How to clean keyboard input?

[mattmao]

Hello everyone, I am studying the programming language C from the scratches, so my question might be too simple for you guys:

How would you clear your previous keyboard strokes while let the program still run?

I did some research online, and yeah, some guy said, just use the

fflush(stdin);

But, it kills everything in the keyboard buffer(excuse me if my understanding is wrong), which is not what I want.

Say, you got a prompt, letting you to input some info, the program must run as a loop waiting for the next input. If the input was invalid, then it must clear all the previous "unread" characters.

I checked with the <stdio.h> and <string.h> libs, but couldn't find a proper answer...

[mohammadazim]

fflush() is the only option. You can use it conditionally when input is invalid. For more help you can show your program.

[mattmao]

fflush() is the only option. You can use it conditionally when input is invalid. For more help you can show your program.

OKay, conceptually talking:

outer loop omitted, which calls the following lengthCheck method

/*to see if there is a '\n' in the first 19 characters, if not, then the input should be cleared and program is ready for the next input.*/

int lengthCheck(char *test)
{
   int i, temp, flag = 0;
   for (i=0; i<19; i++)
   {
      if (test[i]=='\n')
      {
         temp = i;
         flag = 1;
         break;
      }
   }
   if (flag == 0)
      fflush(stdin);
 
   return temp;
}

[mattmao]

Or to clarify my puzzled situation, let's look at this problem:

you use fgets(array, 20, stdin); to grab some user input, everything goes fine if the input doesn't exceed the array length. But some careless user stroke more than 20 characters, and awful things happen: once you loop again for the user input, the "unread" characters would bother you.

Actually I tried this in my do-while loop:

fllush(stdin);
fgets(array, 20, stdin);
fflush(stdin);

It't not working...

Perhaps my understanding of keyboard input is still trivial and novice, but that is really the best effort I can make so far.

[weaknessforcats]

Probably you want fflush(stdin). If you have a prompt and the data you recieve is not what you expect, then you are probably better off discarding the entire buffer rather than fish through it trying to figure out what the user has entered.

[mohammadazim]

You need to know few thing about fgets (char* s, n, stdin)

First, it reads (n - 1) characters from stdin. So if your array is of 20 then you should specify 21.
Second, fgets () waits until you press enter not till stdin gets 20 characters from keybord. You are checking for a new line but you will find it only when you enter less then 20 (if you make 21 in fgets call) characters and pressed enter. If you have written more then 20 and then pressed enter, then s won’t have any new line.

You can define delimiters (space/tab) in your string which you enter on keyboard. This way you can break your string into words. First word you can consider as command and later ones as options or data.