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This article mainly introduces the relevant knowledge of "how to use basic data types, variables and constants in C++". The editor shows you the operation process through an actual case. The method of operation is simple, fast and practical. I hope this article "how to use basic data types, variables and constants in C++" can help you solve the problem.

Variable (variable)-- three elements: variable name-- used in the program, variable type-- opens up memory space size, variable value-- the actual data stored.

Variable definition

Definition syntax: type name variable name = variable value

Int c = 10

Open up memory space for variables, variable declaration will not open up memory space.

Variable declaration

Syntax-- int c; variable definition with no variable value, called declaration, extern int c; add the extern keyword.

Characteristics

If you want to use a variable, you must have a definition. Before using a variable, the compiler must see the variable definition. If not, the compiler will automatically look for a variable declaration and promote it to a definition.

If the extern keyword is added before the variable is declared, it cannot be promoted.

# include

Int main (void)

{

Extern int a; / / variable a declaration

A = 200 position / use variable, which cannot be defined at this time, and the compiler automatically promotes the above declaration to the definition. / / add extern, error reported, a cannot be found

/ / int a = 30; / / variable a definition (redefine variable)

Printf ("a =% d\ n", a)

Return 0

}

Constant (constant): an integer that will not be modified

"hello", "A", 30,-67, 3.67534 (floating point constant)

# define PI 3.1415 (macro definition) syntax: # define macro name, macro value.

Const int a = 5; definition syntax, const type name variable name = variable value, const keyword, the variable modified by this keyword is represented as a read-only variable.

# include

# define PI 3.14

Int main (void)

{

/ / area of the circle: s = PI * r * r

/ / perimeter of the circle: l = 2 * PI * r

Int r = 20

Float s = PI * r * r; / / expression as the value of a variable

Float l = 2 * PI * r

/ / printf ("area of circle:% f\ n", s); / / area of circle: 1256.000000 displays 6 decimal places by default

/ / printf ("perimeter of circle:% f\ n", l); / / perimeter of circle: 125.599998

/ / printf ("area of circle:% .2f\ n", s); / / area of circle: 1256.00

/ / printf ("perimeter of the circle:% .2f\ n", l); / / perimeter of the circle: 125.60 specifies two places after the decimal point, rounded to the third place

Float b = 20.75434

Printf ("b =% 6.2f\ n", b); / / displays a total of 6 digits, including the decimal point, keeping two places after the decimal point, rounding the third place, and filling in less than six places with spaces

Printf ("b = .2f\ n", b); / / displays a total of 6 digits, including the decimal point, keeping two places after the decimal point, rounding the third place, and making up for less than six places with 0

Return 0

}

Identifier identifier: general terms for variables and constants

Identifiers cannot be keywords. Function names: system, printf, int, main, return

It can only be composed of letters, numbers, and underscores: Amurz, Amurz, 0-9, underscore _

Cannot start with a number: int a4ir = 67; (ok), int _ 34d = 43; (ok), float 65ty_7 = 54; (error)

Strict case distinction:

We use uppercase to define constants: # define MAX 100

Variables are usually defined in lowercase

Naming convention

Big hump method: multiple words form a variable name, with the first letter of each word capitalized

Small hump method: multiple words form variable names, the first letter of each word is lowercase, and the first letter of each other word is capitalized.

Lowercase + underscore: for C language only

Sizeof keyword: sizeof is not a function

How to use it:

Sizeof variable name-returns the size of the variable, in whole number sections

Sizeof type name-returns the size of the data type, in whole number sections.

Integer type

Signed integer

-get the minimum and maximum values of the data type. You can use # include

Unsigned integer

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