Knowledge of initial C language 2.0

Write in the foreword ：

This article will briefly introduce the scope and life cycle of variables , Constant , character string , The content such as .

The scope and life cycle of variables ：

The scope of a variable refers to the scope of the variable , That is, the code range that limits the availability of this variable . The scope of a local variable is the local range of the variable , Please look at the chart below. :

Here a stay main Function body creation , namely a Can only be used in this brace .

  These two pictures are the same reason , Use only within your own limits .

For global variables , Its scope is the whole project , Can be used anywhere , Please look at the chart below. :

You can see , There is no error reported by the program here , Explanatory variable a Can be used anywhere .

Classification of constants

c Language constants are divided into the following categories ：

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

For literal constants , That's what we often say ,1,2,3,A,B,C Such numbers and letters

 const Modified constant variable

about const Decorated variable , We say it has constant properties , But it is still a variable , That is, variables , But not exactly , Why does it have variable properties , Please look at the chart below. ：

const Decorated variable a Cannot be modified , When compiling a program, the compiler will also report an error , That is, it has a constant attribute , For variable properties , Please look at the chart below.

We can see , When compiling a program 1 Code can't run past , because vs2019 The syntax of extending arrays is not supported , therefore 1 Code at cannot run through , and 2 The code at can run normally , It also shows that const A modified constant has the properties of a variable .

#define Defined identifier constant

#define The defined identifier constants are usually placed in the header file or at the beginning of the program , The input format is ：

namely #define Variable name assignment

And it is also a thorough constant , You can also use... When defining arrays , Please look at the chart below. ：

There must be no errors when compiling the program .

Enumeration constants

Enumeration constants need to first create enumeration types , This time you need to use keywords enum, use enum To create enumeration types , for instance , We're going to create gender types , Use the following code to implement

It should be noted that , Here MALE You don't need a semicolon at the end of the statement , It's a comma , At the end of the SECRET There is no semicolon after the statement .

After we create the enumeration type , We can print out the value of the enumeration constant

Please look at the chart below. ：

 

  Here MALE Assigned to a variable S, and MALE The value of is actually 0, We can print it out :

  If printed FEMALE, And SECRET Value , Then their values are 1,2, You can try .

String and escape characters and comments

“HELLO,WORLD”, For this kind of character enclosed by double quotation marks, we call it string , And the most important thing is , The end flag of the string is ’\0‘, Look at the following procedure ：

What do you think arr1 And arr2 It's the same , But are they really the same , Let's look at the results of the program :

 2 The number code printed out a garbled code , Why is that , We can open the debug window to see the different .

Find out arr1 Array ratio arr2 There is one more in the array ’\0' character , Oh, yes, we are arr2 Array plus '\0', The printed result is

Conclusion ,'\0' For string end flag , The end of the string will automatically add ‘\0’ To end the string ,printf And strlen The function reads a string with ‘\0' As an end sign , but ’\0' Will not be read . That is to say strlen（”abc“） The length of the string is 3.

About notes , There are two ways of commenting , about c Language notes , We usually use /**/ To comment , Please look at the chart below.

  But there's a drawback to this approach , That is, you cannot nest comments ,

The compiler here reported an error .

The second annotation method is to add two... Before the statement //, This method is fast and practical .

vs You can directly comment all the selected statements .

Escape character ：

If we want to print on the screen c:\code\test.c, You can't directly input this string to output , Please look at the chart below.

Here \t Escaped as horizontal tab , Allied , There are also the following escape characters ：

\? 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 
\xdd dd Express 2 Hexadecimal numbers . Such as ： \x30 0

among , I would like to focus on \ddd, And \xdd, It has been described above \130, Actually \130 It's octal , The escape character it represents is... Corresponding to its decimal system ASCII The characters of the code table , Please look at the chart below. ：

stay strlen When calculating the length of a string , Especially remember ’\ddd' The number in cannot exceed 8, If it exceeds 8, Will calculate... In another way . Here's the picture

  The arrow points to \628 in ,\62 Parsed into an escape character , therefore \628 The string length of is calculated as 2.

function :

function , Separate the functions one by one , For example, when we find the maximum of two numbers , You can write functions to implement , When you want to compare in the future, you can call it directly . As shown in the following figure, find two functions with larger values

Array   

Array , Is a collection used to store the same elements , First , It introduces how to define

int arr[10] = {1,2,3,4,5,6,7,8,9,10}; This code defines an array , among int It refers to the type of data stored in the array ,[10] Refers to the maximum number of... Stored in the array 10 Elements .

int arr[] = {1,2,3,4,5,6,7,8,9,10}; In this code, you can also [10] Medium 10 Get rid of , At this time, the number of storage elements in the array is the number of initialization .

If the array is not initialized , be arr[] The required number must be placed in the .

It should be noted that , If the array is not fully initialized , Other elements of the array are initialized to... By default 0, Here's the picture

  Here, the array initializes only the first element to 0.

Array subscript from 0 Start , That is, if you want to access the elements in the array , Should be accessed by subscript , Here's the picture

The end of this paper .