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Nov 18th, 2007
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IO problem...

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hi, i have the following problem::

i want to read a string from the stdin. The string may contain white spaces so {i think} the scanf family of functions is out of the question... I think the most appropriate function for this case is fgets {if i am wrong please correct me}. Fgets is that it stops when it reads an enter in the input stream and problem arises when i use a menu prior to reading a string that causes an enter to be left in the stream... Is there anyway to circumvent this problem? The only way i found is if i call fgets 2 times in a row{so that the first will discart the enter , and the second will read the string}

here is a sample code::

  1. #include <stdio.h>
  2.  
  3. int Menu()
  4. {
  5.  
  6. 	int choice = 0;		/* menu choice value */
  7. 	/* with the following loop we force the user to choose
  8. 	 * on of the 3 valid answers: 1, 2, or 3
  9. 	 */
  10.  
  11. 	do { 
  12. 		fprintf(stdout, "Please choose one of the following:\n"
  13. 				"1. do1 \n"
  14. 				"2. do2 \n"
  15. 				"3. do3 \n");
  16. 		fscanf(stdin, "%d", &choice);
  17.  
  18. 	} while ( choice != 1 && choice != 2 && choice != 3);	
  19.  
  20. 	return choice;
  21. }	
  22.  
  23.  
  24. int main()
  25. {
  26. 	int mychoice = Menu();
  27. 	printf("\n\n\tyour choice was:: %d\n", mychoice);
  28. 	printf("\nnow lets read a string with fgets\n", mychoice);
  29. 	char buffer[256];
  30. 	fgets(buffer, sizeof(buffer), stdin);
  31. 	fgets(buffer, sizeof(buffer), stdin);
  32. 	//fscanf(stdin, "%s", buffer);	//unfortunatelly it doesn't work because it stops after reading a white space
  33. 	printf("\nhere is your string ::: %s\n", buffer);
  34.  
  35. 	return 0;
  36. }

thanks in advance for your help,
nicolas

PS:: also another weird behaviour is when instead of a number{in the menu response} i type a letter, then i get in an infinite loop.Why is this happening?
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n.aggel is offline Offline
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Nov 18th, 2007
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Re: IO problem...

Use fgets() for everything.

So for example.
  1. int Menu()
  2. {
  3. 	char buff[BUFSIZ];
  4. 	int choice = 0;		/* menu choice value */
  5. 	/* with the following loop we force the user to choose
  6. 	 * on of the 3 valid answers: 1, 2, or 3
  7. 	 */
  8.  
  9. 	do { 
  10. 		fprintf(stdout, "Please choose one of the following:\n"
  11. 				"1. do1 \n"
  12. 				"2. do2 \n"
  13. 				"3. do3 \n");
  14. 		fgets( buff, sizeof buff, stdin );
  15. 		if ( sscanf( buff, "%d", &choice ) != 1 ) {
  16. 			printf( "I asked for a number!\n" );
  17. 		}
  18. 	} while ( choice != 1 && choice != 2 && choice != 3);	
  19.  
  20. 	return choice;
  21. }

This also fixes your "what if they type the wrong thing" problem as well.
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Re: IO problem...

thanks salem, it works! I understand why your code solves the problem "type the wrong thing"...

The thing i can't understand is why scanf can't work together with fgets...i think i 've read somewhere that it isn't the best thing to mix io functions from different families but i can't understand why...
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Re: IO problem...

>>it isn't the best thing to mix io functions from different families
but scanf() and fgets() are of the same family of functions, all declared in stdio.h But it is NOT good to use scanf() for string input because it can (potentially) corrupt and possibly crash your program if you enter a string that is longer then the buffer size you want to hold it.
Last edited by Ancient Dragon; Nov 18th, 2007 at 1:59 pm.
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Re: IO problem...

scanf() uses the bare minimum of characters necessary to perform the conversion, and leaves the first character which cannot be converted on the input stream.

So if you type in
42\n
and scan that with "%d", then 42 will be used and \n will be left.

Now you come along with fgets(), and boom, first character is \n, and it's "thank you and good night from fgets()".
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Re: IO problem...

If you are unsure about user input I would recommend reading the tutorial below:

http://www.daniweb.com/tutorials/tutorial45806.html
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Nov 20th, 2007
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Re: IO problem...

thank you all for your help...
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Nov 20th, 2007
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Re: IO problem...

i read the tutorials on input.Excellent work from Dave Sinkula.

It seems that my biggest mistake concerning fgets was that i didn't know this::
When the incoming text is one less than the size of the buffer, the newline is retained.

PS:: are there any simular tutorials for c++?{ i found the code snipset section but i don't know how can i search it...}

thanks again for all your help,
nicolas
Last edited by n.aggel; Nov 20th, 2007 at 8:56 am.
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Re: IO problem...

>PS:: are there any simular tutorials for c++

There is yes.
http://www.daniweb.com/tutorials/tutorial71858.html
And if you want to fall asleep the below is a must read:
http://www.daniweb.com/forums/thread90228.html

If you want to avoid all the theory you could read everything in as strings then convert to numbers wherever necessary.
Last edited by iamthwee; Nov 20th, 2007 at 6:33 pm.
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