K&R p.97

Discussion in 'C Programming' started by Tom, Feb 11, 2012.

  1. Tom

    Tom Guest

    Guys,

    On page 97 of K&R (2nd ed) there is a function called getint().

    I won't paste it here because I assume most people can look it up, but
    if you want me to I will.

    The 6th line of the body of the function is:

    ungetch(c) /* it's not a number */

    I don't understand this. This pushes the character (c) back to input
    if it is not a number. But this creates an infinite cycle of finding a
    non-number, pushing it back to input, reading the next character on
    the input, finding it to be the same non-digit character and pushing
    it back again.

    Shouldn't this line be omitted, so that we can just skip to 'return
    0', so that we can progress to the next character in the input?

    As the function is written, if we hit a non-digit in the input it will
    just get stuck there until the algorithm hits the MAX limit.

    Am I missing something or is this a mistake?

    Thanks,

    Tom
     
    Tom, Feb 11, 2012
    #1
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  2. On Feb 11, 8:46 pm, Tom <> wrote:
    >
    > The 6th line of the body of the function is:
    >
    > ungetch(c) /* it's not a number */
    >
    > I don't understand this. This pushes the character (c) back to input
    > if it is not a number. But this creates an infinite cycle of finding a
    > non-number, pushing it back to input, reading the next character on
    > the input, finding it to be the same non-digit character and pushing
    > it back again.
    >

    I haven't a copy of the book.
    It's possible that you've found a bug in the K and R example, but it's
    unlikely. ungetc() (it should be ungetc, not ungetch) puts the last
    character read back into the input stream. It's quite commonly used
    when a parser reads to the end of the input it understands. The last
    character after a number is a non-digit, so it knows the integer is
    finished, but it can't understand what that character is doing. So it
    puts it back to enable higher level code to decide what to do.
    --
    Vist my website, lots of free games to play
    http://www.malcolmmclean.site11.com/wwww
     
    Malcolm McLean, Feb 11, 2012
    #2
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  3. Tom

    Ian Collins Guest

    On 02/12/12 09:46 AM, Tom wrote:
    > Guys,
    >
    > On page 97 of K&R (2nd ed) there is a function called getint().
    >
    > I won't paste it here because I assume most people can look it up, but
    > if you want me to I will.
    >
    > The 6th line of the body of the function is:
    >
    > ungetch(c) /* it's not a number */


    Followed by a return. This function has done its job (extracting an
    integer from the stream) so it has to push the last character it read
    back into the stream. If it didn't, that character would be lost.

    --
    Ian Collins
     
    Ian Collins, Feb 11, 2012
    #3
  4. Tom

    Eric Sosman Guest

    On 2/11/2012 3:46 PM, Tom wrote:
    > Guys,
    >
    > On page 97 of K&R (2nd ed) there is a function called getint().
    >
    > I won't paste it here because I assume most people can look it up, but
    > if you want me to I will.
    >
    > The 6th line of the body of the function is:
    >
    > ungetch(c) /* it's not a number */
    >
    > I don't understand this. This pushes the character (c) back to input
    > if it is not a number. But this creates an infinite cycle of finding a
    > non-number, pushing it back to input, reading the next character on
    > the input, finding it to be the same non-digit character and pushing
    > it back again.


    If you call the same function again without doing anything else,
    it will behave as you say. Presumably, though, after you've read
    a number and swallowed all the input up to a non-number, your next
    action would be to read and do something non-numeric with the non-
    number. For example, if the input file looks like

    1+(2-3)*5

    .... you'd probably want to examine and do something with the
    interstitial characters, not just ignore them without even
    knowing what they were.

    > Shouldn't this line be omitted, so that we can just skip to 'return
    > 0', so that we can progress to the next character in the input?


    You'll process the next character(s) in some other way. There's
    a stream of numbers and non-numbers; both components are meaningful.

    --
    Eric Sosman
    d
     
    Eric Sosman, Feb 11, 2012
    #4
  5. Tom

    Tom Guest

    On Feb 11, 9:36 pm, Ian Collins <> wrote:

    > Followed by a return.  This function has done its job (extracting an
    > integer from the stream) so it has to push the last character it read
    > back into the stream.  If it didn't, that character would be lost.
    >
    > --
    > Ian Collins


    Yes, I see your point. Thank you.

    However, if the function is called using the for loop on p.96 then we
    will see the behaviour I described, won't we?

    The code that calls the function should account for a non-numerical
    character, but it seems like the code on p.96 does not.

    Is this right?

    Thanks again,
    Tom
     
    Tom, Feb 11, 2012
    #5
  6. Tom

    Ian Collins Guest

    On 02/12/12 11:18 AM, Tom wrote:
    > On Feb 11, 9:36 pm, Ian Collins<> wrote:
    >
    >> Followed by a return. This function has done its job (extracting an
    >> integer from the stream) so it has to push the last character it read
    >> back into the stream. If it didn't, that character would be lost.


    > Yes, I see your point. Thank you.
    >
    > However, if the function is called using the for loop on p.96 then we
    > will see the behaviour I described, won't we?


    There isn't a loop on that page. But yes, if you where to blindly call
    the function without checking the result you would be in a loop.

    > The code that calls the function should account for a non-numerical
    > character, but it seems like the code on p.96 does not.


    The code is nothing more than a function to read an integer. Whatever
    calls it determines what to do next.

    --
    Ian Collins
     
    Ian Collins, Feb 11, 2012
    #6
  7. Tom

    Tom Guest

    On Feb 11, 10:38 pm, Ian Collins <> wrote:
    > On 02/12/12 11:18 AM, Tom wrote:
    >
    > > On Feb 11, 9:36 pm, Ian Collins<>  wrote:

    >
    > >> Followed by a return.  This function has done its job (extracting an
    > >> integer from the stream) so it has to push the last character it read
    > >> back into the stream.  If it didn't, that character would be lost.

    > > Yes, I see your point. Thank you.

    >
    > > However, if the function is called using the for loop on p.96 then we
    > > will see the behaviour I described, won't we?

    >
    > There isn't a loop on that page.  But yes, if you where to blindly call
    > the function without checking the result you would be in a loop.


    Maybe we have different versions of the book. In my version there is
    the following on page 96:

    for (n = 0; n < SIZE && getint(&array[n]) != EOF; n++)
    ;
     
    Tom, Feb 11, 2012
    #7
  8. Tom <> writes:

    > On Feb 11, 9:36 pm, Ian Collins <> wrote:
    >
    >> Followed by a return.  This function has done its job (extracting an
    >> integer from the stream) so it has to push the last character it read
    >> back into the stream.  If it didn't, that character would be lost.
    >>
    >> --
    >> Ian Collins

    >
    > Yes, I see your point. Thank you.
    >
    > However, if the function is called using the for loop on p.96 then we
    > will see the behaviour I described, won't we?
    >
    > The code that calls the function should account for a non-numerical
    > character, but it seems like the code on p.96 does not.
    >
    > Is this right?


    I, for one, would have to see the loop and getint function to be sure.
    However, the code in my 1st edition version will get "stuck" on
    non-numeric input, but the for loop on the previous page won't loop
    indefinitely because it is also bounded by the array size. Hence an
    input stream of, say,

    1-2 45x 99

    will read 1, -2, 45 and then fill the rest of the array with zeros,
    leaving the x unread. It's maybe not ideal, but it's a reasonable way
    to handle unintelligible input.

    --
    Ben.
     
    Ben Bacarisse, Feb 11, 2012
    #8
  9. Tom

    Ian Collins Guest

    On 02/12/12 11:44 AM, Tom wrote:
    > On Feb 11, 10:38 pm, Ian Collins<> wrote:
    >> On 02/12/12 11:18 AM, Tom wrote:
    >>
    >>> On Feb 11, 9:36 pm, Ian Collins<> wrote:

    >>
    >>>> Followed by a return. This function has done its job (extracting an
    >>>> integer from the stream) so it has to push the last character it read
    >>>> back into the stream. If it didn't, that character would be lost.
    >>> Yes, I see your point. Thank you.

    >>
    >>> However, if the function is called using the for loop on p.96 then we
    >>> will see the behaviour I described, won't we?

    >>
    >> There isn't a loop on that page. But yes, if you where to blindly call
    >> the function without checking the result you would be in a loop.

    >
    > Maybe we have different versions of the book. In my version there is
    > the following on page 96:
    >
    > for (n = 0; n< SIZE&& getint(&array[n]) != EOF; n++)
    > ;


    I do have two versions of the book, the one with my glasses on and the
    one with my glasses off!

    The loop will fill the array with zeros if the input isn't numeric. The
    loop is bound by the array size.

    --
    Ian Collins
     
    Ian Collins, Feb 12, 2012
    #9
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