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[C language] open the door of C

2022-07-04 07:24:00 【InfoQ】

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C What is language

In a nutshell ：C Language is a kind of computer programming language , It has the characteristics of advanced language , It also has the characteristics of assembly language .
C Language is developed by the bell Institute in the United States D.M.Ritchie On 1972 Launched in 2013 , stay 1978 Years later, C The language has been transplanted to a Big in Small On the model Microcomputer ( notes : A microcomputer is actually a computer system ). It can be used as Writing system 、 Design language and application program, etc , It can also realize the design of some languages on the program , Write applications that do not depend on computer hardware , so to speak C In fact, the scope of application of language is very wide , Because it has a very powerful data processing ability , Not just in software development , And all kinds of research need to be done C Language , Like a microcontroller , And embedded development .

C What are the advantages of language over other languages

Less resources
Fast running speed
It is convenient to operate the hardware
If you want to learn Embedded Development , And communications , The first choice is C Language .
C Language is a senior big brother (ง •_•)ง

C Characteristics of language

Here I will use the shortest sentence to express
Efficiency : Did it " And have both ", From here C The history of language can be known .
flexibility : C Language syntax can be created from the original code syntax , So as to give programmers more room to play .
Rich in functions : You can use a lot of rich operators ( Arithmetic operator 、 concatenation operator 、 Relational operator 、 Assignment operator and Logical operators ) And custom structure types , Express any complex data type
Expressive power : linguistic form , Freedom of writing , Structural specifications are complete , After light control .
Good portability : C Under different operating systems , Simply modify or not modify , Cross platform program development .

Print hello word

Next, I'll teach you how to C Print in language hello word, If you can succeed, it means that you have formally embarked on learning C Language process ！
The compiler I use is :
vs 2013
  This compiler can be downloaded on Baidu , Bear in mind ： Official website , Baidu also has... On the key , I won't explain more here . The sample code is as follows ：

#include<stdio.h>

int main(void)
{
 printf(&quot;hello word\n&quot;);

 return 0;
}

So here's why it looks like this (｡･∀･)ﾉﾞ hi
The first 3 That's ok ,  This  
int 
With a watch means shaping , So this
main
Is the name of the main function , It means that this is a main function , Bear in mind ：
every last C Procedures must have main function , And there is and only one in the whole C In the process ,
Any program is from main Function executed .
The first 4 Xing He 6 That's ok ,
{ }
( The use of scope improves the locality of program logic , Enhance program reliability , Reduce code to code conflicts ).
The first 5 That's ok , Function call statements ,printf The functionality ：
Send the content to be output to the display screen to display （ Print ）, Is a standard library function , Can be called between programs .
Be careful ：'\n' It stands for < Line break > It means .
The first 7 That's ok ,return return ,0 plastic , Back to main Among functions , There's nothing to say .
      Be careful ： When writing code, type
Characters must be in English , Characters must be in English , Characters must be in English Say important things three times \(@^0^@)/

complete C Program

You are not required to be a beginner , Just know C What constitutes a language program is ok .
The sample code is as follows ：

#include<stdio.h>
// &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;arr It's essentially a pointer 
int binary_search(int arr[],int k,int sz)
{ &nbsp; // Implementation of algorithm 
&nbsp;&nbsp; &nbsp;int left = 0;// The first subscript of the left array 
&nbsp;&nbsp; &nbsp;// The first subscript of the right array 
&nbsp;&nbsp; &nbsp;int right = sz - 1; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; /* &nbsp;1 2 3 4 5 6 7 8 9 10
&nbsp; &nbsp; &nbsp; // Find the middle element subscript according to the left and right subscripts  &nbsp; 0 1 2 3 4 5 6 7 8 9 */&nbsp;&nbsp; &nbsp;
&nbsp;&nbsp; &nbsp;while (left<=right)// When left be equal to right  It means you've found k The subscript 
&nbsp;&nbsp; &nbsp;{&nbsp;
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;int mid = (left+right)/2;
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;if (arr[mid] < k) &nbsp; &nbsp; // If mid Elements (4) Than the elements I'm looking for (6) want < Words 
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;{
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;left = mid + 1; &nbsp; //4+1=5 This refers to the element 
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;}
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;else if (arr[mid] > k)// If mid Elements (4) Than the elements I'm looking for (6) want > Words 
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;{
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;right = mid - 1; &nbsp;//4-1=3
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;}
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;else
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;{
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;return mid; &nbsp; &nbsp;// return mid Go to the function 
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;}
&nbsp;&nbsp; &nbsp;}
&nbsp;&nbsp; &nbsp;return 1;// Not found. Return to 1
}
int main(void)
{
&nbsp;&nbsp; &nbsp;// Two points search 
&nbsp;&nbsp; &nbsp;// Find a specific number in an ordered array 
&nbsp;&nbsp; &nbsp;// If found, return the index of this array , No return found -1.
&nbsp;&nbsp; &nbsp;int arr[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
&nbsp;&nbsp; &nbsp;int k = 7;
&nbsp;&nbsp; &nbsp;int sz = sizeof(arr) / sizeof(arr[0]);//sz Number of elements 
&nbsp;&nbsp; &nbsp;//ret Receive return value  &nbsp; &nbsp; &nbsp; here arr Only the address of the first element of the array is passed 
&nbsp;&nbsp; &nbsp;int ret = binary_search(arr,k,sz);// call ( Two points search ) function 
&nbsp;&nbsp; &nbsp;if (ret == 1)
&nbsp;&nbsp; &nbsp;{
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;printf(&quot; Array subscript not found &quot;);
&nbsp;&nbsp; &nbsp;}
&nbsp;&nbsp; &nbsp;else
&nbsp;&nbsp; &nbsp;{
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;printf(&quot; Find array subscript :%d\n&quot;, ret);
&nbsp;&nbsp; &nbsp;}
&nbsp;&nbsp; &nbsp;return 0;
}

1. First write how this function should be used .
2. Then write how this function should be implemented .
This is a roughly complete c Language program , This is the realization , Write a function to realize an orderly binary search , Of course, now we are talking about the foundation , This is just an example I show you , Let everyone realize a complete and standardized c How to write language . In my submission ：C The procedure mainly includes the following parts ：
Preprocessor instruction function variable statement & Expression comments, of course , Including one of your thoughts , Logic , So I recommend that every time we write a line of code , You can add notes , Let your line of code , Easier to understand . And the advantage of annotation is , When you want to see the code you've written before, you can look directly at your comment line , Then let's introduce the notes ！

notes

// Single-line comments   With // The first single line comment , This comment can take up a single line .
/* Single-line comments */
/* Multiline comment Multiline comment Multiline comment */
/* */ Comments in this format can be one or more lines . You cannot embed comments in comments ,
Comments cannot also appear in strings or character values .

Print function and input function ( Focus on understanding )

printf： The function is ： Print formatted data for printing , take C The string is written to the standard output in format （ Stout ）. If the format includes a specifier （ from % Starting sub order ）, The other parameters below the format will be formatted and inserted into the generated string , To replace their respective specifiers .
Parameters ： Contains... To be written for thick text C character string . It can optionally contain embedded format specifiers , These specifiers are replaced by values specified in other subsequent parameters , And format as required .
To put it bluntly ： Format output , printf("< Stylized strings >",< Parameter table >); Be careful ： Remember to emphasize that it is English , It's not Chinese ！
scanf： function ： from stdin Read formatted data
from stdin(stdin yes C Standard input stream in language , It is generally used to get what the keyboard inputs into the buffer ) Reading data , And store it in the location indicated by other parameters according to the parameter format . Other parameters should point to the assigned object specified in the corresponding format in its format string .
Parameters ： Format ---- A character sequence containing characters that control the characters extracted from the stream C Character handling ：
Blank character ： This function will read and ignore any white space characters encountered before the next non white space character （ White space characters include space 、 New lines and tab characters - see isspace --isspace, Computer functions , Mainly used to check parameters c Whether it is a blank character .） A single white space in the format string validates any number of white space characters extracted from the stream （ Does not include any white space characters ）.
Nonwhite space character ： Format specifier （%） With the exception of ： Any character that is not white space （ blank 、 New lines or labels ） Or format specifier （ With % The beginning of a character ） A part of the character will cause the function to read the next character from the stream , Compare it with this non white space character , If the match , Throw it away , And this function continues to use the next character of the format . If the characters don't match , The function will fail , The subsequent characters that return and leave the stream are unread .
Format specification ： By the initial percentage symbol （%） The resulting sequence represents the format specifier , Used to specify the type and format of data to be retrieved from the stream and stored in the location indicated by other parameters .
Last , I'm talking about , I / O function （scanf and printf) Is the most frequently used function , You must focus on , Especially format input and output ,C The key and difficult points of language , In fact, this kind of play and learn more in the back , Of course . So stick to it .

C Structural features of language programs

One C Language source programs can consist of one or more source files . Each source file can consist of one or more functions . Preprocessing commands are usually placed at the front of the program （#include）. Every description , Every statement must begin with ;( A semicolon ) ending . Source files, no matter how many files you have ,C Language has and has only one main function . identifier There must be at least... Between keywords + A space as an interval , If there is an obvious separator , Don't have to .

Programming specification

From clear writing , Easy to read , understand , From the perspective of maintenance , The following rules should be followed when writing procedures ：
1. A description or statement takes up a line .
2. use {} The enclosed part , Usually represents a hierarchy of programs .{} It is usually aligned with the first letter of the structure statement , And take a single line .
3. A lower level statement or description can be indented by several boxes than a higher level statement or description . To make it look clearer , Increase the readability of the program .

C Language character set

First , Let's talk about ： Characters are the most basic elements of a language .C Language characters consist of ：
Letter ： Lowercase letters a~z, Capital A~Z, , respectively, 26 individual .
Numbers ： Arabic numerals 0~9 common 10 individual .
Space ： Let's talk about its types and forms here ：
A key on a computer keyboard , Usually in the bottom line of the keyboard , It is the longest key on the computer keyboard . Represents an empty space , It's in GBA The simulator plays a role of acceleration in the default controller .
Computer knowledge , Spaces are counted as one character , So when you fill out a form on the website , You can write nothing , But don't press the spacebar in blank items , When you search in search engines , If you forget a word , Space can be used to temporarily replace . Spaces can also be used as small lattices, hollow lattices .
GBA expand ： A simulation GBA Game software , It can be in Africa GBA Run through simulation in the platform GBA( Nintendo's second-generation portable game console ) game , When running the game, you also need to have the corresponding game ROM( read-only memory ).
Punctuation and identifiers ： Punctuation has nothing to say , Let's focus on the next identifier
finger ： A symbol used to identify an entity , It has different meanings in different application environments .

identifier

Variable name 、 Function name 、 label 、 These are called identifiers . Except that the library function name is defined by the system , The rest are defined by the user .C Language policy , The indicator can only be represented by ： Letter （A~Z、a~z）、 Numbers （0~9） It's the decimal system 、 Underline （_） Composed string , And the first character must be composed of letters or underscores .
Be careful ： In the identifier , Case is distinguished by . for example :Book and book Are two different identifiers . keyword ： Keywords are C The vocabulary of language . They are right C It's special for , You can't use them as identification operator （ Such as , Variable name ）. Many keywords are used to specify different types , Such as int. There are still some customs Key word （ Such as ,if） Used to control the execution order of statements in a program . In the table 2.2 Listed in C Language keywords , Bold indicates C90 Keywords added to the standard , In italics C99 New to the standard keyword , Bold italics indicate C11 Keywords added to the standard .
Of course C Language keywords are also divided into the following categories ：
Type specifier 、 It's like data types that describe variables and functions ,int char Wait for the statement definer 、 Functions used to represent statements, such as if else This preprocessing command word 、 Used to represent a preprocessing command, such as C Language header file include.