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Documentation: Comments in C

Text written in a program that is ignored by the compiler and is used to explain or document the code for humans.
Explains logic of the Code, improves readability, facilitates easier maintenance of code
Single Line Comments:

// This program prints the message Namaste Varanasi

// This is a basic Calculator program

Multi-line Comments: Should be enclosed within /* and */

/* first line

Second line */

Comments can appear anywhere in the program

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Structure of a C Porgram

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Preprocessor Directives (Link)

#include
C Programs are divided into modules and functions
Commonly used functions are grouped and stored in separate files called header files (end with .h)
Ex: printf is stored in stdio.h
To access such functions -

#include <headerfile_name.h> or

#include “headerfile_name.h” is used

Preprocessor directives are generally placed in the beginning of the file

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Preprocessor Directives: Symbolic Constants

#define
It is a preprocessor directive and not a C statement
#define PI 3.14
Replaces the individual occurrence of PI in the program with 3.14
Should not end with a semicolon
float a = PI * 2; (will be preprocessed as float a = 3.14*2;)
PI is a symbolic constant but not a variable. Therefore no memory will be allocated
The constant cannot be reassigned (PI= 3.413 is invalid)
Symbolic constant names should follow same rules as variable names
No type checking & error detection

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Structure of a C Porgram

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C Tokens

Smallest individual units of a program
int a = 10+4;

int - keyword; a - identifier; = , + - operators; 10, 4 - numeric constants

Types of C Tokens

Keywords
Identifiers (names of variables, symbolic constants, functions(ex.printf) etc)
Strings - sequence of characters in double quotes - Ex. “ABC123”
Constants (literals) - Ex: 10, ‘Z’, 4.5 etc
Operators - Ex: +, -, ^ etc
Special symbols - Ex: ( ), [ ]

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Keywords in C

What are keywords in a language?

Keywords in C are words that have special meaning to the C compiler

Reserved words used in programming. Ex: for

Each keyword is associated with a specific feature

Ex: int – the integer datatype

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Keywords in C

Some keywords in C

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Variables in C

A variable is an entity whose value may change during the runtime of the program

Declaration of variables:

Variables are declared by specifying the data type of the variable followed by its name chosen by the programmer.

Ex: int a;

char c;

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Declaration of Variables

When a variable is declared a set of memory locations are assigned to store the value of the variable (during the execution of the program)

Ex: int a; specifies that a is an integer and 4 bytes of memory is assigned to store the value of a.

char c; assigns 1 byte of memory to store the value of c

The amount of memory assigned to store a variable depends on its data type

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Declaration of Variables

All variables must be declared before their use in the program

Multiple variables of the same data type can be declared in a single statement
Ex: int a,b,c; // declares three variable a,b and c as integers

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Initialization of Variables

Assigns a value to the variable

Ex: int a; // declares a as an integer

a=10; // assigns the value 10 to a and the value 10 will be stored (in binary format) in the memory locations assigned to a.

int a=10; //assigns and declares a variable in one statement

char a=‘A’, b=‘B’; //assigns and declares the variables a and b variable in one statement

Uninitialized variables may take the value 0 or some garbage values

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Initialization of Variables

Uninitialized variables may take the value 0 or some garbage values

Ex: int a, b=10; // declares a,b as integers and assigns value 10 to b

b = a + 10; // may re-assign some garbage value to b as the value of a itself is garbage

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Variable Names in C

A variable name is a human-readable name for the locations in memory assigned to the variable

Ex: int x=10; // declares a variable with name x and assigns value 10 to it. Internally, 4 bytes of memory is allocated for the x.

x = 20; // re-writes the same memory locations assigned to x with the new value 20

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Variable Names in C

A variable name

May consist of alphabets (upper and lower case), digits and underscore ‘_’
Must begin with alphabets or ‘_’
No whitespaces or special characters are allowed
Cannot be same as keyword

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printf on variables

int a = 10;

printf(“The value of variable is %d”, a);

// prints “The value of variable is 10”

Here %d is the format specifier for an integer variable

The input to printf within “ “ is called as the format string.

It specifies the format in which the output is printed

The format string “The value of variable is %d” specifies to print the text The value of variable is followed by an integer whose variable name is given next argument/input

More arguments than format specifiers -> Extra arguments are ignored

Lesser arguments than format specifiers -> Undefined behaviour /Prints garbage data

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Data types

The size of each data size is specific to the system configuration. Here are some typically used values

int – to declare integers - %d

long - to declare large integers - %ld

float - declare numbers with fractional parts - %f

double - declares large floating point numbers - %lf

char - declares characters - %c

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scanf on variables

scanf is used to read user-input data into variables

scanf takes multiple inputs - a format string, and addresses of the variables where the input data has to be stored

int a; char b;

Ex. scanf(“%d %c”, &a, &b);

Here %d is the format specifier for the integer variable a

%c is the format specifier for the character variable b

The input to printf within “ ” is called as the format string.

It specifies the format in which the input should be read

&a and &b specify the addresses of the variables a and b respectively. We will addresses in more detail later!

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scanf on variables

More arguments than format specifiers -> Extra arguments are ignored

Lesser arguments than format specifiers -> Undefined behaviour

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Qualifiers

“type qualifiers are” keywords that give extra properties to a type without changing its size or basic representation.
const is a “qualifier” in C that is used to define read only variables in C.
The remaining qualifiers volatile, restrict, _Atomic are out of the scope of this course.

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Const: Read-only Variables in C

An entity whose value cannot be modified once they are defined

They are fixed values in a program

It is useful to define fixed values as constants to avoid programming errors

Read-only variables can be defined using keyword const
const int a =10; // declares a as an integer constant
const char d =’Z’; // declares and initializes d as a character constant
Initialization of a constant should happen along with declaration
const int a=10; // compiles successfully
const int a; a=10; // results in compilation error