Re: NMinimize problem: fct minimized uses FindRoot
- To: mathgroup at smc.vnet.net
- Subject: [mg123703] Re: NMinimize problem: fct minimized uses FindRoot
- From: DrMajorBob <btreat1 at austin.rr.com>
- Date: Fri, 16 Dec 2011 05:45:41 -0500 (EST)
- Delivered-to: l-mathgroup@mail-archive0.wolfram.com
- References: <201112101227.HAA19219@smc.vnet.net>
- Reply-to: drmajorbob at yahoo.com
> This returning Return[something] rather than just something is not the
> result of some sort of confusion in the design
Eh. A sow's ear might turn out to be a purse in disguise, but I wouldn't
count on it.
Bobby
On Thu, 15 Dec 2011 03:51:26 -0600, Andrzej Kozlowski <akoz at mimuw.edu.pl>
wrote:
>
> On 14 Dec 2011, at 12:02, Oleksandr Rasputinov wrote:
>
>> On Tue, 13 Dec 2011 10:41:11 -0000, Andrzej Kozlowski
>> <akoz at mimuw.edu.pl>
>> wrote:
>>
>>>
>>> On 12 Dec 2011, at 12:43, Oleksandr Rasputinov wrote:
>>>
>>>> For example, the erroneous usage:
>>>>
>>>> Block[{a = 1, b = 2}, Return[a]; Return[b]]
>>>>
>>>> gives:
>>>>
>>>> Return[1]
>>>>
>>>> but a correct usage:
>>>>
>>>> Do[Block[{a = 1, b = 2}, Return[a]; Return[b]], {1}]
>>>>
>>>> gives (because of the enclosing Do):
>>>>
>>>> 1
>>>
>>> This is not quite the whole story. For example, while the above use of
>>> Return in Block does not work, this one (in a function call) does:
>>>
>>> f[] := Block[{a = 1, b = 2}, Return[a]; Return[b]]
>>>
>>> f[]
>>>
>>> 1
>>>
>>> Moreover, if you replace the Do loop in your example with a While loop,
>>> you will get:
>>>
>>> n = 1; While[n < 10, Block[{a = 1, b = 2}, Return[a]; Return[b]]]
>>>
>>> Return[1]
>>>
>>> All of this is purposeful design and can be explained and justified but
>>> I don't think it's worth the bother since Mathematica has far superior
>>> means of flow control than this clumsy and arcane construct.
>>>
>>> Andrzej
>>>
>>
>> Thanks for the additional examples. Yes, there are limitations to what
>> Return (or Break, or Continue) will accept as a control flow construct;
>> I
>> imagine most people would have suspected that Block does not count but
>> it
>> is a little surprising to find that While doesn't either.
>
> My view of this is a little different. I think there is a good reason
> why the While example returns Return[1] rather than just 1. To see my
> point, look at the example on memberQ I posted earlier. With a Do loop
> this does not work
>
> memberQ[x_List, y_] := (Do[If[i === y, Return[True]], {i, x}]; False)
>
> Now use the While loop:
>
> Clear[memberQ]
>
> memberQ[x_List, y_] := (i = 1;
> While[i <= Length[x], If[x[[i]] === y, Return[True], i++]]; False)
>
> memberQ[{a, b, c}, b]
>
> True
>
> The While loop works fine. Why is that? Well, I think it is exactly for
> the same reason why my earlier While example (when While was not in the
> body of a function) "did not work" - i.e. when we got Return[1] instead
> of 1. This returning Return[something] rather than just something is not
> the result of some sort of confusion in the design (although it is, no
> doubt, confusing) but it actually the means by which Return in a While
> loop gets through all the control flow structures while in a Do loop it
> only exits the innermost control structure. As I pointed out, to achieve
> the same result in a Do loop, you will need a double Return:
>
> Clear[memberQ]
>
> memberQ[x_List,
> y_] := (Do[If[i == y, Return[Return[True]]], {i, x}]; False)
>
> memberQ[{a, b, c}, b]
>
> True
>
>
> Andrzej Kozlowski
>
>
--
DrMajorBob at yahoo.com
- References:
- NMinimize problem: fct minimized uses FindRoot
- From: "Doug Tinkham" <dtinkham@live.ca>
- NMinimize problem: fct minimized uses FindRoot