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MathGroup Archive 1999

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Re: ? Return[ ] in a Do loop ?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg18555] Re: [mg18469] ? Return[ ] in a Do loop ?
  • From: "Andrzej Kozlowski" <andrzej at tuins.ac.jp>
  • Date: Sat, 10 Jul 1999 02:18:48 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

As they say, it's is not a bug it's a feature! Look at the folowing 
examples:

In[1]:=
While[True, Return[1]]
Out[1]=
Return[1]


In[2]:=
Do[Return[1], {i, 1}]
Out[2]=
1


It is because the While loop returns not 1 but Return[1] that it will exit
the entire function rather than jus the loop. But of course this is not
always convenient: often you  would like to return an actual value rather
than Return[value]. This is more likely to be the case in the case of Do
loops than While loops, because Do loops are more often used alone than
While and For loops.

I any case, you can always get the behavoiour you desire with the Do loop by
using Throw and Catch instead of Return:

In[2]:=
firstPosition[cand_List, target_] :=
Module[{i},
 Catch[Do[ If[cand[[i]] == target, Throw[i]],
     {i, 1, Length[cand]}
 ];
 -1]
]

In[3]:=
firstPosition[{1, 1, 2, 3, 4}, 2]


Out[3]=
3
--
Andrzej Kozlowski
Toyama International University
JAPAN
http://sigma.tuins.ac.jp
http://eri2.tuins.ac.jp


----------
>From: "DongGook Park" <park at isrc.qut.edu.au>
To: mathgroup at smc.vnet.net
>To: mathgroup at smc.vnet.net
>Subject: [mg18555] [mg18469] ? Return[ ] in a Do loop ?
>Date: Thu, Jul 8, 1999, 12:08 PM
>

> Hi,
>
> I have found a strange behaviour of Return[ ] command when it is located in
> a Do[ ] loop. That is, Return[ ] in a Do[ ] loop does not exits the
> "function" but just the "Do loop". Fortunately(?), I have found that
> eturn[ ] behaved desirably in While and For loops. I am quite confused and
> wondering if these different behaviours of Return[] is as designed or just a
> bug. Can anyone explain this behaviour?
>
> Please check the following Mathematica scripts.
>
> -------- Return in Do loop
>
> In[1]:=
> firstPosition[cand_List, target_] :=
> Module[{i},
>  Do[ Print[i]; If[cand[[i]] == target, Return[i]],
>      {i, 1, Length[cand]}
>  ];
>  -1
> ]
>
> In[2]:=
> firstPosition[{1,1,2,3,4},2]
>
> 1
> 2
> 3
>
> Out[2]=
> -1
>
>
> -------- Return in While loop
>
> In[3]:=
> firstPosition[cand_List, target_] :=
> Module[{i=1},
>  While[i <= Length[cand],
>
>        If[cand[[i]] == target, Return[i]];
>        i++
>  ];
>  -1
> ]
>
> In[4]:=
> firstPosition[{1,1,2,3,4},2]
>
> Out[4]=
> 3
>
>
> -------- Return in For loop
>
> In[5]:=
> firstPosition[cand_List, target_] :=
> Module[{i},
>  For[i = 1,
>      i <= Length[cand],
>      i++,
>
>      If[cand[[i]] == target, Return[i]]
>  ];
>  -1
> ]
>
> In[6]:=
> firstPosition[{1,1,2,3,4},2]
>
> Out[6]=
> 3
>
> ----------
>
> Thanks,
> DongGook Park
> 


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