Help with scanf

Discussion in 'C Programming' started by Sivarn, Apr 1, 2004.

  1. Sivarn

    Sivarn Guest

    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...
    }
     
    Sivarn, Apr 1, 2004
    #1
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  2. Sivarn

    Leor Zolman Guest

    On 31 Mar 2004 19:54:06 -0800, (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
     
    Leor Zolman, Apr 1, 2004
    #2
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  3. Sivarn

    Leor Zolman Guest

    On Thu, 01 Apr 2004 04:39:57 GMT, Leor Zolman <> 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
     
    Leor Zolman, Apr 1, 2004
    #3
  4. Sivarn

    Simon Biber Guest

    "Leor Zolman" <> 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.
     
    Simon Biber, Apr 1, 2004
    #4
  5. Sivarn

    Simon Biber Guest

    "Simon Biber" <> wrote:
    > "Leor Zolman" <> 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.
     
    Simon Biber, Apr 1, 2004
    #5
  6. On Thu, 1 Apr 2004, Leor Zolman wrote:
    >
    > On 31 Mar 2004 19:54:06 -0800, (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
     
    Arthur J. O'Dwyer, Apr 1, 2004
    #6
  7. Sivarn

    Leor Zolman Guest

    On Thu, 01 Apr 2004 05:29:51 GMT, "Simon Biber" <>
    wrote:

    >"Leor Zolman" <> 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
     
    Leor Zolman, Apr 1, 2004
    #7
  8. Sivarn

    Leor Zolman Guest

    On Thu, 1 Apr 2004 02:54:31 -0500 (EST), "Arthur J. O'Dwyer"
    <> wrote:

    >
    >On Thu, 1 Apr 2004, Leor Zolman wrote:
    >>
    >> On 31 Mar 2004 19:54:06 -0800, (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
     
    Leor Zolman, Apr 1, 2004
    #8
  9. Sivarn

    olaf Guest

    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



    (Sivarn) wrote in message news:<>...
    > 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...
    > }
     
    olaf, Apr 1, 2004
    #9
  10. Sivarn

    Simon Biber Guest

    "Leor Zolman" <> wrote:
    > "Simon Biber" <> wrote:
    > >"Leor Zolman" <> 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.
     
    Simon Biber, Apr 1, 2004
    #10
  11. Sivarn

    Leor Zolman Guest

    On Thu, 01 Apr 2004 13:45:02 GMT, "Simon Biber" <>
    wrote:

    >"Leor Zolman" <> wrote:
    >> "Simon Biber" <> wrote:
    >> >"Leor Zolman" <> 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.


    Well, then, that's the last straw. I've been driven to actually create a
    checklist, because there's no /way/ I'm going to remember to flush my
    stdout before reading stdin unless I have that written down somehwere
    (grumble...) ;-)
    -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
     
    Leor Zolman, Apr 1, 2004
    #11
  12. Sivarn

    Dan Pop Guest

    In <> (Sivarn) writes:

    >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?


    Try engaging your brain. What is the smallest 4-digit number? 1000.
    What is the largest 4-digit number? 9999. So, the test for 4-digit
    numbers (with no preceding zeros) becomes:

    if (year >= 1000 && year <= 9999) /* good input */ ;
    else /* bad input */ ;

    Now, if your number is supposed to be a year, you may want to restrict
    the valid input range even further, using exactly the same technique.

    Dan
    --
    Dan Pop
    DESY Zeuthen, RZ group
    Email:
     
    Dan Pop, Apr 1, 2004
    #12
  13. Sivarn

    Mike Wahler Guest

    "Leor Zolman" <> wrote in message
    news:...

    > >> 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?


    Yes, if you want to guarantee the output appears before
    the input request.

    > Where'd I get the impression there was some sort of special case for
    > that...


    Probably from C++, where cin/cout are 'tied', causing
    the output to appear first automatically.

    -Mike
     
    Mike Wahler, Apr 1, 2004
    #13
  14. Sivarn

    Mike Wahler Guest

    "Leor Zolman" <> wrote in message
    news:...
    > On Thu, 01 Apr 2004 13:45:02 GMT, "Simon Biber" <>
    > wrote:
    >
    > >"Leor Zolman" <> wrote:
    > >> "Simon Biber" <> wrote:
    > >> >"Leor Zolman" <> 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.
    >
    > Well, then, that's the last straw. I've been driven to actually create a
    > checklist, because there's no /way/ I'm going to remember to flush my
    > stdout before reading stdin unless I have that written down somehwere
    > (grumble...) ;-)


    Or you see your program behave 'wrongly', i.e. it waits
    for input before a 'prompt' appears (I've actually had this
    happen to me on some older DOS platforms -- my lesson
    was a 'real-world' one, so it stuck. :)).

    -Mike
     
    Mike Wahler, Apr 1, 2004
    #14
  15. Simon Biber wrote:
    > "Leor Zolman" <> wrote:
    >> "Simon Biber" <> wrote:
    >> >"Leor Zolman" <> 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.


    Which systems require the flush?

    Jeremy.
     
    Jeremy Yallop, Apr 1, 2004
    #15
  16. Sivarn

    Mike Wahler Guest

    "Jeremy Yallop" <> wrote in message
    news:...
    > > 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.
    >
    > Which systems require the flush?


    It's not required by 'systems', but by the language
    standard.

    -Mike
     
    Mike Wahler, Apr 1, 2004
    #16
  17. Mike Wahler wrote:
    > "Jeremy Yallop" <> wrote in message
    > news:...
    >> > 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.
    >>
    >> Which systems require the flush?

    >
    > It's not required by 'systems', but by the language
    > standard.


    No. The intent of the standard is exactly the opposite.

    5.1.2.3
    The input and output dynamics of interactive devices shall take
    place as specified in 7.19.3. The intent of these requirements is
    that unbuffered or line-buffered output appear as soon as possible,
    to ensure that prompting messages actually appear prior to a
    program waiting for input.

    7.19.3
    Furthermore, characters are intended to be transmitted as a block
    to the host environment when a buffer is filled, when input is
    requested on an unbuffered stream, or when input is requested on a
    line buffered stream that requires the transmission of characters
    from the host environment.

    This comes up rather often: see Chris Torek's article
    <news:> from a few days ago for an
    example.

    Also, your news client is munging quoted text. Could you fix it,
    please?

    Jeremy.
     
    Jeremy Yallop, Apr 1, 2004
    #17
  18. On Thu, 1 Apr 2004, Leor Zolman wrote:
    >
    > On Thu, 1 Apr 2004 02:54:31 -0500 (EST), Arthur J. O'Dwyer wrote:
    > >
    > > 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?


    That's not intrinsic behavior of 'scanf' in general, but of the '%d'
    and other format specifiers. The only reason my code ignores leading
    whitespace (including '\n') is because I told it to. If you want to
    get rid of the leading-whitespace-gobbler, you just remove the whitespace-
    gobbling format specifier from the format string. That is, change

    > > if (1 != scanf(" %4[0123456789]", buffer)) {


    to

    if (1 != scanf("%4[0123456789]", buffer)) {

    or if you're really stubborn and clever,

    scanf("%*[\t ]");
    if (1 != scanf("%4[0123456789]", buffer)) {

    This *must* be two separate 'scanf' invocations, though; if you run the
    format specifiers together, it *demands* leading whitespace, which is
    certainly not what we want! :)

    -Arthur
     
    Arthur J. O'Dwyer, Apr 1, 2004
    #18
  19. Sivarn

    Leor Zolman Guest

    On Thu, 1 Apr 2004 17:21:35 -0500 (EST), "Arthur J. O'Dwyer"
    <> wrote:

    >
    >On Thu, 1 Apr 2004, Leor Zolman wrote:
    >>
    >> On Thu, 1 Apr 2004 02:54:31 -0500 (EST), Arthur J. O'Dwyer wrote:
    >> >
    >> > 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?

    >
    > That's not intrinsic behavior of 'scanf' in general, but of the '%d'
    >and other format specifiers. The only reason my code ignores leading
    >whitespace (including '\n') is because I told it to. If you want to
    >get rid of the leading-whitespace-gobbler, you just remove the whitespace-
    >gobbling format specifier from the format string. That is, change
    >
    >> > if (1 != scanf(" %4[0123456789]", buffer)) {

    >
    >to
    >
    > if (1 != scanf("%4[0123456789]", buffer)) {


    Ha! I must admit I had to stare at that new version for about 30 seconds
    before I noticed the difference. I honestly didn't know you could do that
    with scanf (If my own implementation of scanf from BDS C did that right, I
    don't remember ever consciously thinking about it). So that's good to
    know, and one point back to scanf's corner.
    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
     
    Leor Zolman, Apr 1, 2004
    #19
  20. Sivarn

    Dan Pop Guest

    In <> Leor Zolman <> writes:

    >don't remember ever consciously thinking about it). So that's good to
    >know, and one point back to scanf's corner.


    In competent hands, scanf is a very powerful beast. Could have been even
    better if the committee tried harder (the * feature of printf conversion
    specifications is sometimes badly needed).

    Dan
    --
    Dan Pop
    DESY Zeuthen, RZ group
    Email:
     
    Dan Pop, Apr 2, 2004
    #20
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