C• Character Constants

• Character constant is a character or symbol from the character set enclosed between two single quotes e.g., „a‟

• Escape characters

• A character preceded by a backslash \ enclosed in a pair of single quotes e.g., „\n‟

• The backslash is known as an escape character

• It is used when the represented character does not have any graphic associated with it i.e., when it cannot be printed or entered from a keyboard

• The escape character signifies that what follows is not a normal character but something else

• For example „\n‟ inserts a line feed also known as new-line character

Constants in CExamples of representing integer constants

Representation Value Type

+123 123 int

-378 378 int

-32271L -32,371 long int

76542LU 76,542 unsigned long it

12789845LL 12,789,845 long long int

Constants in C• Real Constants

• Default form of real constant is

double

• To have the resulting data type to

be float or long double,

corresponding code must be used

to specify the desired data type

• f and F are used for float and l

and L are used for long double

• Upper case alphabets are

recommended for representing

the codes

Representation Value Type

0. 0.0 double

.0 0.0 double

2.0 2.0 double

3.1416 3.1416 double

-2.0f -2.0 float

3.1415926536L 3.1415926536 long

double

Constants in C• Complex constants

• Are widely used in engineering

• Coded as two parts – real part and imaginary part, separated by a plus sign

• Real part is coded using the real format rules

• Imaginary part is coded as a real number times (*) the imaginary constant (_Complex_I)

• If the complex.h library is included, the imaginary part can be abbreviated as I

Representation Value Type

12.3 + 14.4 * I 12.3 + 14.4 * (-1)1/2 double complex

14F + 16F * I 14 + 16 * (-1)1/2 float complex

1.4736L + 4.56756L * I 1.4736 + 4.56756 * (-1)1/2 long double

complex

Constants in C• String Constants

• A string constant is a sequence of zero or more characters enclosed in double quotes

• Examples of strings

“” // a null string

“h” // a string containing only a single letter h (not a //character)

“Hello world\n”

“HOW ARE YOU”

“Good Morning!”

• Difference between a null character „\0‟ and a null string “”

• Null character represents no value

• As a character it is 8 zero bits

• An empty string on the other hand is a string containing nothing

• Single quotes are used for character constants and double quotes are used for string constants

Constants in C• Coding constants

• There are three different ways to code constants in programs – literal constants, defined constants and memory constants

• Literal Constants

• A literal is an unnamed constant used to specify data

• If it is known that the data value cannot be changed, the data value is itself coded in a statement

• Literals are coded as a part of a statement using the constant format described previously

• For example, the literal 5 is used in the following statement

a = b + 5;

Constants in C

• Defined Constants

• Another way to designate a constant using the preprocessor #define

• Like all preprocessor commands, it is prefixed with # (pound sign)

• The defined constants are usually placed at the beginning of a program (although they are legal anywhere in the program)

• Placing them at the beginning of the program makes them easy to find and change

• An example for a typical define command may be

#define SALES_TAX_RATE .0825

The sales tax rate may change more often and it and other similar constants at the beginning of the program enables one to find and change them easily

Constants in C• When the preprocessor formats the program for the language translator, it

replaces each defined name SALES_TAX_RATE with its defined value (.0825) wherever it is found in the source program

• The preprocessor does not evaluate the definition in any way – it just blindly makes the substitution

• Memory Constants• Memory constants use a C type qualifier – const to indicate that the data

cannot be changed

• The syntax or format of a memory constant is

const type identifier = value;

Constants in C

• A variable declaration does nothing more than giving a type and size to a named

object in memory

• A Memory constant – const fixes the contents of the memory location such that

it cannot be changed

• This is similar to the concept of literal but it is named

• The following code creats a memory constant cPi

• To indicate that it is a constant, it is prefixed with c

Example: const float cPi = 3.14159;

• Points to be observed

• The type qualifier comes first

• There must be an initializer – If there is no initializer then cPi would contain

whatever was present in the memory location assigned to cPi

• Finally since cPi is a constant, it cannot be changed

What has been discussed?• Background of C Language

• Writing, Compiling and Executing C Programs

• Comments in C language

• How to make valid and good identifiers in C

• Basic data types in C

• Declaring and defining variables in C

• Different types of constants in C