Rough notes of C language (1)

C Language is a computer language —— The language of human and machine communication （c Language ,c++,JANA,Python）
Computers have to develop —— Early recognition is binary .

Computers are hardware , Electrify , Electricity Division , Positive and negative , Used in computers 0 Means negatively charged ,1 Indicates positive electricity , All computers can handle are electrical signals ,0,1 Such a signal , Binary language . So to run a program, you just need to give it a binary sequence .
At the beginning of a program, you need to type a large string of code, such as ：100001110110
It's too hard to remember these , So people wonder if they can give a name to a program instruction represented by a piece of code
Such as 1000011——ADD（ Mnemonic symbol ）
In the future, just fight ADD Just fine , It knows or turns into 1000011.
And this mnemonic is assembly language .
In the later development, people wanted one sentence to represent many strings of code , There was also B Language ——C Language ,C++,……（ High-level language ）

C Language —— Early immaturity —— mature —— popular
Early immaturity ： Each company has its own customized functions , So every company has its own customized functions , My house c Language can't pass in your compiler software , I can't pass in his compiler . Then international standards appeared .
ANSICC（ The National Bureau of standards ）——C89（ Launched in 2013 ）/C90 （89,90 be similar ）
later ISO cognizance C89 For thorough International Standards .
C99/C11 It's not popular , Many compilers do not support , Only by the same standard can it become popular , In order to run in various compilers .

C Language is a process oriented computer programming language , And C++,JAVA, Object oriented programming languages such as , Its compiler mainly includes Clang、GCC、WIN-TC、SUBLIME、MSVC、Turbo C etc. .

first C Language program

.c—— Source file
.h—— The header file

Main

： The main function ： Program entrance , The program from main The first line entry of the function , No addition main Function, it doesn't know which line to start with .
How many more ？
Also, there are too many entrances. It doesn't know where to start . So the main function has and only has one , It can't be without .

data type

To put A There is room for saving , Now let's apply for a space from the computer memory , Give Way ch Take the application and place the space name ch, To store characters A.

Common data types correspond to print symbols

%c// Print data types in character format ;
%d// Print integer decimal data types ;
%f// Print floating point numbers ;
%p// Print in the form of an address ;
%x// Print in hexadecimal data type ;
Why are there so many types ？
What is the size of each type ？
answer ： The range of values is different , To reduce memory , The corresponding number corresponds to the type , Different space occupied .

C Language standards sizeof（long）>=sizeof(int)
The current operating system has developed to 64 Bit operating system , However, due to the different process of program compilation , The two show different differences ：
32 Bit compiler system ：int Four bytes , And long identical .
64 Bit compiler system ：int Four bytes ,long Occupy 8 byte ,long The data range becomes ：-2^63~263-1

Sizeof():

Calculation xx The amount of space taken up , Unit byte ;
Units in a computer
A computer is a piece of hardware , Electrify , It recognizes electrical signals , Positive and negative electricity Division , Positive for 1, Negative is 0, The electrical signal is converted into a signal that the computer can recognize 1 and 0. Computer can recognize positive and negative , Prove that it can recognize 1 and 0 Such a signal .
that 1 and 0, Such instructions need space to store , Deposit 1 or 0 Such a space size is a bit .（ Store a binary bit ）.

A byte 8 A bit ;
For example, there are three bits 2^{3 There are two possibilities and the largest data is 2}3-1;
Sixteen bits have 2^{16 There are two possibilities and the largest data is 2}16-1;

Report errors ,

float

It's single precision ,95.6 It is regarded as double precision
So sometimes in 95.6 After add f Just fine . But sometimes you can't just change .

Variable

Global variables ： Defined in the code block （{}） Other variables ;
local variable ： Defined in the code block （{}） The variables within ;

&： Address fetch , Tell him to put this input data here .

The computer memory is very large, so it is convenient to divide the memory into small pieces , Easy to find , For example, get the address directly num1;

Local variables can only be used in {} Use in ;
It is recommended that the names of local variables and global variables should not be the same . Easy to produce bug
When local variables and global variables exist at the same time, local variables have priority ;
The scope and life cycle of variables
Scope ： The concept of programming , Generally speaking , The names used in a program are not always valid / Usable . The scope of the available code that limits the name is the scope of the name .

Scope of variable

：

1. The scope of a local variable is the local scope of the variable ;
2. The scope of global variables is the whole project ;

extern Declaration of external symbols

shazi, Put it in the code block {} Calculate the global variables ;

Life cycle ：

The life cycle of a variable is the period between the creation of a variable and its destruction .
4. The life cycle of a local variable is ： Enter the beginning of the scope lifecycle , Work or end of life cycle ;
5. The life cycle of global variables is ： The whole life cycle of the program .