1.1 Introduction to C Language
1 Department of CSE Objectives • 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
2 Department of CSE 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
3 Department of CSE Background of C • 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
4 Department of CSE 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
5 Department of CSE Structure of a C Program
Preprocessor Directives To include standard input/output library file in the program
Global Declarations Describes the data which is visible to all parts of the program (optional)
• main function is the starting point of every C int main (parameter list) program { • One and only one function must be named main in a C program Local Declarations •Describes the data used in the function •Visible only inside the function (optional) Statements } •Follows the local declaration section •Contains the instructions for the computer to do something (e.g., add two numbers) Other functions as required Optional functions also called modules similar to main to accomplish specific tasks
6 Department of CSE Dissecting a minimal C program #include
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 >.
7 Department of CSE Dissecting the minimal C program • General Syntax (writing format) of a Preprocessor Directive is #include
• 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 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 #include
• 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 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*/
ignored Matches the first /* Left without an opening /*
• 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 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
• Write a program to print the following figure using suitable characters Solutions to Exercises #include
• 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 Sum-salary //Hyphen not allowed system identifier pattern _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
void Integral Floating Point Derived
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 No of Minimum value Maximum Value Type Bytes that can be stored 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 No of Minimum value Maximum Value Type Bytes that can be stored 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 Types in C • 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 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 float imaginary, double imaginary, long double imaginary y 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; Variable’s identifier int i; long long natl_debt; Variable’s type 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 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 B option char option =„y‟; 14 i int i = 25; long long natl_debt = 1000000000000; 1000000000000 natl_debt float payRate = 25.50; 25.50 payRate double pi = 3.1415926536; 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 C Examples 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 Representation Value Type • Default form of real constant is double 0. 0.0 double • To have the resulting data type to be float or long double, .0 0.0 double corresponding code must be used to specify the desired data type • f and F are used for float and l 2.0 2.0 double and L are used for long double • Upper case alphabets are 3.1416 3.1416 double recommended for representing the codes -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