scanf in a for loop

**skaoth** · Nov 14 '05, 04:40 PM

Re: scanf in a for loop

Yes, the scanf should cause the for loop to pause and allow you to
enter your integer.
The syntax error that I see is that
scanf("i%", &grade);
should be
scanf("%i", &grade);

**Malcolm** · Nov 14 '05, 04:40 PM

Re: scanf in a for loop

wrote[color=blue]
> for(i = 1; i <= numOfGrades; ++i) {
> printf("Enter grade #%i: ", i);
> scanf("i%", &grade);
> ...
> }
>
> Does the scanf command pause the loop until the user input is received?
> When I run this program the loop keeps on going without pausing for the
> user input.
>
> What is going on here? Can anyone please help me?
>[/color]
Yoy've mistyped the "%i" as "i%". This means that scanf() is looking for the
letter I followed by the field specifier, followed by a NUL, which is
illegal. On your system it is simply ignoring it and returning.

When calling scanf() you should check the return value, which gives the
number of fields successfully converted, mainly to guard against bad user
input but also to catch this sort of error.

**reachsachin@gmail.com** · Nov 14 '05, 04:40 PM

Re: scanf in a for loop

"" <> wrote:[color=blue]
> In Stephen G. Kochan's book "Programmin g in C" there is a program[/color]
with the[color=blue]
> following:
>
> for(i = 1; i <= numOfGrades; ++i) {
> printf("Enter grade #%i: ", i);
> scanf("i%", &grade);
> ...
> }
>
> Does the scanf command pause the loop until the user input is[/color]
received?[color=blue]
> When I run this program the loop keeps on going without pausing for[/color]
the[color=blue]
> user input.
>
> What is going on here? Can anyone please help me?
>
> JoshO.[/color]

**sachin** · Nov 14 '05, 04:40 PM

Re: scanf in a for loop

hi josho
first of all, the format specifier in the scanf statement is incorrect,
as mentioned by others.

Secondly, the scanf statement is not waiting for further values of
grade as expected just because it always find a character in its input
buffer.
So one probable solution is to clear that character, which you enter
for one scanf statement from the input buffer, so that next scanf
statement DOES NOT find the previously keyed in character, and waits
for the new fresh value of 'grade'.

for(i = 1; i <= numOfGrades; ++i) {
printf("Enter grade #%i: ", i);
fflush(stdin);
scanf("i%", &grade);
....
}
I tested the code, its running on solaris platform. Moreover, I believe
fflush is a generic syscall independent of the OS.

Regards
Sachin

**Keith Thompson** · Nov 14 '05, 04:40 PM

Re: scanf in a for loop

"sachin" <reachsachin@gm ail.com> writes:[color=blue]
> hi josho
> first of all, the format specifier in the scanf statement is incorrect,
> as mentioned by others.
>
> Secondly, the scanf statement is not waiting for further values of
> grade as expected just because it always find a character in its input
> buffer.
> So one probable solution is to clear that character, which you enter
> for one scanf statement from the input buffer, so that next scanf
> statement DOES NOT find the previously keyed in character, and waits
> for the new fresh value of 'grade'.
>
> for(i = 1; i <= numOfGrades; ++i) {
> printf("Enter grade #%i: ", i);
> fflush(stdin);
> scanf("i%", &grade);
> ...
> }
> I tested the code, its running on solaris platform. Moreover, I believe
> fflush is a generic syscall independent of the OS.[/color]

fflush(stdin) invokes undefined behavior.

<http://www.eskimo.com/~scs/C-faq/q12.26.html>

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

**Mac** · Nov 14 '05, 04:40 PM

Re: scanf in a for loop

On Sat, 25 Dec 2004 20:14:48 +0000, Keith Thompson wrote:
[color=blue]
> "sachin" <reachsachin@gm ail.com> writes:[/color]
[snip][color=blue][color=green]
>> printf("Enter grade #%i: ", i);
>> fflush(stdin);[/color][/color]
[snip][color=blue]
>
> fflush(stdin) invokes undefined behavior.[/color]

Yes, but sachin should have written "fflush(stdout) ", obviously.

--Mac

**Keith Thompson** · Nov 14 '05, 04:40 PM

Re: scanf in a for loop

Mac <foo@bar.net> writes:[color=blue]
> On Sat, 25 Dec 2004 20:14:48 +0000, Keith Thompson wrote:[color=green]
>> "sachin" <reachsachin@gm ail.com> writes:[/color]
> [snip][color=green][color=darkred]
>>> printf("Enter grade #%i: ", i);
>>> fflush(stdin);[/color][/color]
> [snip][color=green]
>>
>> fflush(stdin) invokes undefined behavior.[/color]
>
> Yes, but sachin should have written "fflush(stdout) ", obviously.[/color]

Actually, I think sachin meant to clear the input characters. The way
to do that is to read and discard them. (I'm too lazy to work out the
details.)

Merry Christmas!

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

**infobahn** · Nov 14 '05, 04:40 PM

Re: scanf in a for loop

Mac wrote:[color=blue]
> On Sat, 25 Dec 2004 20:14:48 +0000, Keith Thompson wrote:
>
>[color=green]
>>"sachin" <reachsachin@gm ail.com> writes:[/color]
>
> [snip]
>[color=green][color=darkred]
>>>printf("Ente r grade #%i: ", i);
>>>fflush(stdin );[/color][/color]
>
> [snip]
>[color=green]
>>fflush(stdi n) invokes undefined behavior.[/color]
>
>
> Yes, but sachin should have written "fflush(stdout) ", obviously.[/color]

And he should have written corrected the OP's i% to %i in the
scanf call. And he should have demonstrated how to test the
return value from scanf properly...

**Mac** · Nov 14 '05, 04:40 PM

Re: scanf in a for loop

On Sat, 25 Dec 2004 23:06:42 +0000, Keith Thompson wrote:
[color=blue]
> Merry Christmas![/color]

Merry Christmas to you, too!

;-)

--Mac

**Mac** · Nov 14 '05, 04:40 PM

Re: scanf in a for loop

On Sun, 26 Dec 2004 02:47:06 +0000, infobahn wrote:
[color=blue]
> Mac wrote:[color=green]
>> On Sat, 25 Dec 2004 20:14:48 +0000, Keith Thompson wrote:
>>
>>[color=darkred]
>>>"sachin" <reachsachin@gm ail.com> writes:[/color]
>>
>> [snip]
>>[color=darkred]
>>>>printf("Ent er grade #%i: ", i);
>>>>fflush(stdi n);[/color]
>>
>> [snip]
>>[color=darkred]
>>>fflush(stdin ) invokes undefined behavior.[/color]
>>
>>
>> Yes, but sachin should have written "fflush(stdout) ", obviously.[/color]
>
> And he should have written corrected the OP's i% to %i in the
> scanf call.[/color]

Actually, he did. It's still visible above...

--Mac

**infobahn** · Nov 14 '05, 04:40 PM

Re: scanf in a for loop

Mac wrote:[color=blue]
> On Sun, 26 Dec 2004 02:47:06 +0000, infobahn wrote:
>
>[color=green]
>>Mac wrote:
>>[color=darkred]
>>>On Sat, 25 Dec 2004 20:14:48 +0000, Keith Thompson wrote:
>>>
>>>
>>>
>>>>"sachin" <reachsachin@gm ail.com> writes:
>>>
>>>[snip]
>>>
>>>
>>>>>printf("En ter grade #%i: ", i);
>>>>>fflush(std in);
>>>
>>>[snip]
>>>
>>>
>>>>fflush(stdi n) invokes undefined behavior.
>>>
>>>
>>>Yes, but sachin should have written "fflush(stdout) ", obviously.[/color]
>>
>>And he should have written corrected the OP's i% to %i in the
>>scanf call.[/color][/color]
^^^^^
[color=blue]
> Actually, he did. It's still visible above...[/color]

No, he didn't, and no, it isn't.

**Emmanuel Delahaye** · Nov 14 '05, 04:40 PM

Re: scanf in a for loop

sachin wrote on 25/12/04 :[color=blue]
> fflush(stdin);[/color]

No. Please, read the FAQ.

--
Emmanuel
The C-FAQ: http://www.eskimo.com/~scs/C-faq/faq.html
The C-library: http://www.dinkumware.com/refxc.html

"Clearly your code does not meet the original spec."
"You are sentenced to 30 lashes with a wet noodle."
-- Jerry Coffin in a.l.c.c++

**Mac** · Nov 14 '05, 04:41 PM

Re: scanf in a for loop

On Sun, 26 Dec 2004 07:25:14 +0000, infobahn wrote:
[color=blue]
> Mac wrote:[color=green]
>> On Sun, 26 Dec 2004 02:47:06 +0000, infobahn wrote:
>>
>>[color=darkred]
>>>Mac wrote:
>>>
>>>>On Sat, 25 Dec 2004 20:14:48 +0000, Keith Thompson wrote:
>>>>
>>>>
>>>>
>>>>>"sachin" <reachsachin@gm ail.com> writes:
>>>>
>>>>[snip]
>>>>
>>>>
>>>>>>printf("E nter grade #%i: ", i);
>>>>>>fflush(st din);
>>>>
>>>>[snip]
>>>>
>>>>
>>>>>fflush(std in) invokes undefined behavior.
>>>>
>>>>
>>>>Yes, but sachin should have written "fflush(stdout) ", obviously.
>>>
>>>And he should have written corrected the OP's i% to %i in the
>>>scanf call.[/color][/color]
> ^^^^^
>[color=green]
>> Actually, he did. It's still visible above...[/color]
>
> No, he didn't, and no, it isn't.[/color]

My apologies. You are correct on both counts. I was looking at the
printf() call, as you seem to have surmised.

--Mac

**king arthur** · Nov 14 '05, 04:42 PM

Re: scanf in a for loop

On Sat, 25 Dec 2004 02:48:27 -0500, "" <> wrote:
[color=blue]
>In Stephen G. Kochan's book "Programmin g in C" there is a program with the
>following:
>
>for(i = 1; i <= numOfGrades; ++i) {
> printf("Enter grade #%i: ", i);
> scanf("i%", &grade);
> ...
> }
>
>Does the scanf command pause the loop until the user input is received?
>When I run this program the loop keeps on going without pausing for the
>user input.
>
>What is going on here? Can anyone please help me?
>
>JoshO.[/color]

First of all I believe the scanf statement as listed above has a
slight error. The "i%" should be "%i".

Retry the program with this correction and see what happens.

scanf in a for loop

scanf in a for loop

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