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Chapter 3: Constants, Variables and Data Types

3.1 Character Set and C Tokens

The C character set consists of:

• Letters: A-Z, a-z
• Digits: 0-9
• Special characters: + - * / % = & | ! ~ ^ ? : ; , . ( ) { } [ ] < > #
• White spaces: Space, tab, newline

C programs are written using tokens. Tokens are the smallest individual units. C has six types of tokens:

• Keywords
• Identifiers
• Constants
• Strings
• Operators
• Special symbols

3.2 Keywords and Identifiers

Keywords are reserved words with fixed meanings. They must be written in lowercase. ANSI C has 32 keywords:

Identifiers are user-defined names for variables, functions, and arrays. Rules:

• First character must be a letter or underscore
• Only letters, digits, or underscore are allowed
• Only first 31 characters are significant
• Cannot use a keyword
• Case-sensitive (total, Total, TOTAL are different)

3.3 Constants

Constants are fixed values that do not change during program execution.

#include <stdio.h>

int main() {
    printf("Integer values\n\n");
    printf("%d %d %d\n", 32767, 32768, 32769);
    printf("%ld %ld %ld\n", 32767L, 32768L, 32769L);
    return 0;
}

3.4 Data Types

ANSI C supports three classes of data types:

• Primary (fundamental) data types: int, char, float, double, void
• Derived data types: arrays, functions, pointers
• User-defined data types: structures, unions, enum, typedef

Primary Data Types

Integer Type Modifiers

We can use qualifiers to modify the range:

• short int: smaller range (usually 2 bytes)
• long int: larger range (usually 4 bytes)
• unsigned int: only positive values (range 0 to 65,535 for 16-bit)
• signed int: explicitly signed (default)

3.5 Declaration of Variables

Variables must be declared before use. Syntax:

data_type variable_list;

Examples:

int count;
float price, discount;
double ratio;
char grade;

int count = 0;
float price = 99.95;
char grade = 'A';

3.6 User-Defined Type Declaration (typedef)

We can create new type names using typedef:

typedef int units;
typedef float marks;

units batch1, batch2;    // batch1, batch2 are int
marks maths, science;     // maths, science are float

3.7 Declaring a Variable as Constant

Use the const qualifier to make a variable read-only:

const int class_size = 40;
// class_size cannot be modified

3.8 Defining Symbolic Constants (#define)

Symbolic constants are defined using the #define preprocessor directive:

#define PI 3.14159
#define STRENGTH 100
#define PASS_MARK 50

Rules:

• No blank space between # and define
• # must be the first character in the line
• No semicolon at the end
• Symbolic names are written in CAPITALS by convention

Chapter Exercises

Int x;
float letter, DIGIT;
double -p, q;
short char c;
long int m, count;
long float temp;