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Re: Re: Assignment problem

  • To: mathgroup at smc.vnet.net
  • Subject: [mg86383] Re: [mg86362] Re: Assignment problem
  • From: danl at wolfram.com
  • Date: Mon, 10 Mar 2008 02:02:19 -0500 (EST)
  • References: <fqo8o1$suo$1@smc.vnet.net> <fqqs3b$kf5$1@smc.vnet.net>

> Helen Read wrote:
>> Albert Retey wrote:
>>> Note that usually using a Do-Loop is the better choice in Mathematica:
>>
>> (as compared with a For loop).
>>
>> This is the second post I've seen today that says that Do loops are
>> better than For loops in Mathematica. What is better about them (aside
>> from slightly simpler syntax)?
>
> If it were up to me, I would ban the For construct from the Mathematica
> language.  We wouldn't loose anything by eliminating For.

"We" is a bit too general. Me, I'd lose a lot.


> 1.  It is completely redundant.[...]
> 2.  When newcomers to the language want a looping construct, the first
> thing they search for is "for", because this is what they are familiar
> with from other languages.  This results in abominations like this:
>
> l = {};
> For[i = 1, i <= 10, i++,
>      AppendTo[l, f[i]];
> ]
> [...]
> 3. For[] usually results in slow code, mainly because assignment (and
> changing the values of variables) is slow in Mathematica.[...]
> 4.  Using For[] is error-prone.
>
> When using For[], one is forced to create an otherwise useless global
> variable, "i".[...]
> Another possiblity of error is to just run the For[] loop without the
> initialization l = {}, which will create unexpected results with no
> error or warning message.
>
> 5.  Once a newcomer has found For[], s/he might just stick to it,
> thinking that it is "the main looping construct".[...]

All of these are good points: they indicate ways in which For[] is often
used to disadvantage.


> I think that For[] is completely useless for experienced users (in the
> sense that its absence would not make a difference), but it can cause
> considerable damage when newbies start using it.  I don't understand at
> all why it was included in the language.[...]

If it were to go away, believe me, I would miss it.


> Maybe it was just a marketing
> decision, so those coming from other languages will feel more at home,
> and Mathematica can be advertised as a multiparadigm programming system
> (which it is even without For!).  I don't think that a C programmer who
> is transliterating from C to Mathematica will feel at home in
> Mathematica ...
>
> Note that I am *not* arguing against the use of a procedural style in
> Mathematica.  In fact, a procedural approach is the best solution to
> *certain* problems.  But when newbies find For[] they tend to not look
> further and not learn the style of programming which makes Mathematica
> really effective.  For[] also brings out most of the disadvantages of
> using a procedural style in Mathematica: bad performance because of
> constantly changing variables; too much verbosity and a too "low-level"
> approach to problems; and leaking of temporary variables into the global
> namespace (because of the way scoping works in Mathematica).

There are procedural constructs for which For[] just seems well suited to
handle. At least that's how it seems to arise in code I write. This might
be from a C-and-related background, I'm not sure. I doubt there is a "one
size fits all" approach to procedural programming, so maybe it's a
construct that suits some individual styles and not others.

I guess I should make full disclosure so I will mention that I have even
used Goto/Label in Mathematica. Not often; five times in 16+ years. But
that's probably five more times than virtually everyone else.

Daniel Lichtblau
Wolfram Research






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