Master C Constants and Variables: Types, Definitions, and Common Pitfalls

Constants

In C/C++, a constant is a value that cannot be changed during program execution.

Integer constants

Decimal integer : e.g., 7, 20, 1000.

Octal integer : starts with 0, digits 0‑7, e.g., 07, 024, 01750.

Hexadecimal integer : starts with 0x or 0X, digits 0‑9 and a‑f/A‑F, e.g., 0x07, 0x14, 0x3e8.

Long integer constant : ends with l or L, occupies twice the storage of a basic integer, e.g., 7L, 20L, 1000L.

Example: Convert decimal 7, 20, 1000 to octal, hexadecimal and long integer constants.

Floating‑point constants

Two forms:

Decimal form : a number with a decimal point, e.g., 21.290000.

Exponential form (scientific notation) : mantissa, exponent indicator E/e, and integer exponent, e.g., 2.129000e+001.

NOTE:

Floating‑point constants default to double.

Suffix F/f makes it a float constant.

Suffix L/l makes it a long double constant.

12.3f   // float
12.3    // double
12.3e4f // float
12.3e4  // double
12.3e4l // long double
2.1E+5  // double (2.1×10^5)
2.1E+5f // float
123E-3  // double (123×10^-3)
123E-3f // float

Character constants

Ordinary character constant : a single character enclosed in single quotes, e.g., 'a'.

Escape character constant : a backslash followed by a special character, e.g., '\n'.

NOTE: Character constants occupy 1 Byte and are stored as ASCII codes, so they can be interchanged with integer values.

String constants

A sequence of characters enclosed in double quotes. In memory the characters are stored consecutively and terminated by the null character '\0', which also occupies 1 Byte.

#define CharA 'a'   // 'a' is a character constant, occupies 1 Byte
#define StringA "a"  // "a" is a string constant, occupies 2 Bytes
#define StringB "Hello World!" // occupies 13 Bytes

Symbolic constants

Defined using #define or const. #define creates a macro, while const defines a typed constant that must be initialized.

#define MAX 100
const double PI = 3.1415926;

NOTE: Symbolic constants cannot be modified after definition, are usually written in uppercase, and their defining expressions cannot contain variables or functions.

Variables

A variable is a named storage location whose value can change during program execution.

Three elements of a variable: name, data type, and value.

Definition syntax: data_type variable_name_list; Assignment uses the = operator; initialization assigns a value at definition.

int x;        // declaration
x = 10;       // assignment

char c = 'x'; // initialization
long w = 8L, l = 10L; // multiple initialization
double area, radius = 20; // radius initialized, area only declared

NOTE: Declaring a variable without initializing leaves its memory containing indeterminate data.