First acquaintance with C language (1)

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Preface

c Language is almost the basis of programming , As a foundation , It must be handled carefully , Now take a look at this article ---- To introduce the c The basis of , Very friendly to Xiaobai , Clear and easy to understand .
This article mainly introduces the commonly used functions and data types , Constant variable , Escape character , String, etc.

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One 、c What is it? ？

For many people who have not been exposed to programming, and even some people who have been exposed to it c The concept of may not be understood so clearly , So let's start from here c.
c Language is a computer language , And computer language is the language that people communicate with computers （c It's just one of them , Computer language and python,java etc. ）, meanwhile c Language is a process oriented language （ Pay more attention to process ）,c++,java And so on are object-oriented languages （ Object oriented ）. In the face of c When programming , I'm using vs2019 edition , Note that at this time .c At the end is the source file ,.h The end is the header file , When compiling, we need to use the source file ！

Two 、main function

Let me first introduce main function , because main Function is the entry of a program , Especially important ,main The general format of the function is as follows ：return 0 Means to return normally , When return When the following number is non-zero, it means abnormal return , therefore main In the function is return 0.

int main()
{
    
return 0;
}

3、 ... and 、printf function

To learn about the second common function —printf function （ It is also a library function ）, When using it, you need to import header files （ The use of library functions requires the introduction of header files ）------stdio.h, This function indicates that you want to print data on the screen （ Present to us on the console ）, The format is as follows （ The output is as follows ）：

#include <stdio.h>
int main(){
    
printf("hello c");
return 0;
}

Add ： When printing ：
If it is a decimal integer , The front is %d
If decimal , The front is %f
If character , The front is %c
If it's a string , The front is %s

Four 、scanf function

Enter data from the keyboard , At this time, pay attention to vs Use... Directly scanf Will report a mistake , It will prompt you to use scanf_s This function , Although the program can work normally , but scanf_s This function is vs Provided , Therefore, it does not have cross platform , So it's not recommended , A better solution is ： In the first line of all code in the source file , first line , first line （ Important things are to be repeated for 3 times ） Add a piece of code ：#define _CRT_SECURE_NO_WARNINGS 1 It's working scanf Function will not be wrong , The specific code is as follows （ give the result as follows ）：
Input format ( Looking at the type in front, it changes , Take integer as an example )：scanf(“%d”,& The variable where you want to put the input number );

#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
int main()
{
    
	int n;
	// Make a statement 
	printf(" Please enter a number from the keyboard ：");
	// Input 
	scanf("%d", &n);
	// Can output n See if it is entered 
	printf("%d", n);
	return 0;
}

Four 、 data type

1.char // Character type （ It can also be understood as describing integer , Its ASCALL The code is an integer ）
2.int // integer （ For integers ）
3.long // Long integer （ For integers ）
4.long long // Longer integers （ For integers ）
5.short // Short （ For integers ）
6.float // Single precision floating point number （ For decimals ）
7.double // Double precision floating point （ For decimals ）（ If it is a decimal , The compiler defaults to double type ）
Why are there so many types of integers and decimals ？
The allowable range of each type is different （ Data in the computer is stored in binary form , A binary representation unit is one bit （1b） and 1B（ byte ）=8b）, We can use sizeof Function to see how wide the type is （ It's in bytes ）, The code is as follows ：（ Notice I'm here 64bit Operation of , If 32bit The result is different from this ）

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

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5、 ... and 、 Variables and constants

You can know from the name , Variables represent variable quantities , A constant represents an immutable quantity .
1. Variable

A. Variable definition method ： type + Variable name
eg: int age;（ uninitialized ）
int n=18;( Initialize the )
Be careful ： Variable names are not given casually , There is a naming rule ：
a. Only letters （ Uppercase or lowercase ）, Numbers and underscores （_） form .
b. The number can't start
c. Cannot use keyword
d. Variable names are case sensitive
e. The length cannot exceed 63 Characters （64bit In the system ）;

B. Variables are divided into local variables and global variables
You can simply distinguish ： Inside the braces are local variables , The external variables are global variables .
（ When local variables and global variables have the same name , Local variables are preferred .）
eg:

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>

int m = 100;// Global variables 
int main() {
    
	int n = 20;// local variable 
	return 0;
}

Add a point of knowledge ： Life cycle concept ： The time period between the creation of variables and the destruction of variables .
Global variables can be applied to the entire c project （ self-created , Below I will have code examples ）, Local variables can only be applied to their own local range .
The life cycle of global variables is the life cycle of the whole program .
The declaration cycle of a local variable begins with the range it enters , Out of that range, it ends .
Application examples of global variables ：
a. The most basic application

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>

int m = 100;// Global variables 
int main() {
    
	printf("%d\n", m);
	return 0;
}

b. Second usage （ This is mainly to show that it can be applied to the whole c project ）
In other .c Use another in the file .c You only need to declare the global variables of the file , You can use it .（eg:extern int m;）

test01.c in ：
#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>

int m = 100;// Global variables 

test02.c in ：
#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
int main() {
    
	extern int m;
	printf("%d\n", m);
	return 0;
}

2. Constant
a. Literal constants （ A constant value written directly ）
eg:30,“abcdefg”,2.45
b.const Modified constant variable （ But it is not allowed to be modified , It is still a variable ）
eg:const int m=88;
c.#define Defined identifier constant （ Cannot be modified , Constant in nature ）
The format is ：#define name The number
eg:

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>

#define max 100
int main() {
    
	int a = max;
	printf("%d\n", a);
	return 0;
}

d. Enumeration constants
The format is as follows ：
enum name {
(eg:)male,// Can be initialized ,eg:male=4,( It is not allowed to change after initialization )
(eg:)female,
(eg:)age
};
male,female,age Are enumeration constants , When not initialized , The default value of the enumeration constant at the beginning is 0, Add one next .

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>

enum sex
{
    
	male,
	female,
	age
};
int main() {
    
	printf("%d\n", male);
	printf("%d\n", female);
	printf("%d\n", age);
	return 0;
}

6、 ... and 、 character string

Someone can't distinguish between characters and strings , Briefly , The output on the keyboard is all characters , Enclosed in single quotation marks are characters , Enclosed in double quotation marks is a string . stay c in
eg:“asdfgg”-----> character string
‘g’------> character
String end flag \0, When calculating the length of a string ,\0 Not included in the length
It can be used strlen To calculate the string length , but strlen For a library function , You need to import header files when using
#include<string.h>

A. First, distinguish whether there is \0
char arr1[]=“abcdef”;// At the end \0
char arr2[]={‘a’,‘b’,‘c’,‘d’,‘e’,‘f’};// There is no \0
The output result after printing is ：( Because the second defined array is not followed by \0, So I will always look back , Until I met \0 Will stop , So the second print will show the results shown in the figure )
The second solution ：
char arr2[]={‘a’,‘b’,‘c’,‘d’,‘e’,‘f’,‘\0’};
B. Use strlen Calculate string length
\0 Previous characters , not \0 In itself

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<string.h>
int main() {
    
	char arr1[] = "abcdef";
	char arr2[] = {
     'a','b','c','d','e','f'};
	char arr3[] = {
     'a','b','c','d','e','f','\0' };
	printf("%d\n", strlen(arr1));
	printf("%d\n", strlen(arr2));
	printf("%d\n", strlen(arr3));
	return 0;
}

7、 ... and 、 Escape character

Change the meaning of the original characters .
The approximate table is as follows ：

Three letter words ----- A three letter word is a sequence of several characters , Together, it represents another character .

This escape character is often used for paths ：
The output path ：
Output as shown in the figure , The result is not quite right , Pictured ：

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int main() {
    
	char arr[] = "c:\test\code";
	printf("%s\n", arr);
	return 0;
}

After improvement ：

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int main() {
    
	char arr[] = "c:\\test\\code";
	printf("%s\n", arr);
	return 0;
}

8、 ... and 、 notes

1. //--------> This comment can be used for multiple lines or single lines
2. /…/ Comment on multiple statements

summary

This article is mainly about c Part of the basic knowledge of the language is explained in detail , For example, common data types , String, etc. ,c See the following notes for other basic knowledge in , Now it is c The basis of , You must understand thoroughly , In this way, I can understand later c It's helpful in the deep layer of .