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May 29th, 2007

What input reading functions are there in C?

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I've been attempting to teach myself some C, as I have been tasked with maintaining a program written for a handheld barcode scanner that appears to be written in this language. (At least, I'm assuming that for the moment, considering the fact one of the first statements I saw when I started skimming the source code was a malloc).

I've decided that for my first test program (to be shifted to the scanner to see if I can get it to work there) should be a four-function calculator. The scanner has a numeric keypad setting, so the numbers and symbols shouldn't be any problem.

My problem here is a bit simpler. So far, the only input reading method I've found in the text I'm using for this is the getchar() function. From what I can see, that doesn't look as though it's going to be able to handle more than one input character at a time; ergo, any mathematical equation involving numbers 10 and up would be (as far as I can see) impossible to directly deal with.

What other methods of input reading (from command line; I'm assuming that's stdin) are there? Is there something designed to read in multiple characters, or do I need to look into developing a read/store loop pattern for my numbers?

All input highly welcomed.

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May 29th, 2007

Re: What input reading functions are there in C?

>any mathematical equation involving numbers 10 and up would be (as
>far as I can see) impossible to directly deal with.
You can perform any kind of input using single character input. In fact, that's basically what all of the other input functions do: they read one character at a time, store it somewhere, and optionally do some kind of formatting. For example, let's say you want very basic integer input:

C Syntax (Toggle Plain Text)
#include <stdio.h>
#include <ctype.h>
 
int getint ( void )
{
  char buff[BUFSIZ];
  int result = 0;
  int ch;
 
  /* Pull all digits and add them to the result value */
  while ( ( ch = getchar() ) != EOF && isdigit ( ch ) ) {
    result = 10 * result + ( ch - '0' );
  }
 
  /* Return the unconverted character to the stream */
  ungetc ( ch, stdin );
 
  return result;
}
#include <stdio.h>
#include <ctype.h>

int getint ( void )
{
  char buff[BUFSIZ];
  int result = 0;
  int ch;

  /* Pull all digits and add them to the result value */
  while ( ( ch = getchar() ) != EOF && isdigit ( ch ) ) {
    result = 10 * result + ( ch - '0' );
  }

  /* Return the unconverted character to the stream */
  ungetc ( ch, stdin );

  return result;
}

However, most of the time you don't need to do that. Not to mention that it takes a great deal of skill and effort to do it properly. For example, you should never use the above function in real code because it lacks decent bulletproofing. In your case, you should look up the fgets function for reading a line of input. That will give you a string that you can then parse. You can parse it with sscanf, or a number of functions from string.h.

Last edited by Narue; May 29th, 2007 at 12:14 pm.

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May 29th, 2007

Re: What input reading functions are there in C?

And don't try to shortcut the fgets()/sscanf() pair with scanf(). If someone suggests it, stare at him in horror and cast the daemon away!

Here's why. Read the entire scanf() series...

WaltP

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May 29th, 2007

Re: What input reading functions are there in C?

A little example of using what it has been said so far.
Sorry, the proper c tagging is not working at this time.

C Syntax (Toggle Plain Text)
#include <stdio.h>
#include <ctype.h>
 
int getinteger( int *result )
{
    int send = 0;
    char ch;
    char buffer [ 13 ] = { '\0' };
    if( fgets( buffer, sizeof buffer, stdin ) && !isspace( *buffer ) &&  sscanf( buffer, "%d%c", result, &ch ) == 2 && ( ch == '\n' || ch == '\0' ) )
    {
        send = 1;
    }
    else
    {
       if( buffer[ strlen( buffer ) - 1 ] != '\n' )  /* if newline is not found in string */
       {
           while( getchar() != '\n' );                /* clear stdin */
       }
       send = 0;
    }
    return send;
}
 
int main( void )
{
    int number = 0;
 
    do
    {
       fputs( "Enter an integer: ", stdout );
       fflush( stdout );
    }
    while ( !getinteger( &number ) );
 
    printf( "number = %d\n", number );
 
    getchar();
    return 0;
}
#include <stdio.h>
#include <ctype.h>

int getinteger( int *result )
{
    int send = 0;
    char ch;
    char buffer [ 13 ] = { '\0' };
    if( fgets( buffer, sizeof buffer, stdin ) && !isspace( *buffer ) &&  sscanf( buffer, "%d%c", result, &ch ) == 2 && ( ch == '\n' || ch == '\0' ) )
    {
        send = 1;
    }
    else
    {
       if( buffer[ strlen( buffer ) - 1 ] != '\n' )  /* if newline is not found in string */
       {
           while( getchar() != '\n' );                /* clear stdin */
       }
       send = 0;
    }
    return send;
}

int main( void )
{
    int number = 0;

    do
    {
       fputs( "Enter an integer: ", stdout );
       fflush( stdout );
    }
    while ( !getinteger( &number ) );

    printf( "number = %d\n", number );
    
    getchar();
    return 0;
}

Last edited by Aia; May 29th, 2007 at 8:45 pm.

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May 30th, 2007

Re: What input reading functions are there in C?

Click to Expand / Collapse Quote originally posted by Aia ...

Sorry, the proper c tagging is not working at this time.

would you please explain what you mean by that? you can use [ code=c ] if you want to see line numbers, but I assume you already know that. If something is broken maybe you should start a thread in DaniWeb Community Feedback.

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May 30th, 2007

Re: What input reading functions are there in C?

I try to use the [code=C] tagging way like I always use, but this time
didn't work not matter what I try. Here I attached a snap-shot I took of it.
I will do like you say and start a thread in the Community Feedback.

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Last edited by Aia; May 30th, 2007 at 6:09 pm.

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Re: What input reading functions are there in C?

I just tried it and it was ok -- If it persists for you please report it to Dani so that she can fix it.

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May 30th, 2007

Re: What input reading functions are there in C?

There was a bug yesterday. It was fixed early this morning.

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May 31st, 2007

Re: What input reading functions are there in C?

Click to Expand / Collapse Quote originally posted by Aia ...

C Syntax (Toggle Plain Text)
    if( fgets( buffer, sizeof buffer, stdin ) && !isspace( *buffer ) &&  sscanf( buffer, "%d%c", result, &ch ) == 2 && ( ch == '\n' || ch == '\0' ) )
if( fgets( buffer, sizeof buffer, stdin ) && !isspace( *buffer ) && sscanf( buffer, "%d%c", result, &ch ) == 2 && ( ch == '\n' || ch == '\0' ) )

That's an unreadable and grotesque line of code.... Yuck! It certainly can be make much more readable and less convoluted.

Could you try to clean it up?

Remember, these are new programmers and showing an example that is too complex for them to understand is simply useless to them...

Last edited by WaltP; May 31st, 2007 at 12:54 am.

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May 31st, 2007

Re: What input reading functions are there in C?

>Remember, these are new programmers and showing an example that is
>too complex for them to understand is simply useless to them...
Not to mention that no self-respecting programmer would be caught writing that. It's harder to follow the logic, harder to maintain, and harder to document. Don't try to do too much in the loop condition.

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