Unit 2: Elements of C

C tokens

A token is the smallest individual unit of a C program. The C language has six types of tokens:

1. Keywords — reserved words with predefined meaning (e.g., int, return, if).
2. Identifiers — names given to variables, functions, arrays, etc.
3. Constants — fixed values that do not change during execution.
4. Strings — sequences of characters enclosed in double quotes.
5. Operators — symbols that perform operations (e.g., +, -, *).
6. Special symbols — punctuation such as { }, ( ), ;, ,.

Example:

int sum = a + b;   /* int, sum, =, a, +, b, ; are tokens */

Escape sequences

Escape sequences are character combinations beginning with a backslash (\) that represent special characters inside strings.

Escape	Meaning
\n	New line
\t	Horizontal tab
\b	Backspace
\r	Carriage return
\\	Backslash
\'	Single quote
\"	Double quote
\0	Null character
\a	Alert (bell)

Example:

#include <stdio.h>

int main(void)
{
    printf("Name:\tRam\n");
    printf("Course:\t\"C Programming\"\n");
    return 0;
}

Variables

A variable is a named memory location used to store a value that can change during execution.

Rules for naming variables:

— The first character must be a letter or an underscore.
— The remaining characters may be letters, digits, or underscores. — Keywords cannot be used as variable names.
— C is case sensitive (Sum and sum are different).

Declaration and initialization:

int age;          /* declaration            */
age = 20;         /* assignment             */

float pi = 3.14;  /* declaration + init.    */
char grade = 'A';

Data types

C provides several built-in data types.

Type	Typical size	Example values
char	1 byte	'A', '5'
int	2 or 4 bytes	10, -50
float	4 bytes	3.14, -0.5
double	8 bytes	3.141592653589
void	—	Represents no value

Type modifiers: short, long, signed, unsigned.

Example:

#include <stdio.h>

int main(void)
{
    int age = 20;
    float height = 5.7f;
    char grade = 'A';

    printf("Age: %d, Height: %.1f, Grade: %c\n", age, height, grade);
    return 0;
}

Constants / Literals

A constant is a value that does not change during program execution.

Types of constants:

— Integer constant — 10, -25, 0x1A (hex), 017 (octal).
— Floating-point constant — 3.14, 1.2e3.
— Character constant — 'A', '9'.
— String constant — "Hello".

Constants can be defined using #define or the const keyword.

#include <stdio.h>
#define PI 3.14

int main(void)
{
    const int MAX = 100;
    printf("PI = %f, MAX = %d\n", PI, MAX);
    return 0;
}

Expressions

An expression is a combination of variables, constants, and operators that evaluates to a single value.

int a = 5, b = 3, c;
c = a + b * 2;      /* expression: a + b * 2 */

Statements and comments

A statement is an instruction that tells the computer to perform an action. Every C statement ends with a semicolon (;).

int x = 10;          /* declaration statement */
x = x + 1;           /* assignment statement  */
printf("%d\n", x);   /* function-call stmt    */

Comments are text ignored by the compiler. They describe code for human readers.

/* This is a multi-line comment.
   It can span several lines. */

// This is a single-line comment.