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Help with scanf

 
 
Sivarn
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      04-01-2004
I'm writing a program for that takes dates as input using scanf. I
want to verify that the user is inputting a full 4 digits for the
year. How do I do this? I know that the return value on printf is the
number of printed characters; so if I could somehow get my year
variable to store the leading zeros, I could just run a check:

int dummy = 0;

dummy = printf("%d", year);

if (dummy != 4)
{
...do something...
}
 
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Leor Zolman
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      04-01-2004
On 31 Mar 2004 19:54:06 -0800, http://www.velocityreviews.com/forums/(E-Mail Removed) (Sivarn) wrote:

>I'm writing a program for that takes dates as input using scanf. I
>want to verify that the user is inputting a full 4 digits for the
>year. How do I do this? I know that the return value on printf is the
>number of printed characters; so if I could somehow get my year
>variable to store the leading zeros, I could just run a check:
>
>int dummy = 0;
>
>dummy = printf("%d", year);
>
>if (dummy != 4)
>{
> ...do something...
>}


Wait a minute...you started by asking about scanf, and you say you want to
verify the input? Then I'm sure you'd rather not have to print the value
just in order to be able to validate the user input, right?

You certainly don't have to. Let's go ahead and use scanf, because that's
how you say you're doing it (but scanf is essentially useless if you need
serious validation of input. More on that later...):

int count;
int year;
count = scanf("%d", &year);

Note that you must provide the & on year, since scanf has to know /where/
to put the value. count tells you how many "items" were scanned; it could
be 0 if the user didn't type a number. So let's test that:

if (count != 1)
{
printf("Bad input. You lose.\n");
exit(1);
}

(Of course you might choose to do something other than exit here.)

So, how to test for 4 digits? Well, the easiest way I can think of is to
just test for a valid (or invalid) range:

if (year < 1900 || year > 2030) /* or whatever */
{
printf("Bad year. You're outa here.\n");
exit (1);
}

If you /really/ want "any 4 digit number", just test that it is between
1000 and 9999.

So that takes care of that. Now we can improve the input. sscanf is a royal
pain to work with, because if the user just keeps pressing enter, it'll
merrily echo blank lines till doomsday (it ignores leading whitespace,
including newlines). And if the user presses, say, a letter when scanf is
looking for a number, that letter persists on the input stream the /next/
time you try to read a number, further wreaking havoc.

Better to force a single line of input, and then process it. I like to use
fgets to read a line into a buffer, and then sscanf to do the conversion.
sscanf is a lot like scanf, but it reads input from a string buffer instead
of from the standard input. Putting all of that together with some
re-prompting logic, here's a program for you:

#include <stdio.h>

int main()
{
int year;
char buffer[100];

while (1)
{
printf("Please enter a year: ");
fgets(buffer, 100, stdin);
if (sscanf(buffer, "%d", &year) != 1)
printf("that wasn't a number! Try again....\n");
else if (year < 1900 || year > 2030)
printf("Bad year! Must be between 1900 and 2030."
" Try again...\n");
else
break;
}

printf("Success! your year is %d\n", year);

return 0;
}

That is something you can now expand to support as much processing on that
line of input as you please, without running into a slew of problems you'd
get from using just scanf.

Good luck,
-leor



--
Leor Zolman --- BD Software --- www.bdsoft.com
On-Site Training in C/C++, Java, Perl and Unix
C++ users: Download BD Software's free STL Error Message Decryptor at:
www.bdsoft.com/tools/stlfilt.html
 
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Leor Zolman
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Posts: n/a
 
      04-01-2004
On Thu, 01 Apr 2004 04:39:57 GMT, Leor Zolman <(E-Mail Removed)> wrote:

>So that takes care of that. Now we can improve the input. sscanf is a royal
>pain to work with


That should have read: /scanf/ is a royal pain...oops.
-leor

--
Leor Zolman --- BD Software --- www.bdsoft.com
On-Site Training in C/C++, Java, Perl and Unix
C++ users: Download BD Software's free STL Error Message Decryptor at:
www.bdsoft.com/tools/stlfilt.html
 
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Simon Biber
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      04-01-2004
"Leor Zolman" <(E-Mail Removed)> wrote:
> if (count != 1)
> {
> printf("Bad input. You lose.\n");
> exit(1);
> }


1 is not a portable exit code (termination status); it might have unexpected
effects. If you want to indicate failure, do so explicitly with the macro
EXIT_FAILURE. The only portable exit codes are 0, EXIT_SUCCESS and
EXIT_FAILURE. The value of EXIT_SUCCESS may or may not be zero, but has the
same effect.
exit(EXIT_FAILURE);
EXIT_FAILURE is defined in <stdlib.h> which you should already have included
for the exit function itself. It might expand to 1 or to any other value
other than zero.

Also, please don't post tabs to newsgroups. Indenting should be done with
spaces.

[...]

> #include <stdio.h>
>
> int main()
> {
> int year;
> char buffer[100];
>
> while (1)
> {
> printf("Please enter a year: ");


Here you need to flush the stdout buffer to ensure that the prompt is
displayed to the screen before you wait for user input.
fflush(stdout);

> fgets(buffer, 100, stdin);


I'd avoid magic numbers and check for success/failure
if(fgets(buffer, sizeof buffer, stdin) == NULL)
{
printf("There was a problem reading your input.\n");
exit(EXIT_FAILURE);
}

At this point if the user has entered a number outside the range of int, the
behaviour of sscanf will be undefined. This is a good point to validate the
input before continuing.
if(strlen(buffer) != 5 || strspn(buffer, "0123456789\n") != 5)
{
printf("That wasn't a four digit number! Try again...\n");
}
else

> if (sscanf(buffer, "%d", &year) != 1)
> printf("that wasn't a number! Try again....\n");
> else if (year < 1900 || year > 2030)
> printf("Bad year! Must be between 1900 and 2030."
> " Try again...\n");
> else
> break;
> }
>
> printf("Success! your year is %d\n", year);
>
> return 0;
> }


--
Simon.


 
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Simon Biber
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Posts: n/a
 
      04-01-2004
"Simon Biber" <(E-Mail Removed)> wrote:
> "Leor Zolman" <(E-Mail Removed)> wrote:
> > #include <stdio.h>


I forgot to add that my changes to the code below require
#include <stdlib.h> /* for exit and EXIT_FAILURE */
#include <string.h> /* for strlen and strspn */

> > int main()
> > {
> > int year;
> > char buffer[100];
> >
> > while (1)
> > {
> > printf("Please enter a year: ");

>
> Here you need to flush the stdout buffer to ensure that the prompt is
> displayed to the screen before you wait for user input.
> fflush(stdout);
>
> > fgets(buffer, 100, stdin);

>
> I'd avoid magic numbers and check for success/failure
> if(fgets(buffer, sizeof buffer, stdin) == NULL)
> {
> printf("There was a problem reading your input.\n");
> exit(EXIT_FAILURE);
> }
>
> At this point if the user has entered a number outside the range of int,

the
> behaviour of sscanf will be undefined. This is a good point to validate

the
> input before continuing.
> if(strlen(buffer) != 5 || strspn(buffer, "0123456789\n") != 5)
> {
> printf("That wasn't a four digit number! Try again...\n");
> }
> else
>
> > if (sscanf(buffer, "%d", &year) != 1)
> > printf("that wasn't a number! Try again....\n");
> > else if (year < 1900 || year > 2030)
> > printf("Bad year! Must be between 1900 and 2030."
> > " Try again...\n");
> > else
> > break;
> > }
> >
> > printf("Success! your year is %d\n", year);
> >
> > return 0;
> > }


--
Simon.


 
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Arthur J. O'Dwyer
Guest
Posts: n/a
 
      04-01-2004

On Thu, 1 Apr 2004, Leor Zolman wrote:
>
> On 31 Mar 2004 19:54:06 -0800, (E-Mail Removed) (Sivarn) wrote:
> >I'm writing a program for that takes dates as input using scanf. I
> >want to verify that the user is inputting a full 4 digits for the
> >year. How do I do this?


> Let's go ahead and use scanf, because that's how you say you're
> doing it (but scanf is essentially useless if you need
> serious validation of input. More on that later...):


Now, just a minute there! I'll grant you that scanf is less than
completely useful for *complex* validation, but *serious* validation
can indeed be performed, if you're willing to put up with the syntax.
I like scanf.

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>

int main(void)
{
char buffer[5] = {0};
char *bp;
int k;
int year;

puts("Enter a 4-digit year now.");
if (1 != scanf(" %4[0123456789]", buffer)) {
puts("No digits entered!");
}
else {
year = strtol(buffer, &bp, 10);
if (bp != buffer+4) {
puts("Fewer than 4 digits entered!");
}
else {
printf("You entered the (valid) year %d.\n", year);
}
}

k = getchar();
if (!isspace(k)) {
puts("You left some input at the end of your line.");
}
ungetc(k, stdin);

return 0;
}


> So, how to test for 4 digits? Well, the easiest way I can think of is to
> just test for a valid (or invalid) range:
>
> if (year < 1900 || year > 2030) /* or whatever */


Presumably the OP's problem was that he wanted *four digits of input*,
not merely a number in the range 1000-9999; i.e., counting leading
zeroes.

> So that takes care of that. Now we can improve the input. sscanf is a royal
> pain to work with, because if the user just keeps pressing enter, it'll
> merrily echo blank lines till doomsday (it ignores leading whitespace,
> including newlines). And if the user presses, say, a letter when scanf is
> looking for a number, that letter persists on the input stream the /next/
> time you try to read a number, further wreaking havoc.


OTOH, sometimes that's a nice behavior, not least because it's easy
to predict its behavior. As opposed to...

> Better to force a single line of input, and then process it.


....fgets, which reads either up to the first newline, *or* until its
buffer runs out, whichever comes first. Not that it's impossible
to do good stuff with fgets; it's not. But scanf's behavior isn't
entirely illogical, either.

HTH,
-Arthur

 
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Leor Zolman
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Posts: n/a
 
      04-01-2004
On Thu, 01 Apr 2004 05:29:51 GMT, "Simon Biber" <(E-Mail Removed)>
wrote:

>"Leor Zolman" <(E-Mail Removed)> wrote:
>> if (count != 1)
>> {
>> printf("Bad input. You lose.\n");
>> exit(1);
>> }

>
>1 is not a portable exit code (termination status); it might have unexpected
>effects. If you want to indicate failure, do so explicitly with the macro
>EXIT_FAILURE. The only portable exit codes are 0, EXIT_SUCCESS and
>EXIT_FAILURE. The value of EXIT_SUCCESS may or may not be zero, but has the
>same effect.
> exit(EXIT_FAILURE);
>EXIT_FAILURE is defined in <stdlib.h> which you should already have included
>for the exit function itself. It might expand to 1 or to any other value
>other than zero.


Sorry, old habits die hard

>
>Also, please don't post tabs to newsgroups. Indenting should be done with
>spaces.


Arghh. I forgot this time; but generally I've been pretty good. In fact,
I'd just come up with an Epsilon macro that with one keystroke a)
untabifies my edit buffer, b) copies it into the clipboard, and c)
re-tabifies. Now I just have to remember to /use/ it.

>
>[...]
>
>> #include <stdio.h>
>>
>> int main()
>> {
>> int year;
>> char buffer[100];
>>
>> while (1)
>> {
>> printf("Please enter a year: ");

>
>Here you need to flush the stdout buffer to ensure that the prompt is
>displayed to the screen before you wait for user input.
> fflush(stdout);


Is that actually required for stdout if you then pause to read from stdin?
Where'd I get the impression there was some sort of special case for
that...

>
>> fgets(buffer, 100, stdin);

>
>I'd avoid magic numbers and check for success/failure
> if(fgets(buffer, sizeof buffer, stdin) == NULL)
> {
> printf("There was a problem reading your input.\n");
> exit(EXIT_FAILURE);
> }
>
>At this point if the user has entered a number outside the range of int, the
>behaviour of sscanf will be undefined. This is a good point to validate the
>input before continuing.
> if(strlen(buffer) != 5 || strspn(buffer, "0123456789\n") != 5)
> {
> printf("That wasn't a four digit number! Try again...\n");
> }
> else
>
>> if (sscanf(buffer, "%d", &year) != 1)
>> printf("that wasn't a number! Try again....\n");
>> else if (year < 1900 || year > 2030)
>> printf("Bad year! Must be between 1900 and 2030."
>> " Try again...\n");
>> else
>> break;
>> }
>>
>> printf("Success! your year is %d\n", year);
>>
>> return 0;
>> }


Thanks!
-leor

--
Leor Zolman --- BD Software --- www.bdsoft.com
On-Site Training in C/C++, Java, Perl and Unix
C++ users: Download BD Software's free STL Error Message Decryptor at:
www.bdsoft.com/tools/stlfilt.html
 
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Leor Zolman
Guest
Posts: n/a
 
      04-01-2004
On Thu, 1 Apr 2004 02:54:31 -0500 (EST), "Arthur J. O'Dwyer"
<(E-Mail Removed)> wrote:

>
>On Thu, 1 Apr 2004, Leor Zolman wrote:
>>
>> On 31 Mar 2004 19:54:06 -0800, (E-Mail Removed) (Sivarn) wrote:
>> >I'm writing a program for that takes dates as input using scanf. I
>> >want to verify that the user is inputting a full 4 digits for the
>> >year. How do I do this?

>
>> Let's go ahead and use scanf, because that's how you say you're
>> doing it (but scanf is essentially useless if you need
>> serious validation of input. More on that later...):

>
> Now, just a minute there! I'll grant you that scanf is less than
>completely useful for *complex* validation, but *serious* validation
>can indeed be performed, if you're willing to put up with the syntax.
>I like scanf.


I think the way it ignores leading newlines alone is enough to justify
dispensing with it for interactive use. Do you really want to have to
include text such as "If you press Enter first and nothing happens, don't
keep wailing on the Enter key! Try something /else/!" as part of your
prompt?

>
>#include <stdio.h>
>#include <stdlib.h>
>#include <ctype.h>
>
>int main(void)
>{
> char buffer[5] = {0};
> char *bp;
> int k;
> int year;
>
> puts("Enter a 4-digit year now.");
> if (1 != scanf(" %4[0123456789]", buffer)) {
> puts("No digits entered!");
> }
> else {
> year = strtol(buffer, &bp, 10);
> if (bp != buffer+4) {
> puts("Fewer than 4 digits entered!");
> }
> else {
> printf("You entered the (valid) year %d.\n", year);
> }
> }
>
> k = getchar();
> if (!isspace(k)) {
> puts("You left some input at the end of your line.");
> }
> ungetc(k, stdin);
>
> return 0;
>}
>
>
>> So, how to test for 4 digits? Well, the easiest way I can think of is to
>> just test for a valid (or invalid) range:
>>
>> if (year < 1900 || year > 2030) /* or whatever */

>
> Presumably the OP's problem was that he wanted *four digits of input*,
>not merely a number in the range 1000-9999; i.e., counting leading
>zeroes.


I'm not sure that's what he really wanted; It looks like he only wanted the
leading zeros as part of his printf-related validation scheme for the
input. I don't know, but the fact he's putting the year into an int pretty
much excludes his possible use for those leading zeros...

>
>> So that takes care of that. Now we can improve the input. sscanf is a royal
>> pain to work with, because if the user just keeps pressing enter, it'll
>> merrily echo blank lines till doomsday (it ignores leading whitespace,
>> including newlines). And if the user presses, say, a letter when scanf is
>> looking for a number, that letter persists on the input stream the /next/
>> time you try to read a number, further wreaking havoc.

>
> OTOH, sometimes that's a nice behavior, not least because it's easy
>to predict its behavior. As opposed to...
>
>> Better to force a single line of input, and then process it.

>
>...fgets, which reads either up to the first newline, *or* until its
>buffer runs out, whichever comes first. Not that it's impossible
>to do good stuff with fgets; it's not. But scanf's behavior isn't
>entirely illogical, either.


A sane user interface has always been /much/ easier to get working for me
using fgets/sscanf, but of course YMMV.

>
>HTH,
>-Arthur


I expected to get some critique of this from folks, and wasn't
disappointed. I still don't quite have all the "modern C" idioms down
(EXIT_FAILURE seems to come real hard, perhaps because I just balk at
having to type it out...), but I'm slowly getting there
-leor


--
Leor Zolman --- BD Software --- www.bdsoft.com
On-Site Training in C/C++, Java, Perl and Unix
C++ users: Download BD Software's free STL Error Message Decryptor at:
www.bdsoft.com/tools/stlfilt.html
 
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olaf
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Posts: n/a
 
      04-01-2004
Hello

If you want some string to be has leading zero's

use
printf("%04d",1); => "0001"
printf("%04d",10001); => "10001"
printf("%04.4d",10001); => "1000"
printf("%4d",1); => " 1"


for a complete set of pssiblelities look in a good C manual.

Greetings



(E-Mail Removed) (Sivarn) wrote in message news:<(E-Mail Removed). com>...
> I'm writing a program for that takes dates as input using scanf. I
> want to verify that the user is inputting a full 4 digits for the
> year. How do I do this? I know that the return value on printf is the
> number of printed characters; so if I could somehow get my year
> variable to store the leading zeros, I could just run a check:
>
> int dummy = 0;
>
> dummy = printf("%d", year);
>
> if (dummy != 4)
> {
> ...do something...
> }

 
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Simon Biber
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Posts: n/a
 
      04-01-2004
"Leor Zolman" <(E-Mail Removed)> wrote:
> "Simon Biber" <(E-Mail Removed)> wrote:
> >"Leor Zolman" <(E-Mail Removed)> wrote:
> > > printf("Please enter a year: ");

> >
> > Here you need to flush the stdout buffer to ensure that the
> > prompt is displayed to the screen before you wait for user
> > input.
> > fflush(stdout);

>
> Is that actually required for stdout if you then pause to read
> from stdin? Where'd I get the impression there was some sort
> of special case for that...


Although many systems do implement a special case that they flush stdout's
buffer when you read from stdin, it is still required on some systems so
it's a good idea to include the fflush(stdout) when writing portable code.

--
Simon.


 
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