Elements of C: Character Set, Keywords, Identifiers, Data Types, and Constants

1. Character Set

The C character set is the fundamental alphabet of the language. A C compiler recognizes the following categories:

• Letters: Uppercase (A-Z) and lowercase (a-z).
• Digits: 0-9.
• Special Characters: Symbols like !, @, #, $, %, ^, &, *, (, ), -, +, =, {, }, [, ], :, ;, ", ', <, >, ,, ., ?, /, \, |, ~, _.
• White Spaces: Blank space, horizontal tab, carriage return, new line, and form feed.

2. Keywords

Keywords are reserved words in C that have a special meaning to the compiler. They cannot be used as variable names or identifiers. C (ANSI C) has 32 keywords:

3. Identifiers

Identifiers are names given to various program elements such as variables, functions, and arrays.

• Rules for naming:
  1. Must begin with a letter or underscore (_).
  2. Can contain letters, digits, and underscores.
  3. Cannot be a keyword.
  4. C is case-sensitive (Total and total are different).
  5. No special characters or spaces are allowed.

4. Data Types

Data types specify the type of data a variable can hold and the memory allocated for it.

• Modifiers: short, long, signed, and unsigned can be used with int and char to change their range and size.

5. Constants

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

• Integer Constants: e.g., 10, -500, 0 (decimal, octal, or hexadecimal).
• Real (Floating-point) Constants: e.g., 3.14, -0.005, 2.5e3.
• Character Constants: Enclosed in single quotes, e.g., 'A', 'z', '5'.
• String Constants: Enclosed in double quotes, e.g., "Hello", "TU".
• Backslash Character Constants (Escape Sequences):
  • \n (New line)
  • \t (Horizontal tab)
  • \b (Backspace)
  • \\ (Backslash)

6. Variables

A variable is a named memory location used to store data.

• Declaration: int age;
• Initialization: int age = 20;

Worked Example: Variable Declaration and Usage

#include <stdio.h>

int main() {
    int age = 21;           // Integer variable
    float gpa = 3.85;       // Floating point variable
    char grade = 'A';       // Character variable

    printf("Age: %d\n", age);
    printf("GPA: %.2f\n", gpa);
    printf("Grade: %c\n", grade);

    return 0;
}

Trace:

1. Memory is allocated for age (4 bytes), gpa (4 bytes), and grade (1 byte).
2. Values are assigned to these memory locations.
3. printf uses format specifiers to display the values stored in these locations.

7. Comparison: Constants vs. Variables

Exam Tip

In TU exams, questions from this unit are usually short-answer questions (2.5 or 5 marks). Focus on:

1. Rules for identifiers: Always provide examples of valid vs. invalid identifiers.
2. Data types: Be prepared to write the size and range of int and float.
3. Escape sequences: Memorize at least four common ones (\n, \t, \a, \\).
4. Distinction: Be ready to differentiate between keywords and identifiers, or constants and variables.