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From: tell@oscar.cs.unc.edu
Newsgroups: comp.lang.perl
Subject: #elements in assoc array / array in scalar context
Message-ID: <15429@thorin.cs.unc.edu>
Date: 30 Jul 90 00:03:58 GMT
Sender: news@thorin.cs.unc.edu
Reply-To: tell@oscar.cs.unc.edu ()
Organization: University Of North Carolina, Chapel Hill
Lines: 71


What is the recommended way to find out how many elements are in an
associative array?  I don't see any method that looks particularly
efficient.

Here's some playing around I did on some real code:

#
  DB<3> s
read_libdata(289):              $nvalues = keys(%needvalues);
  DB<3> s
read_libdata(291):              while($libfile = pop(@libfiles) && $nvalues) {
  DB<3> p $nvalues
      
#
# Hmmm. $nvalues isn't set to anything useful.  How do we find how many
# items we're looking for?
# First he takes a stab in the dark:
#
  DB<5> p $#keys(%needvalues)
syntax error in file (eval) at line 1, next 2 tokens "$#keys("

  DB<6> p $#(keys(%needvalues))
syntax error in file (eval) at line 1, next 2 tokens "$#("

# Then he remembers the manual page:
#     If you evaluate an array in a scalar context, it returns the
#     length of the array.  The following is always true:
#
#          @whatever == $#whatever - $[ + 1;
# and tries to "force a scalar (or numeric) context"
#
  DB<8> p keys(%needvalues) + 0
0
  DB<9> p (keys(%needvalues)) + 0
0
  DB<10> p 0+keys(%needvalues)
0
#
# This works, but copying that big array around seems inefficient.
# or is the optimizer that good?
#
  DB<11> @foo = keys(%needvalues); print $#foo
5
#
# Just make sure that our test data is really ther
#
  DB<12> p join(';', keys(%needvalues))
CRYSTAL;74LS04;74HCT573;74LS08;8052;LED
		  

So, how about making keys(%foo) return the number of elements in foo
if evaluated in a scalar context?    What does it produce in this context
now?

If I'm missing somthing, (and I probably am) please let me know.



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Patch level: 18

Copyright (c) 1989, 1990, Larry Wall

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--------------------------------------------------------------------
Steve Tell      e-mail: tell@wsmail.cs.unc.edu usmail:  #5L Estes Park apts
CS Grad Student, UNC Chapel Hill.                       Carrboro NC 27510
Former chief engineer, Duke Union Community Television, Durham, NC.