C language case: password setting and login > clear solution getchar and scanf

（1）getchar It is specially used for input and output of character data . And by default, only one character is processed at a time, but you can also get strings . The characters entered will return ASII value .48-57 Is a digit . Can be between 0~127 A decimal integer between （ contain 0 and 127）( exceed 127 It's not ASCII code )

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2. How to use getchar Get string

char src[10];char input=getchar(); Whether it's scanf still getchar still gets, Are the last to \0 a null-terminated string .

\\ The first input 
char  ch[]="abcde";( The last one behind defaults \0, Because of the definition , Default exists )
\\ The second input 
char  ch[]={"'a' ,'b','c','d','e'}";
\\ Output 
printrf("%d\n",strelen(char));

The second input method , Is a character array ,strelen String length cannot be caught \0, So it will be printed randomly , Stop at random value . Will produce many random values that do not meet expectations .

3. Principle introduction ：scanf and getchar There are buffers when entering content while waiting , And take the input characters from the buffer to complete the reading .

The difference is that ,scanf You can take more ,getchar Only one can be taken and the access type is limited to character .

4. application

Here are a few situations ：

（1） Input complete ,ctrl+z End input ;

（2） When we log in to the system, we often encounter , Enter a character in the keyboard to select , because getchar() The limitation that only one character can be obtained , therefore scanf Left after taking the string \n , Will be getchar Getting causes the next selection operation to fail .

[scanf Function encountered while reading a string tab And spaces stop reading , But you can enter , encounter \n End input and reading . That is why we press enter after entering .] but gets() The function will receive the entire string of input until it encounters Line break until , You can try this .

Our thinking is , Clear the missing characters in the buffer \n. So we need to use a cleanup operation . If the system does not come with , We need to use our brains to write by ourselves . Here we take getchar To pick \n,

The code is as follows ：

while((temp=getchar())!='\n')  // The explanation is that when the result we pick up again is not empty ,
                               // Think the buffer has been cleared to empty 
{   ;/// Logic is empty 
}

（3）EOF（End  Of  File） End of file flag . We tend to be in putchar I will judge first , Whether our status is entered and the file is read successfully .

while（（ch=get char()!=EOF））
{  putchar(char);
}

(4)int char=get char();

Here we use int The reason is that ,getchar The essential type of the return value is ASII value , If there is a read error, it will also return EOF, namely -1.

(5) Example show ：

#include<stdio.h>
#include<stdlib.h>
int main()
{
	char input[100] = { 0 };
	printf(" Please enter your password :>");\\ Set the password 
	scanf("%s",input);
	int temp = 0;
	while ((temp = getchar()) != '\n')
	{
		;
	}
	printf(" Please confirm your password , Input from keyboard (Y/N) choice ");\\ Confirm password 
	int ch = getchar();
	if (ch == 'Y')
	{
		printf(" Confirm success !\n");
	}
	else { printf(" Confirmation failed !"); }
	again:  printf(" Please enter your password again to login :>\n");\\ Second login 
	char put[100] = { 0 };
	scanf("%s", put);
	if ((strcmp("20020225", put)) == 0)
	{
		printf(" The password is correct ");
	}
	else
		 goto again;
		   
		
	
}