Data Types and Variables

Most values in a computer program have a type. The type tells C what the value represents and what operations make sense for it.

Common C types include:

• Integer types: short, int, long, and long long
• Floating-point types: float, double, and long double
• Character and Boolean types: char, plus bool from <stdbool.h>
• Derived and user-defined types: arrays, pointers, enums, structures, and unions

Variables

A variable is a named storage location. C is statically typed, so every variable declaration states what kind of data the variable can store.

For example, int num = 10; gives a type, name, and initial value, followed by a semicolon.

int num = 10;
printf("%d\n", num);      // 10

double salary = 8232.56;
printf("%f\n", salary);   // 8232.56

Here, num is declared as int, while salary is declared as double. C checks later assignments against those types.

Naming variables

Good variable names are clear and valid:

• Use letters, digits, and underscores, but do not start with a digit.
• Do not use C reserved keywords.
• Prefer meaningful English words.

Names like age, student_name, and total_score are easier to read than a or x1.

C keywords

Keywords are reserved by C and cannot be used as ordinary variable names.

Common keywords include:

• char, int, double, and void
• if, else, switch, and case
• for, while, break, and continue
• struct, union, enum, and typedef
• const, static, extern, and return

Using a keyword as a variable name would make C unable to tell whether you mean a language feature or an ordinary value.