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C Language is a universal language Computer programming language , Widely used in the underlying development .C The design goal of the language is to provide a way to compile 、 Processing low-level memory 、 Generate a small amount of machine code and a programming language that can run without any support of the running environment .
Even though C Language provides many low-level processing functions , But it still has good cross platform characteristics , Written in a standard specification C Language programs can be compiled on many computer platforms , It even includes some embedded processors （ Single chip computer MCU） And super Level computer and other operating platforms .
The 1980s , In order to avoid the use of C There are differences in language grammar , By the National Bureau of standards C Language has developed a complete set of American national standard grammar , be called ANSI C, As C The original standard of language . [1] at present 2011 year 12 month 8 Japan , International Organization for Standardization （ISO） And the International Electrotechnical Commission （IEC） released C11 standard yes C The third official standard of language , It's also C The latest standard of language , The standard better supports Chinese character function names and Chinese character identifiers , To a certain extent, Chinese character programming is realized .
C Language is a language Process oriented Computer programming language , And C++,Java etc. object-oriented Different programming languages .
Its compiler mainly includes Clang、GCC、WIN-TC、SUBLIME、MSVC、Turbo C etc. .

Why study C Language ？

Before understanding programming , Most people always think that programming is the business of programmers , It has nothing to do with your study and work . But that's not the case ： For most science and engineering subjects , With programming, the computer can deal with a large number of 、 Complex data , Time saving and labor saving . And liberal arts students can also exercise their logical thinking by learning programming , And use some simple functions to record 、 Deal with a large number of documents .

In the undergraduate graduation stage , If you can master a programming language , Will be able to stand out from many candidates ; And during graduate study , Programming is the foundation of learning , All theoretical knowledge should be realized on the computer through programming in the end .

C The basis of language learning content 、 System , also C The fast running speed of the language can meet the needs of most science and engineering majors . As the saying goes C All living things , Through the study C Language has a good programming foundation , Then it will be very easy to transfer to other programming languages .

Any learning process is not achieved overnight ,C Language learning is even more so . I'm learning C You need to practice repeatedly during the language 、 Constantly consolidate and review 、 And cultivate blogging , A good habit of recording your learning process .C Language learning takes about a year , I believe that this year's efforts will eventually bring you rich returns .

2. first C Language program

#include<stdio.h>// Import header file 

int main()// Main function entry , A project can only have one main function 
{
	printf("Hello World\n");// Print “Hello World”
	return 0;// Returns an integer , And int echo 
}

The header file ： If you want to call a function in the header file , for example “printf”, You need to start with #include Import header file , If the header file is not referenced, execute the program , The compiler will report an error ：“printf” Undefined .

The main function ： The main function represents the entry of the program , A project can only have one main function main, new edition C Language use int To define the main function , Return an integer value at the end to echo , The general default return value is 0, return 0 It means that the program is successful , Not 0 False report .（ In the old textbook void main() No longer applicable ）

3. data type

char // Character data type  
short // Short  
int // plastic  
long // Long integer  
long long // Longer plastic surgery  
float // Single-precision floating-point  
double // Double precision floating point

So many data types , To cope with various values in life . In the process of writing code , Try to assign appropriate data types to our variables . adopt sizeof Operators can query the size of these data types , Unit is byte .

#include <stdio.h>
int main()
{
	printf("%d\n", sizeof(char));//1
	printf("%zd\n", sizeof(short));//2
	printf("%zd\n", sizeof(int));//4
	printf("%zd\n", sizeof(long));//4
	printf("%zd\n", sizeof(long long));//8
	printf("%zd\n", sizeof(float));//4
	printf("%zd\n", sizeof(double));//8
	printf("%zd\n", sizeof(long double));//8
	return 0;
}

notes ：sizeof The returned value is an unsigned integer , When printing, you can use %d, The program can run successfully without error , But there will be a warning . All you have to do is %d Change it to %zu that will do .（ This is a C99 standard ）

4. Variable 、 Constant

seeing the name of a thing one thinks of its function , The amount of change is called a variable （ height 、 weight 、 Age 、 balance ）, Constant quantities are called constants （ PI , ID number ）.

How to define variables

#include <stdio.h>

int main()
{
	char ch = 'a';// Single characters in single quotes 
	char name1[] = " Wang Jiujiu ";// String in double quotation marks 
	int age = 23;
	return 0;
}

notes ： When you define variables , If you don't need to assign values to variables at first , It is best to initialize variables . example ：int age =0; Some old textbooks like to use int age; This method , Do not initialize variables , It is easy to appear in the later programming process bug.

Classification of variables

Variables are divided into global variables and local variables .

Global variables ： Variables defined outside the function are global variables .

local variable ： Variables defined inside a function are called global variables .

#include<stdio.h>

int num = 100;// Global variables 
int main()
{
	int num = 99;// local variable 
	printf("%d\n", num);// When local variables and global variables conflict , Local variables take precedence , This shows num Yes. 99
	return 0;
}

Use of variables

#include<stdio.h>

int main()
{
	int a = 0;// Defining variables , And initialization 
	int b = 0;
	int c = 0;
	scanf("%d %d", &a, &b);// use scanf The function inputs two integers , The two numbers are separated by spaces .& To get the address symbol , Pass the input parameters to the variables a,b
	c = a + b;
	printf("%d\n", c);

}

notes ： When you name a variable , It's best to give meaning , Convenient for future use without confusion .

The scope and life cycle of variables

Scope ：

Scope （scope） It's a programming concept , Generally speaking , The names used in a program are not always valid / Usable .

Scope of global variables ： For the whole project , Global variables can be used anywhere in the entire project , And modify it .

The scope of a local variable ： Is the local range of the variable , That is, the function in which the variable is located .

Life cycle ：

The life cycle of a variable is the period between the creation of a variable and its destruction .

The life cycle of global variables ： For the whole project life cycle .

The life cycle of a local variable ： Enter the scope lifecycle begins , Out of scope life cycle ends .

notes ： When programming , Global variables should be used as little as possible , Use more local variables . Global variables can be accessed by the whole project 、 modify , Poor safety ; Local variables can only be used within the scope , Out of scope, you cannot access 、 modify , High safety .

Constant

C Constants in languages are divided into the following ：

Literal constants
#define Defined identifier constant
Enumeration constants
const Modified constant variable

Literal constants ：

100;
3.14;// Constant cannot be changed

#define Defined identifier constant ：

#include <stdio.h>


#define pass 60   
#define good 80
#define excellent 90
//define  Variables defined 

int main()
{
	int num = 0;// initialization 
	scanf("%d", &num);// Input 
	if (num<pass)
	{
		printf(" fail, ");
	}
	else if (num >=pass && num <good)
	{
		printf(" pass ");
	}
	else if (num >= good && num < excellent)
	{
		printf(" good ");
	}
	else if (num >= excellent)
	{
		printf(" good ");
	}
	return 0;
}

notes ： If the full score of roll noodles is 150 branch , Just change the above define Part of the definition , There is no need to change the value inside the function , Very convenient .

Enumeration constants ：

#include<stdio.h>

enum color// Enumeration constants 
{
	red,//0      Enumeration constants are from... By default 0 Start , Increase in descending order 1
	yellow,//1
	blue//2
};

int main()
{    

	
	enum color a = red;// use enum color Create a variable a, The assignment is red
	enum color b = yellow;
	enum color c = blue;

	printf("%d\n", a);//0   
	printf("%d\n", b);//1
	printf("%d\n", c);//2

	return 0;
}

notes ： Except for color , Gender 、 hobby 、 Regions and so on can be expressed by enumerating constants .

const Modified constant variable ：

#include<stdio.h>

int main()
{
	int a = 10;

	a = 20;// Regular defined variables can be modified 

	const int b = 10;//const Defined constant variables have constant properties , Can't change , But its essence is constant .
	return 0;
}

5. character string 、 Escape character

character string

A string of characters enclosed in double quotation marks is called a string

The end flag of the string is ‘\0’, When calculating the length of string, it will not be included . When entering a string with double quotation marks , The default ending is \0, No manual input . however , If you enter a string of characters in consecutive single quotation marks , The result must add ‘\0’.

#include<stdio.h>

int main()
{

	char arr1[] = "hello";// You can create an array without defining space , Automatically define according to the input 
	char arr2[] = { 'h', 'e', 'l', 'l', 'o', '\0' };
	char arr3[] = { 'h', 'e', 'l', 'l', 'o' };

	printf("%s\n", arr1);
	printf("%s\n", arr2);
	printf("%s\n", arr3);

	return 0;
}

The result of the above code execution is shown in the following figure ：

Array arr3 Because there is no ‘\0’, The compiler does not recognize the end flag , After printing hello Will continue to print garbled .

Besides , When defining space for an array , The two methods are also different .

char arr1[5] = "hello";
char arr2[6] = { 'h', 'e', 'l', 'l', 'o', '\0' };// When entering in this way , When defining space, you need to consider ‘\0’ Take up a byte , and arr1 You don't need to

Escape character

Escape character	paraphrase
\?	Use... When writing multiple consecutive question marks , Prevent them from being parsed into three letter words
\'	Used to represent character constants '
\“	Double quotation marks used to represent the inside of a string
\\	Used to indicate a backslash , Prevent it from being interpreted as an escape sequence character .
\a	Warning characters , Beep
\b	Back space
\f	Paper in
\n	Line break
\r	enter
\t	Horizontal tabs
\v	Vertical tabs
\ddd	ddd Express 1~3 Eight octal numbers . Such as ： \130 X
\xdd	dd Express 2 Hexadecimal numbers . Such as ： \x30 0

Compiler at compile time , It is read from left to right , Sometimes the content you want to print is confused with the content of escape characters , Result in failure to compile , Then join in front \ that will do .

for example ：

Print a character ’, How to print ？

Print a string , The content of the string is ”, How to print ？

#include <stdio.h>
int main()
{
	printf("%c\n", '\'');// Print a single character '
	printf("%s\n", "\"");// Print string , Its content is "
	return 0;
}

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