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Incrementing a Hash Value

by arunhorne (Pilgrim)
on Jun 14, 2002 at 10:51 UTC ( #174449=perlquestion: print w/replies, xml ) Need Help??

arunhorne has asked for the wisdom of the Perl Monks concerning the following question:

Hi... I want to increment the value of a hash by one on a certain condition. I thought the following would work:

$thes_name{$fields[1]} = thes_name{$fields[1]}++;

However the value always remains as 1 (the value for each key is initialised to 1). The following code works though:

$count = $thes_name{$fields[1]}; $count++; $thes_name{$fields[1]} = $count;

Can anyone tell me why this is... I would like to use the former code style if possible for compactness/to avoid creating another variable.



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Re: Incrementing a Hash Value
by Abigail-II (Bishop) on Jun 14, 2002 at 10:57 UTC
    $thes_name{$fields[1]} = thes_name{$fields[1]}++;
    This is just a version of the canonical:
    $i = $i ++;
    DO NOT DO THAT. Now you have an expression in which you are modifying $i twice. It's behaviour is undefined. If you want to increment the value, just use:
    thes_name {$fields [1]}++;
    That's all.


      Is it undefined, or does do this:
      # basic form $i = $i++; # long hand $k = $i; $i++; $i = $k;

      ¤ Steve Marvell

        Is it undefined, or does do this: $k = $i; $i++; $i = $k;


        $i++ stores the old value, increases the value, and then sets the return value to the stored old value. Although that is true for all versions of Perl, its behaviour is not documented. Actually, it is documented, but wrong:

        [...] increment or decrement the variable after returning the value. [...] -- perlop
        It doesn't increment after returning. It increments first, and then returns the old value.

        - Yes, I reinvent wheels.
        - Spam: Visit eurotraQ.

          A reply falls below the community's threshold of quality. You may see it by logging in.

      $i = $i ++; Its behaviour is undefined.

      It is defined. The ++ operator has nothing to do with this, in Perl. You're right about perlop, and with multiple increments/decrement the order in which things happen is undefined (well, not always. But it is not clear either). But that isn't really what we're dealing with. This is an assignment, just like any $i = EXPR. In this case, EXPR is $i++, which causes $i to be incremented, but returns the old value. The value of EXPR is assigned to $i, so as an end result nothing happens. If $i is tied, FETCH, STORE and STORE are called.

      $i = 5; $i = $i++; print $i, "\n"; # 5\n
      1. $i is set to 5
      2. $i is incremented, new value is 6
      3. old value (5) is used as the rhs of the assignment operator
      4. $i is set to that value, old value was 6, new value is 5
      5. $i and newline are printed
      $i = 5; $i = ++$i; print $i, "\n"; # 6\n
      1. $i is set to 5
      2. $i is incremented, new value is 6
      3. new value (6) is used as the rhs of the assignment operator
      4. $i is set to that value, old value was 6, new value is 6
      5. $i and newline are printed

      I have tried this with Perl 5.005 and 5.6.1.

      Note: I agree that $i = $i++ is bad style and should never ever be used in non-obfu.

      - Yes, I reinvent wheels.
      - Spam: Visit eurotraQ.

        Proof by experimentation's not valid for programming languages. Bad, bad idea. (This isn't physics here--the rules of the universe are subject to change from version to versions) Only what the standard, or the documentation, guarantees is valid to count on. Everything else should be considered a quirk of the implementation.

        This bit, in particular, could easily be changed with a small cahnge to the optimizer, or optree generator. That it hasn't happened is mainly because nobody's bothered. (Well, because that part of the code's reasonably scary) Because of the way that perl works internally, both the pre and post increment versions of that code could easily resolve to 6.

        Trust Abigail here. Don't count on the behaviour of multiple manipulations to a variable without an intervening sequence point.

        (I really should know better than to walk into this firestorm, but I just can't help myself...)

        The two of you obviously mean different things by "defined".

        Abigail says that $i = $i++ is undefined because it is not declared in the Perl (or C) language specs. Juerd says that it is defined because it has consistently behaved in the same way for as long as anyone can remember.

        In my experience, Abigail's definition of definedness is the one most commonly (and, many would argue, most properly) used in this context. While the behaviour of $i = $i++ may be deterministic, the fact remains that its behaviour is merely an artifact of how it is implemented and should not be relied upon, because, without any implementation-independent specification of its behaviour, next release of perl is free to arbitrarily change it for any reason (or no reason at all).

        This is so utterly wrong, and it's unbelievable that after so many years, people keep iterating the wrong myths.

        Please do provide a pointer to the documentation that garantees things will happen this way. All $i ++ is saying that $i will be incremented after the value is returned. But it is nowhere specified that $i will be incremented before or after an assignment. Its behaviour is UNDEFINED.

        (And if you consider this a flame, please take your question to comp.lang.c (before you say "Perl isn't C", look up the discussion of ++ in perlop. It says that it works as in C) and really learn what being flamed is.)


      Why is this difficult to grasp? It seems like good common sense to me.
Re: Incrementing a Hash Value
by marvell (Pilgrim) on Jun 14, 2002 at 10:54 UTC
    Try simply: $thes_name{$fields[1]}++;

    Steve Marvell

Re: Incrementing a Hash Value
by Sifmole (Chaplain) on Jun 14, 2002 at 11:05 UTC
    marvell has a correct answer for you. I wanted to explain for you why your choice was not working.
    // I am making the assumption that the syntax // error here is just a typo ( missing a $ in front // of the second thes_name) . $thes_name{$fields[1]} = thes_name{$fields[1]}++;

    What this code does is effectively the following:
    Start) $thes_name{$fields[1]} has a value of 1
    1) Resolves $thes_name{$fields[1]} to its value "1".
    2) Increments $thes_name{$fields[1]} to "2"
    3) Sets $thes_name{$fields[1]} to the previously resolved value, "1".
    End) Thus $thes_name always stays at "1".

    Because you used a "post-increment" it happens after the variable is already resolved. If you had used a "pre-increment", ++$thes_name{$fields[1]}, if would have done what you expected -- but marvells answer for that is better.

      It's a common mistake to assume that it works this way. And with some compilers and some versions of Perl it will work on some platforms for some values of the hash. But in other cases, pink monkeys may fly out of your nose, your dog may die and your piano suddenly walks away.

      The behaviour of $i = $i ++ in all its variants is undefined. Anything could happen.


        And with some compilers and some versions of Perl it will work on some platforms for some values of the hash.

        Compilers and platforms have absolutely nothing to do with the order in which Perl walks its ops.

        Perl code (code written in Perl) is not compiled by a C compiler.

        - Yes, I reinvent wheels.
        - Spam: Visit eurotraQ.

        Please provide a single instance on any platform or version of Perl where it does not work the way I described. Not theory, but a specific example.
Re: Incrementing a Hash Value
by Juerd (Abbot) on Jun 14, 2002 at 17:34 UTC

    $thes_name{$fields[1]} = thes_name{$fields[1]}++;

    This being a no-op, and all postincrement stuff aside, you're missing a $ before thes_name.

    19:29 < ua> did anyone bother mentioning to the guy that he was missing a '$'
                in his postdec?
    (No, just placing the $ there won't do what you want. The correct example has already been given.)

    - Yes, I reinvent wheels.
    - Spam: Visit eurotraQ.

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