Add an article ----- scanf usage

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        Hello！！！ I'm here again , To write an article about scanf Usage of , Why put it here ？ harm ---- Because there is a question about scanf The problem of , Make Xiaotian （ I ） Tangled for more than half an hour . Now let me show you my question , I hope you don't appear ！！！

error

 

  What's wrong （ After half an hour of observation ........)

  correct

  Blame yourself for not being good at learning …………（ I want to understand it ！！！！）

scanf

        scanf() yes C An input function in a language . And printf The function is the same , Are declared in The header file stdio.h in , So in use scanf Function with #include <stdio.h>.（ In some implementations ,printf Function and scanf Functions can be used without precompiled commands #include .） It's a format input function , That is to say, input the data into the specified variables from the keyboard according to the format specified by the user . 

The function prototype  

        int scanf(const char * restrict format,...);

    The first argument to the function is the format string , It specifies the format of the input , According to the format specifier, the information of the corresponding position is parsed and stored in the corresponding pointer position in the variable parameter list . Each pointer must be non null , And it corresponds to the format characters in the string one by one .

          scanf The first parameter of is the string to be input and the placeholder to be read .scanf Subsequent parameters of , Is the variable address that is read and assigned in turn . The type and quantity of placeholders need to correspond to the type and quantity of subsequent parameters .

Return value  

        scanf The function returns the number of successfully read data items , When I read in the data, I encountered “ End of file ” Then return to EOF.

scanf("%d %d",&a,&b);

         The return value of the function is int type . If a and b Have been successfully read into , that scanf The return value of 2; If only a Successfully read into , The return value is 1; If a Read failed , The return value is 0; If you encounter an error or encounter end of file, The return value is EOF.end of file by Ctrl+z perhaps Ctrl+d

    

  Format specifier

        

        c Read a character sequence consisting of a specified number of characters in the field width （ There will be no empty bytes after it ）, If the width is omitted, a single character is read . Such as %c or %1c Read in single characters ,%2c Read in two characters （ There will be no empty bytes after it ）, And so on .

        s Read in a sequence of characters , It will be followed by bytes , Encountered white space character (\t \r \n Spaces, etc. ) Finish read .

        d Read in optional signed （ Optional symbols indicate that you can enter with or without symbols , If there is no sign, it is regarded as non negative ） Decimal integer . The input format should be like strtol Functional base The argument is 10 The character sequence recognized when calling is the same .

        u Read in unsigned decimal integers . The input format should be like strtol Functional base The argument is 10 The character sequence recognized when calling is the same .

           p Read into one The pointer value . The read character sequence should match fprintf Of %p The resulting character sequence has the same form .

Just know the following （ above Understand and be able to use ）

        i Read in optional signed integers . The input format should be like strtol Functional base The argument is 0 The character sequence recognized when calling is the same .

        a,e,f,g,A,E,F,G Read in optional signed floating-point numbers , The input format should be like strtod The character sequence recognized by the function is the same .

        o Read optional signed octal integers . The input format should be like strtoul Functional base The argument is 8 The character sequence recognized when calling is the same .

        x,X Read optional signed hexadecimal integers . The input format should be like strtoul Functional base The argument is 16 The character sequence recognized when calling is the same .

        n Don't read any characters , Instead, the number of characters read to this position is stored in the corresponding int* Point to . If this conversion specifier has * Or with domain width information （ Such as ：%*n or %3n etc. ）, Then the consequences are undefined .

Scan character sets

        % Read in % Symbol ( Percent sign )

         An invalid conversion specifier will cause undefined behavior .

Length modifier          

        

        hh And d, i, o, u, x, X, or n In combination with , Means corresponding to a signed char or unsigned char data .

        h And d, i, o, u, x, X, or n In combination with , Means corresponding to a short int or unsigned short int data .

        l And d, i, o, u, x, X, or n In combination with , Means corresponding to a long int or unsigned long int data ;

        ll And d, i, o, u, x, X, or n In combination with , Means corresponding to a long long int or unsigned long long int data .

long a = 10;
scanf("%ld",&a);

Blank character

         White space makes scanf Function omits one or more white space characters in the input during a read operation .

         The white space in the control string makes scanf() Skip one or more blank lines in the input stream . Whitespace can be a space (space)、 tabs (tab) And the new line (newline). Essentially , The white space in the control string makes scanf() Read... In the input stream , But don't save the results , Until you find non blank character until .

#include<stdio.h>

int main()
{
	int a = 0;
	int b = 0;
	scanf("%d %d", &a, &b);// Data cannot be separated by commas , Only white space characters can be used （ Space or tab Key or enter key ） Separate , for example ：2 3
	printf("%d %d\n", a, b);

	return 0;
}

Nonwhite space character

         A non white space character will make scanf() Function to erase the same character as the non whitespace character on read in .

         The non blank character makes scanf() Read a matching character in the stream and ignore it . for example ,"%d,%d" send scanf() Read in an integer first , Discard commas in reading , Then read another integer . If no match is found ,scanf() return .

#include<stdio.h>

int main()
{
	int a = 0;
	int b = 0;
	scanf("%d,%d", &a, &b);// When inputting data, you must add “,”, for example ：2,3
	printf("%d %d\n", a, b);

	return 0;
}

The format command can describe the maximum Domain width . At the percent sign (%) The integer between and format code is used to limit the maximum number of characters read in from the corresponding field . for example , Hope to address Read no more than 3 In characters , It can be written in the following form ：

scanf("%3s",arr);

  If the content of the input stream is more than 3 Characters , The next time scanf() Read in from the stop . If a blank character has been encountered before reaching the maximum field width , Then the read to the field stops immediately ; here ,scanf() Jump to the next domain .

Although the space 、 tabs And newline are used as field segmentation symbols , But in the operation of reading single character, it is treated as normal character . for example , For the input stream "x y" call ：

scanf("%c%c%c",&a,&b,&c);

After the return ,x In variables a in , Spaces in variables b in ,y In variables c in .

matters needing attention  

•   For string arrays or string pointer variables , Because the array name can be converted into array and pointer, the variable name itself is the address , Therefore use scanf() Function time , You don't need to add... Before them "&" The operator .
•   Can be in the format string "%" Add an integer between the format specifiers , Represents the maximum number of digits in any read operation .
•  scanf There is no similar in the function printf Precision control of .
• Such as ： scanf("%5.2f",&a); It's illegal. . You cannot attempt to enter a decimal as 2 The real number of bits .

•  scanf Variable address is required in , If you give a variable name, there will be an error

  Such as scanf("%d",a); It's illegal. , Should be changed to scanf("%d",&a); It's legal .

•   When inputting multiple numerical data , If there is no non Format characters Make the interval between input data , You can use a space ,TAB Or enter to make an interval .

  C The compiler encountered a space ,TAB, Carriage return or illegal data ( If yes “%d” Input “12A” when ,A It's illegal data ) The data is considered to be over .

• In the input character data (%c) when , If there are no non format characters in the format control string , All the characters entered are considered to be valid . Such as ："%c" Input ' ' The space is also called

  
  scanf("%c%c%c",&a,&b,&c);

  Input is ：d e f

  Then put 'd' give a, ' '（ Space ） give b,'e' give c. because %c Only one is required to read character , There is no need to use a space as the space between two characters , So the ' ' Send it as the next character b.                                          Only if the input is ：def（ No spaces between characters ） when , To put 'd' be endowed with a,'e' give b,'f' give c. If you add spaces as intervals in the format control .

  scanf("%c %c %c",&a,&b,&c);

  Space can be added between the data when input .

problem  

• How to make scanf() Function correctly accepts a string with spaces ？ Such as : I love you! 

  #include <stdio.h>
   
  int main(void)
   
  {
   
  char str[80];
       
  scanf("%s",str);
   
  printf("%s",str);
       
  return 0;
   
  }

Input ：

        I love you!

Output ：

        I

The above procedure does not achieve the expected purpose . because scanf Scan to "I" The following space is right str End of scan （ Spaces are not scanned ）, And ignore the following " love you!".

reason  

#include<stdio.h>
 
#include<windows.h>
 
int main(void)
 
{
 
char str[80],str1[80],str2[80];
     
scanf("%s",str);/* Input here :I love you!*/
 
printf("%s\n",str);
 
Sleep(5000);/* Wait here 5 second , Tell you where the program runs */
     
/**
 
* No sleep(5)
 
*1, The function name is Sleep No sleep.
     *
2,Windows API in ,unsigned Sleep(unsigned) It should be milliseconds ms.
 
*/
 
scanf("%s",str1);/* You don't have to type in these two sentences , It's right stdin Stream rescan */
     
scanf("%s",str2);/* You don't have to type in these two sentences , It's right stdin Stream rescan */
 
printf("%s\n",str1);
     
printf("%s\n",str2);
 
return 0;
 
}

Input ：

        I love you!

Output ：     

  I

  love

  you!

Okay , I see why , So the conclusion is ： Residual information love you It exists in stdin Streaming , Not on the keyboard buffer in .

correct

#include<stdio.h>
int main(void)
{
char str[50];
scanf("%[^\n]",str);/*scanf("%s",string); Cannot receive space characters */
printf("%s\n",str);
return 0;
}

• keyboard buffer Residual information problem                                                                                                            

  #include<stdio.h>
  int main(void)
  {
  int a;
  char c=0;
  while(c!='N')
  {
  scanf("%d",&a);
  scanf("%c",&c);
  printf("a=%dc=%c\n",a,c);/*printf("c=%d\n",c);*/
  }
  return 0;
  }

  scanf("%c", &c); This sentence can't receive characters properly , What's the reason ？                                                      We use it printf("c = %d\n", c); take C use int Express it , Enable printf("c = %d\n", c); This sentence , have a look scanf() Function to C What is it , The result is c=10 ,ASCII The value is 10 What is it? ？ The new line is \n. by the way , Every time we hit "Enter" key , To keyboard buffer Send one “ enter ”(\r), One “ Line break "(\n), ad locum \r By scanf() The function takes care of （ Let's just say that ^_^）, and \n By scanf() function “ error ” The earth bestows on c.                      terms of settlement                                                                                                                               Can be in two scanf() Add getchar(), But it depends scanf() Sentence plus that

  #include<stdio.h>
  int main(void)
  {
  	int a;
  	char c=0;
  	while (c != 'N')
  	{
  		scanf("%d", &a);
  		getchar();
  		scanf("%c", &c);
  		printf("a=%d c=%c\n", a, c);/*printf("c=%d\n",c);*/
  	}
  	return 0;
  }

•    The input type does not match the format string, resulting in stdin Blocking of flow .                                                      

  #include<stdio.h>
  int main(void)
  {
  int a=0,b=0,c=0,ret=0;
  ret=scanf("%d%d%d",&a,&b,&c);
  printf(" First read in quantity ：%d\n",ret);
  ret=scanf("%d%d%d",&a,&b,&c);
  printf(" The second read in quantity ：%d\n",ret);
  return 0;
  }

  Input correctly ：

• But when the input content does not match the format string , The result will be surprising

  Go to the first scanf when , When entering characters ’b’ Time and ret=scanf("%d%d%d",&a,&b,&c); The format string in does not match ,stdin The flow is blocked ,scanf Function is not reading the following part , Direct will 1 return , It means only will stdin Streaming 1 Read in the variable a in .

  Go to the second scanf when , character ’b’ Still does not match the format string ,stdin The flow is still blocked , So there is no prompt to enter ,scanf Function will 0 return .                                                                                        correct

  #include<stdio.h>
  int main(void)
  {
  int a=0,b=0,c=0,ret=0;
  ret=scanf("%d%d%d",&a,&b,&c);
  fflush(stdin);//fflush(stdin) Clear input buffer .
  printf(" First read in quantity ：%d\n",ret);
  ret=scanf("%d%d%d",&a,&b,&c);
  fflush(stdin);
  printf(" The second read in quantity ：%d\n",ret);
  return 0;
  }

• stay scanf("%d ",&a) %d Put a space after it           stay "%d"  There is a %d Add one   A space , This is the format when the program reads data , What does this format mean ? This format means after reading - After an integer ,  Spaces in the format also need to match secondary valid input ,  Just need to read another integer .  Therefore, input 6 Data only read 5 individual . So remove the spaces in the format .      

Conclusion ：         

       There are only two truths in the world ：1、 People are bound to die .

                                             2、 The program must have Bug.

        Thank you for watching ！！！