Thread: what to clear with clearerr() ?

First, thanks for all the help with my other questions, learning a lot fairly quickly. I read basically when you get an error from input, the error stays until you clear it and enters an infinite loop (this describes EXACTLY the problem I was having). So I'm trying to clear it using clearerr() but I can't seem to figure out what I'm trying to clear. I thought I understood but I don't, I commented out the various combinations I've tried but don't work. The error occurs if I enter something like ff55. If I enter 55lb or 503lb or 4kg the program works fine.

Code:

#include <stdio.h>
#include <string.h>

int main()
{
   int currentNumber;
   int testForEOF;
   char currentUnit[3];
	
   printf("Enter a number between 0-200 followed by it's units: ");
   testForEOF = scanf("%d%2s", &currentNumber, currentUnit);
   while (testForEOF != -1)
   {
      if (strcmp("lb", currentUnit) == 0 && currentNumber >= 0 && currentNumber <= 200)
      {
         printf("Valid lbs\n");
      }
      else if (strcmp("kg", currentUnit) == 0 && currentNumber >= 0 && currentNumber <= 100)
      {
         printf("Valid kgs\n");
      }
      else
      {
	/* I've tried
	clearerr(testForEOF); 
	clearerr(&testForEOF); 
	clearerr(*testForEOF);
	clearerr();
	clearerr(stdin);
        clearerr(scanf);
	*/
        printf("Invalid format\n");
      }
      testForEOF = scanf("%d%s", &currentNumber, currentUnit);
   }
	
   return (0);
}

Originally Posted by jephthah

to try and clearerr() on anything other than a filestream is pointless... see clearerr.

your real problem here is that you should cease using "scanf()". learn to use "fgets()" instead.

using scanf() to get string input is nothing but trouble, unless you really have an advanced understanding of the function.


.

I have 2 issues with this the first is that from the link you posted they say
char *fgets(char *restrict s, int n, FILE *restrict stream); I don't want to read from a file, I want the input to be from the keyboard. The 2nd, Im guessing using fgets dumps the whole input into some sort of array of characters, Im not sure how I could do what Ive done in the middle of my code with checking the units and the integer size in 1 line of code like I have written. ie how can I do
(strcmp("lb", currentUnit) == 0 && currentNumber >= 0 && currentNumber <= 200) with a giant character array?

stdin is a stream. it's the standard input. typically from teh keyboard.

Code:

fgets(buffer, sizeof buffer, stdin);

once you've got the buffer input, parse it out using various functions like strncmp, strstr, strtok, strtol, strtod, and so forth

hey, this sure is sounding a lot like the last thread we had this same discussion. were you not paying attention?

Originally Posted by jephthah

stdin is a stream. it's the standard input. typically from teh keyboard.

Code:

fgets(buffer, sizeof buffer, stdin);

once you've got the buffer input, parse it out using various functions like strncmp, strstr, strtok, strtol, strtod, and so forth

hey, this sure is sounding a lot like the last thread we had this same discussion. were you not paying attention?

I was originally told to use getchar() then to use scanf() now to use fgets() so I don't know how many more inputs C has that I shouldn't be using because they don't work but I'm waiting to be told that I should be using getchar() again so that I'll have come full circle.

Originally Posted by quzah

As long as you understand how to correctly use each of them, use whatever one your assignment requires, or whatever works best for you for the task. C doesn't really care what you use, as long as you use it correctly.


Quzah.

That's probably the problem, I don't really see why I can't use scanf to do what I want since it basically does exactly what I want except I can't clear the error when I enter hh88. Using fgets seems like I'm going to have to write a hundred lines of quasi regex code to get it to accept 1kg or 200lb or 54kg. If there's no better way than using fgets and writting some huge nasty like 30 case if / switch statement I guess it has to be done, but scanf was so close, is there honestly no way to clear the error being cause by hh66 and I really have to use fgets and write my own regex for it?

you can use scanf all the livelong day, if you like.

it's a beauty when everything is input perfectly and according to plan.

just be prepared for it to fail miserably if your user fatfingers the keyboard and inputs something unexpected.

fgets, on the other hand, will allow you much cleaner control of the user input, because it's all in a string, and you get to be in charge of ensuring it's parsed correctly

Originally Posted by jephthah

you can use scanf all the livelong day, if you like.

it's a beauty when everything is input perfectly and according to plan.

just be prepared for it to fail miserably if your user fatfingers the keyboard and inputs something unexpected.

fgets, on the other hand, will allow you much cleaner control of the user input, because it's all in a string, and you get to be in charge of ensuring it's parsed correctly

So the jist is no I can't use scanf I'm going to have to use fgets and write a parser for it since I know there is going to be purposely bad formatted entries. I feel like all I do in C is code things that high level languages allow you to implement in 1 line. I've been coding for like 15 hours and I've gotten 0 accomplished when I could've written this extremely simple assignment in 2 minutes and in 10 lines of code in any else but this, assembly, and direct machine code.

Originally Posted by quzah

As per above. You can use whatever you like. You just need to pay attention to the return value of scanf, and if it isn't what it should be, you need to clean up after your user.


Quzah.

That's the heart of my question. How do I clean up after the user? How can I stop the infinite loop when they enter bad input? That's EXACTLY what I'm asking you guys. How do I clean up bad input from scanf????

nm. quzah gots it covrered.