| Re: Old Timers ?? [message #22297] |
Thu, 02 November 2000 09:55 |
John-David T. Smith
Messages: 384
Registered: January 2000
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Senior Member |
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Paul van Delst wrote:
> O.k., following the advice that "he who asks a question is a fool for 5 minutes, he who
> does not ask a question is a fool forever" I have to ask:
>
> What is a hash?
>
> I have seen the term used before regarding string searches but, being a mostly fortran
> number crunchy type programmer, I've never been one for searching through strings and what
> not.
Also known as a "named associative list", or "associative array", hashes
(or "hash tables") are specialized lookup tables. You can think of them
as arrays indexed, not with numeric indices, but with string values (or
even other data types for some languages). In the example in this
thread, a hash element would exist for each keyword... e.g.
hash["file ID"]="test"
hash["otherkeyword"]=12.22
etc. Hashes are like infinitely malleable structures, which is why they
are so nice. They are somewhat complicated to make (not for us, but for
the language designers), but easy and fast to use. Here's a link I
found: http://www.math.utah.edu/~beebe/software/hash/hash.html.
Hashes are built into Perl, which is why they are so popular these days,
I think (no offense to the lisp programmers out there).
JD
--
J.D. Smith | WORK: (607) 255-6263
Cornell Dept. of Astronomy | (607) 255-5842
304 Space Sciences Bldg. | FAX: (607) 255-5875
Ithaca, NY 14853 |
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| Re: Old Timers ?? [message #22300 is a reply to message #22297] |
Thu, 02 November 2000 08:31  |
Paul van Delst
Messages: 364
Registered: March 1997
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Senior Member |
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"J.D. Smith" wrote:
>
> Eric Kihn wrote:
>> 2) It's clear this code is simulating a HASH with Arrays, does IDL 5.3 have
>> native hash support of some kind?
>
> 2) No, sadly. The get_keyword routine probably does a linear search
> through the header line array, looking for "file ID" in this case. This
> is of course just the type of thing hashes were made for. The
> get_keyword routine I found just uses "where", as you might expect.
> This means not only is the search linear, all elements are considered,
> even after a match is found. It could have been made more efficient in
> IDL 5.4 if array_equal() had been implemented not just to return true as
> soon as a match is found, but return the index of that match. Oh well.
> Maybe an optional variable will be added to array_equal for this in
> 5.4.1. (hint)
>
> It is possible to make a better simulation of a hash with a structure,
> but that is somewhat inflexible since adding new keywords (fields) is
> difficult, without copying the whole thing, though create_struct() can
> do it. Then there is the matter of indexing the structure with a
> variable field (the essence of what hashes do). Possible, but not
> efficient... in fact nearly as inefficient as a "where" on the full
> array. Unfortunately, it's not that easy to write a good hash type that
> scales well for small and large hashes, and doesn't eat too much
> memory.
O.k., following the advice that "he who asks a question is a fool for 5 minutes, he who
does not ask a question is a fool forever" I have to ask:
What is a hash?
I have seen the term used before regarding string searches but, being a mostly fortran
number crunchy type programmer, I've never been one for searching through strings and what
not.
The only dictionary definitions I found were:
1) To chop up meat/potatoes into small pieces
2) To confuse or muddle
3) To stick in one's pipe and smoke it.
:o)
paulv
--
Paul van Delst Ph: (301) 763-8000 x7274
CIMSS @ NOAA/NCEP Fax: (301) 763-8545
Rm.207, 5200 Auth Rd. Email: pvandelst@ncep.noaa.gov
Camp Springs MD 20746
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| Re: Old Timers ?? [message #22301 is a reply to message #22300] |
Thu, 02 November 2000 08:01 |
John-David T. Smith
Messages: 384
Registered: January 2000
|
Senior Member |
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Eric Kihn wrote:
>
> Hello all,
>
> I've go some legacy code I'm trying to update and it looks like:
>
> ; Read text header from file
> ; until 'end header' reached.
> ;-----
> line = ''
> header = ''
> repeat begin
> readf, unit, line
> if (n_elements(header) eq 0) then $
> begin
> header = line
> endif else $
> begin
> header = [header,line]
> endelse
> endrep until (strtrim(line,2) eq 'end header')
>
> ;-----
> ; Close file.
> ;-----
> close, unit & free_lun, unit
>
> ;-----
> ;Get relevant info from the header.
> ;-----
> dda_header.file_id = $
> strtrim( get_keyword( header, 'file ID'), 2)
>
> This code is circa 95 and it's balking at the get_keyword() function. So my
> questions are two:
> 1) Did previous versions of IDL have a get_keyword() (I'm 5.3 now)?
> 2) It's clear this code is simulating a HASH with Arrays, does IDL 5.3 have
> native hash support of some kind?
1) No, but a search at
http://www.astro.washington.edu/deutsch/idl/htmlhelp/index.h tml reveals:
;+
; Project : SOHO - CDS
;
; Name : GET_KEYWORD
;
; Purpose : Extract values in a string array that appear after
; keyword construct such as: KEYWORD=VALUE
; (e.g. extract all time values following
STARTIME=time_value)
;
Could be from that, but then again, it might be something else
altogether.
2) No, sadly. The get_keyword routine probably does a linear search
through the header line array, looking for "file ID" in this case. This
is of course just the type of thing hashes were made for. The
get_keyword routine I found just uses "where", as you might expect.
This means not only is the search linear, all elements are considered,
even after a match is found. It could have been made more efficient in
IDL 5.4 if array_equal() had been implemented not just to return true as
soon as a match is found, but return the index of that match. Oh well.
Maybe an optional variable will be added to array_equal for this in
5.4.1. (hint)
It is possible to make a better simulation of a hash with a structure,
but that is somewhat inflexible since adding new keywords (fields) is
difficult, without copying the whole thing, though create_struct() can
do it. Then there is the matter of indexing the structure with a
variable field (the essence of what hashes do). Possible, but not
efficient... in fact nearly as inefficient as a "where" on the full
array. Unfortunately, it's not that easy to write a good hash type that
scales well for small and large hashes, and doesn't eat too much
memory.
JD
--
J.D. Smith | WORK: (607) 255-6263
Cornell Dept. of Astronomy | (607) 255-5842
304 Space Sciences Bldg. | FAX: (607) 255-5875
Ithaca, NY 14853 |
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