Logical error I think.

Jack.Sparrow · 03-18-2008, 03:53 PM

Hi

I've got this little program (it doesn't do much) but I can't find where the error is in it

Code:

#include <stdio.h> 
#include <stdlib.h> 
   
int main( void )
{
    double a;
    char ch;
       
    printf("a = ");
    scanf("%lf", &a);
    scanf("%c", &ch); 
   
    return EXIT_SUCCESS;
}

It's prints the "a =" part and scans in the input, then it quits. Even if I insert another line to print "ch=", it prints that line, then quits. I think I know why this is happening, but I can't figure out how to stop it.

Can anyone see where the error is? (Just a nudge in the right direction would be very helpful)

Thanks for reading.

**shuuhen** · 03-18-2008, 08:52 PM

It's a problem with scanf(). I have generally avoided using it in anything, so I can't really help with the exact scanf() issue. A lot of programmers avoid that function because it can cause issues. A good way to deal with input is to use fgets() to read into a large buffer, then process the data from there.

You can declare a large buffer with: char tmpbuf[BUFSIZ];

BUFSIZ is a constant defined in C, but feel free to use something other than tmpbuf if you want for the temporary buffer. There is an actual limit to how much can be stored in that temporary buffer, but it is a fairly large number.

Jack.Sparrow · 03-18-2008, 09:51 PM

Thanks for the reply shuuhen. That question is actually from a practical for one of my subjects. We've just started learning the basics. The lecturer has mentioned that scanf is bad but hasn't taught us an alternative. That piece of code is what we were given and were told that we can use any means to discover the error expect asking the lecturer for help. I also thought that the second scanf might be the issue.

How do you think it should be implemented?

**shuuhen** · 03-18-2008, 11:22 PM

Well, without scanf() is how I'd implement it. Since you're required to figure out what's wrong, try playing with larger numbers of scanfs. Hint: printf() is something I use a lot for debugging and the char datatype is fantastic. What exactly is happening when you call scanf()?

If your textbook doesn't have enough information about the function, a reference may help.

Jack.Sparrow · 03-18-2008, 11:33 PM

Thanks for the link and the reply. I think I see what the issue is now. I'll go play with it for now and if I can't figure it out, I'll post back.

03-18-2008, 03:53 PM	#1 (permalink)
Jack.Sparrow Resident Pirate Join Date: Jan 2007 Location: Qld, Aus Posts: 16,717 OS: XP SP2/Vista SP1 My System Blog Entries: 6	Logical error I think. Hi I've got this little program (it doesn't do much) but I can't find where the error is in it Code: #include <stdio.h> #include <stdlib.h> int main( void ) { double a; char ch; printf("a = "); scanf("%lf", &a); scanf("%c", &ch); return EXIT_SUCCESS; } It's prints the "a =" part and scans in the input, then it quits. Even if I insert another line to print "ch=", it prints that line, then quits. I think I know why this is happening, but I can't figure out how to stop it. Can anyone see where the error is? (Just a nudge in the right direction would be very helpful) Thanks for reading. __________________ //PC Wizard//Speedfan//Can you run it?//Tweak Guides//Rules//Donations//

03-18-2008, 08:52 PM	#2 (permalink)
shuuhen Moderator Join Date: Sep 2004 Location: Colorado Posts: 700 OS: Mac OS 9.1, Mac OS X 10.5.4, WinXP Pro, FreeBSD 6.0, Gentoo Linux	Re: Logical error I think. It's a problem with scanf(). I have generally avoided using it in anything, so I can't really help with the exact scanf() issue. A lot of programmers avoid that function because it can cause issues. A good way to deal with input is to use fgets() to read into a large buffer, then process the data from there. You can declare a large buffer with: char tmpbuf[BUFSIZ]; BUFSIZ is a constant defined in C, but feel free to use something other than tmpbuf if you want for the temporary buffer. There is an actual limit to how much can be stored in that temporary buffer, but it is a fairly large number. __________________ Has it been a few days since I replied to your thread? Don't panic! I'm a busy college student and may forget a post if I'm extra busy (or it might just take me a while to be able to do a decent reply). If you still need help and are awaiting my reply after a few days, PM me about it. When posting what errors you get, please give the full message. It makes helping you much easier.

03-18-2008, 09:51 PM	#3 (permalink)
Jack.Sparrow Resident Pirate Join Date: Jan 2007 Location: Qld, Aus Posts: 16,717 OS: XP SP2/Vista SP1 My System Blog Entries: 6	Re: Logical error I think. Thanks for the reply shuuhen. That question is actually from a practical for one of my subjects. We've just started learning the basics. The lecturer has mentioned that scanf is bad but hasn't taught us an alternative. That piece of code is what we were given and were told that we can use any means to discover the error expect asking the lecturer for help. I also thought that the second scanf might be the issue. How do you think it should be implemented? __________________ //PC Wizard//Speedfan//Can you run it?//Tweak Guides//Rules//Donations//

03-18-2008, 11:22 PM	#4 (permalink)
shuuhen Moderator Join Date: Sep 2004 Location: Colorado Posts: 700 OS: Mac OS 9.1, Mac OS X 10.5.4, WinXP Pro, FreeBSD 6.0, Gentoo Linux	Re: Logical error I think. Well, without scanf() is how I'd implement it. Since you're required to figure out what's wrong, try playing with larger numbers of scanfs. Hint: printf() is something I use a lot for debugging and the char datatype is fantastic. What exactly is happening when you call scanf()? If your textbook doesn't have enough information about the function, a reference may help. __________________ Has it been a few days since I replied to your thread? Don't panic! I'm a busy college student and may forget a post if I'm extra busy (or it might just take me a while to be able to do a decent reply). If you still need help and are awaiting my reply after a few days, PM me about it. When posting what errors you get, please give the full message. It makes helping you much easier.

03-18-2008, 11:33 PM	#5 (permalink)
Jack.Sparrow Resident Pirate Join Date: Jan 2007 Location: Qld, Aus Posts: 16,717 OS: XP SP2/Vista SP1 My System Blog Entries: 6	Re: Logical error I think. Thanks for the link and the reply. I think I see what the issue is now. I'll go play with it for now and if I can't figure it out, I'll post back. __________________ //PC Wizard//Speedfan//Can you run it?//Tweak Guides//Rules//Donations//