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

I have an array reference with various undef values within it. I want to initialise all undef's so as a test I wrote 
#!/usr/local/bin/perl -w
use strict;

my $x = ['1','2',undef,undef,undef,'4'];
@$x = map { $_ ||= "" } @$x;

while (@$x) {
    print ">", shift @$x, "<\n";
}

### This gives
>1<
>2<
><
><
><
>4<

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That works as I wanted so I ported the code into my main program.... 
    $res = $dh->prepare($sql);
    $res->execute();

    my @dataset = ();
    # Iterate over result set building up the result set
    do {
        while (my $data = $res->fetch()) {
            if ($res->{syb_result_type} == CS_ROW_RESULT) {
                if (defined $data) {
                    @$data = map {$_ ||= ""} @$data;

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This bombs instead with.. 
Modification of a read-only value attempted at TradeExtract.pl line 10
+3 (#1)

    (F) You tried, directly or indirectly, to change the value of a
    constant.  You didn't, of course, try "2 = 1", because the compile
+r
    catches that.  But an easy way to do the same thing is:

        sub mod { $_[0] = 1 }
        mod(2);

    Another way is to assign to a substr() that's off the end of the s
+tring.

Uncaught exception from user code:
    Modification of a read-only value attempted at TradeExtract.pl lin
+e 103.

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I'm missing something simple here.. Help!

Replies are listed 'Best First'.
(tye)Re: Modification of a read-only
by tye (Sage) on Sep 11, 2001 at 21:05 UTC

    The code @$x = map { $_ ||= "" } @$x; modifies the original values of @$x and the replaces @$x with those modified values. It is a bit redundant, like doing: 

    for(  @$x  ) {
    $_ ||= "";
}
@$x= @$x;

    [download]
    Two good heuristics when using map:
    1. Don't use map in a void context
    2. Don't modify $_ inside of map
    and you've broken rule 2 here.

    This code @$x= map { defined($_) ? $_ : "" } @$x; avoids that problem.

            - tye (but my friends call me "Tye")
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Re: Modification of a read-only
by derby (Abbot) on Sep 11, 2001 at 23:13 UTC
    Jonathan,

    While <node>tye</node> makes some valid points, I don't think that's what the problem is here. I believe deep down in the DBD::Sybase (that's what driver you're using correct?) there is some little magic that is marking the return'ed ref as read only (disclaimer - I'm not an internals expert).

    If we try some of the other fetch methods, we see some interesting things: 

    my $data;
do {
   while ($data = $res->fetchrow_arrayref()) {
      if ($res->{syb_result_type} == CS_ROW_RESULT) {
         if (defined $data) {
            @$data = map { defined($_) ? $_ : "" } @$data;
         }
      }
   }
}

    [download]

    same problem. However: 

    my @data;
do {
   while (@data = $res->fetchrow_array()) {
      if ($res->{syb_result_type} == CS_ROW_RESULT) {
         if (@data) {
            @data = map { defined($_) ? $_ : "" } @data;
         }
      }
   }
}

    [download]

    works. The funny thing is by stepping through the map statement in the debugger, the assignments to $_ are fine, it's just the final assigment to @$data that is barfing. Any Inline or XS writers out there care to comment?

    -derby

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[select]
      DBI internals actually allocate the array that is returned by fetch_xxxref() - DBD::Sybase just fills each item as needed.

      DBI also sets this array "read-only", which I had to work-around as Sybase can return multiple result sets with varying number of columns.

      So DBD::Sybase has this code in the fetch routine: 

          av = DBIS->get_fbav(imp_sth);
    num_fields = AvFILL(av)+1;

    /* XXX
       The code in the if() below is likely to break with new versions
       of DBI!!! */
    if(num_fields < imp_sth->numCols) {
    int isReadonly = SvREADONLY(av);
    if(isReadonly)
        SvREADONLY_off(av);        /* DBI sets this readonly  */
    i = imp_sth->numCols - 1;
    while(i >= num_fields)
        av_store(av, i--, newSV(0));
    num_fields = AvFILL(av)+1;
    if(isReadonly)
        SvREADONLY_on(av);        /* protect against shift @$row etc *
+/

      [download]
      To summarize - the array that is referenced by $data in the code snippet above is read-only, but each of the items in the array are read-write (hence the assignment to @$data fails, but setting $_ for each undef item works).

      Michael

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[d/l]
      tye makes a valid point (I'm cutting too many corners lately) but your response appears much closer. Yes it is a Sybase database. I'm using fetchrow_arrayref for performance reasons (I'm processing half a million records, about 350Mb of data). I've still got the problem. I want to change the data in the array reference to get rid of warnings but don't want the performance hit of creating another array. Doh!
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        I'd use a 

        local $^W = 0;

        [download]
        to get rid of the warnings...

        Michael

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[d/l]

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