C language - Introduction - Foundation - syntax - [identifier, keyword, semicolon, space, comment, input and output] (III)

What is an identifier ？

stay C In language , Symbolic constant , Variable , Array , Functions and so on need a certain name , We call this name an identifier

Identifier partition ？

keyword , Predefined identifiers and user identifiers ！

Naming rules for identifiers ？

• Only by letters , Numbers , Underline , Or dollar sign ($) form , And can't start with a number ;

• C Languages are case sensitive , Usually, variable and function names are written in lowercase letters ;

• Although the length of the name is not specified , But don't write too long , It's best to see the name and know the meaning , such as int max At first glance, it is used to store the maximum value ！ also , Not with the following 32 individual C The keyword name of the language is the same ！ as well as C The identifier defined by the system in the language , Such as system class library , System constant name system function name , Such as : Format output function :printf(); The nuptial ！

Identifier hump naming

• See the name and know the meaning , It can improve the readability of the code
• Hump named , It can improve the readability of the code
• Hump naming is when variable names or function names are connected by multiple words , When constituting an identifier , The first word begins with a lowercase letter ; The first letter of the second word is capitalized .
• for example : myFirstName、myLastName Such variable names look like humps
  

What is a keyword ？

C An English word with a special meaning in a language , Usually used to construct statements , Store the data , Define data types and so on

A semicolon

stay C In the program , Semicolons are statement terminators . in other words , Each statement must end with a semicolon , Otherwise, there will be compilation problems . It shows the end of a logical entity . for example

printf(“Hello, World!\n”);

Space

Lines that contain only spaces , It's called a blank line , Maybe with comments ,C The compiler will completely ignore it . stay C in , Spaces are used to describe spaces 、 tabs 、 Line breaks and comments . Spaces separate parts of a statement , Let the compiler recognize an element in a statement , Where to end , Where does the next element start . therefore , In the following sentence ：

int age;

ad locum ,​int​ and ​age​ There must be at least one space character between （ It's usually a blank character ）, So that the compiler can distinguish them . On the other hand , In the following sentence ：

fruit = apples + oranges;   //  The total number of fruits obtained 

fruit​ and ​=​, perhaps ​=​ and ​apples​ The space character between is not required , But to enhance readability , You can add some spaces as needed .

notes

What is annotation for , Comments are for programmers to see , It's not for the computer . In learning , You can add comments next to the code you write , Let yourself know what this code is for now or in the future , What's the usage? , What do you need to pay attention to ; At work , The program is not in the charge of itself , It is the joint efforts of multiple programmers , At this time, comments are needed to help others understand the code they write . Don't underestimate comments , Large companies pay great attention to the specification of code and comments .

I added some comments on it . Notes can be expressed in two ways ,

• The first is //, Indicates this line // All the following characters are comments ;
• The second is /* */, This is a multi paragraph comment , When you want to express a long comment , It is more convenient to use multi paragraph comments .

Other symbols

• Curly braces { } ： The body of the function , Compound statements and initialization of arrays ;

• parentheses () ： Function definition is used to enclose parameters , Or it can be used to modify the operation order , such as ：(a + 1) 2 and a + (1 2)

• square brackets [] Define array types and application elements ;

• Order number . ： Reference member in structure ;

It's on the Internet , We will introduce it in detail later in our study , And repeated use , Here is only for understanding

Input and output

I / O function （printf and scanf） yes C Two very important functions in language , It's also learning C Two functions that language must learn . stay C Language program , Almost no program doesn't need these two functions , Especially the output function （printf）, So these two functions have to be mastered .

If you want to use in the program printf perhaps scanf, Then you must include the header file stdio.h. Because these two functions are included in the header file .
The function of the output function is to output the results of the program running to the screen , The function of the input function is to assign values to variables in the program through the keyboard . It can be said that input-output function is the interface between user and computer . among printf Is very powerful , The usage is very flexible , It's hard to master ; and scanf The usage of is relatively fixed , But there's a lot to pay attention to .

printf Output

printf Function is a formatted output function , Used to output data to the screen .printf Function is called by ： printf( Formatted string , parameter list );
Column ： printf(" My heart is full of bandits , You can't roll it , My heart is full of stones , Don't turn around .\n"); among \n The meaning of a new line

printf There are four formats

• printf(" character string \n");
• printf(" Output controller ", Output parameters );
• printf(" Output controller 1 Output controller 2…", Output parameters 1, Output parameters 2, …);
• printf(" Output controller Non output controller ", Output parameters ); All with % The beginning is basically the output control character , Non output controller , It's the characters that will be output as is

demonstration ：

int age=18;
char xb='x';
double weight=62.5;
char name[21]; // A character array 
memset(name,0,sizeof(name)); // Apply for a piece of memory with a length of name Size 
strcpy(name, " Xi Shi "); // strcpy The function copies the string 
printf(" My name is ：%s, Sex ：%c, Age ：%d year , weight %lf kg .\n",name,xb,age,weight);

There are some functions above that we may , No contact , No problem , It will be explained later , At present, you can

Common controllers

Controller	explain
%d	Output according to the actual length of decimal integer data .
%ld	Output long integer data .
%md	m The width of the specified output field . If the number of digits in the data is less than m, Then fill in the space at the left end , If more than m, Then output according to the actual number of digits .
%u	Output unsigned integer （unsigned）. It can also be used to output unsigned integers %d, This is to convert unsigned numbers to signed numbers , Then the output . But it's better not to write this when programming , Because it's a conversion , send CPU Do one more idle .
%c	Used to output a character .
%f	Used to output real numbers , Including single precision and double precision , Output in decimal form . Do not specify field width , Automatically assigned by the system , The whole integer part is output , Decimal part output 6 position , exceed 6 The rounding of bits . %lf It is used for input Equivalent to %f If you use %f Data chaos
%.mf	Output real number after decimal point m position , Be careful m There's a point in front . have access to %.2f To keep it 2 position perhaps %.3f Retain 3 position …… Pay attention to the decimal point Not less 了
%o	Output as octal integer , This one is rarely used , Just get to know .
%s	Used to output string . use %s The output string is the same as the previous direct output string . But first define character array or character pointer to store or point to string , Let's talk about this later .
%x（ or %X or %#x or %#X）	Output integer in hexadecimal form , This is very important .
%p	Print address data

%x、%X、%#x、%#X The difference between

Be sure to master %x（ or %X or %#x or %#X）, Because when debugging, it is often necessary to output all binary codes in memory , Then display it in hexadecimal . Let's write a program to see what the four differences are ：

# include <stdio.h>
int main(void)
{
    
    int i = 47;
    printf("%x\n", i); //2f
    printf("%X\n", i);  //2F
    printf("%#x\n", i); //0x2f
    printf("%#X\n", i);  //0X2F
    return 0;
}

As can be seen from the output ： If it's lowercase x, The output letters are lowercase ; If it's capital X, The output letters are uppercase ; If you add one #, It's output in standard hexadecimal form . It's better to add one #, Otherwise, if there is no letter in the output hexadecimal number, it will be mistaken for a decimal number ！ All in all , No addition # Easy to misunderstand . But if the output 0x2f or 0x2F, Then people will know it's hexadecimal . and %#x and %#X in , I think capital is better , Because capitalization is the absolute standard hexadecimal notation .

How to output %d、\ And double quotes

printf There is an output controller in %d, The escape character is preceded by a backslash \, And double quotes . Have you ever thought about such a question ： How to pass these three symbols printf Output to the screen ？ To export %d Just add one more... To the front %, To export \ Just add one more... To the front \, To output double quotation marks, just add a \ that will do . The procedure is as follows ：

# include <stdio.h>
int main(void)
{
     
    printf("%%d\n");  //%d
    printf("\\\n");   //\ printf("\"\"\n"); //""
    return 0;
}

printf yes C A very important function in language . After the above study, we found that , It's not hard . Just program more and practice more , I will soon master .

Secondly, after learning this section, you should know why you need “ Output controller ”. Because all the data in the computer is binary 0、1 Code , So when outputting, you should use “ Output controller ” Tell the computer how to display binary data .

In output controller ,%d、%f、%s、%c Is the most commonly used , They are output integers 、 The set of real Numbers 、 String and character control characters . of %x、%X、%#x、%#X The difference between the four usages , Just need to know .

scanf Input

Program is the medium of human-computer interaction , There must be input as well as output stay C In language , There are multiple functions to get user input from the keyboard ：

• scanf()： and printf() similar ,scanf() You can enter multiple types of data .
• getchar()、getche()、getch()： All three functions are used to enter a single character .
• gets()： Get a row of data , And treat it as a string .

scanf() Is the most flexible 、 The most complicated 、 The most common input function , But it can't completely replace other functions , Everyone should know something .

scanf Function is a format input function , Used to accept data entered from the keyboard , After user input data is completed , press return （Enter） End input .

scanf Function is called by ：scanf( Format control string , Address table column );

Be careful , Not in scanf The last addition of the formatted string of \n. among , The function of format control string and printf The same function , But you can't display non formatted strings , That is, the prompt string cannot be displayed , The address of each variable is given in the address table column . The address is determined by the address operator “&” Followed by variable names . for example ：&a、&b Each represents a variable a And variables b The address of . and scanf Functions are essentially assigning values to variables , But you need to write the address of the variable , Such as &a This address is given by the compiler system in memory a The address of the variable assignment . stay C In language , Using the concept of address , This is different from other languages .

Input integer

For the format of input integer %d Express , List integer variable names in parameters , Used to save input data . Before entering data , Usually output a sentence to prompt the user . The same below .

int age=0;
printf(" Please enter your age ：");   //  Prompt text not to wrap , Let the user type later , The same below .
scanf("%d",&age);     //  Sign before variable name &, Don't ask why , Later on .

The input character

For the format of input characters %c Express , List character variable names in parameters , Used to save input data .

char xb=0;
printf(" Please enter your family name ："); 
scanf("%c",&xb);     //  Sign before variable name &

Enter floating point number

Input floating-point number format %lf Express , List floating point variable names in parameters , Used to save input data .

double weight=62.5;
printf(" Please enter your weight ：");  
scanf("%lf",&weight);     //  Sign before variable name &.

Input string

Format of input string %s Express , List string variable names in parameters , Used to save input data .

char name[21];
memset(name,0,sizeof(name));
printf(" Please enter your name ："); 
scanf("%s",name);     //  Pay attention , String variable name can be unsigned &, Don't ask why , Later on .

Enter multiple content

Call once scanf Function can input multiple data , After all data input, press enter to end the input .

    int  c = 0, d = 0;
    printf(" Please enter 2 Integer user spaces separated ：\n");
    scanf("%d %d", &c, &d);  // Enter two integers and assign them to c、d
    printf("c*d=%d\n", c*d);  // Calculation c*d And output 
    return 0;

scanf() There are two spaces separated by %d, Followed by two variables , This requires us to enter two integers at once , And assign them to c and d. Be careful "%d %d" There are spaces between , So there should also be spaces when entering data . about scanf(), The format of input data should be consistent with that of control string .

scanf and printf Compare

Actually scanf and printf Very similar , It's just the opposite ：

scanf("%d %d", &a, &b);  //  Get two integers entered by the user , Assign values to variables  a  and  b
printf("%d %d", a, b);  //  Put the variable  a  and  b  The value of is output on the display 

They all have format control strings , There's a list of variables . The difference is ,scanf You have to bring a & Symbol .& It's called an address character , That is, get the address of the variable in memory . Then the value we input on the console will be stored in , In the corresponding memory address , Using variables directly is equivalent to taking out the value corresponding to the memory address .

putchar and getchar

putchar: Output a character to the screen

getchar: Get a character from the keyboard

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