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Re: Double evaluation in recursive function


On 28 Aug 1999 16:33:23 -0400, "Benoit Debaque"
<bdebaque at cybercable.fr> wrote:

>Hi all !
>
>Does somebody know how to use 2 calls at a time in a recursive function...?
>
>for example I want to evaluate the following
>
>myf[x_] : =  If[ x > 5, i = x ; j = x ; i--; j--; myf[i]; myf[j] ]
>
>I get the answer from
>myf[7]
>
>i=6
>i=5
>j=5
>j=5
>
>apparently Mathematica doesn't like much the second term myf[j], because it
>start from 5 to 5 ! and not from 6 to 5 like variable i............how can I
>manage such a problem ?
>

It is not entirely clear what this code is meant to do, since your
'If' construction does not specify the result for x <= 5, however, one
obvious problem is that i and j are global variables so the value of j
will have been destroyed before the second call is executed. So write
myf[x_]:=Module[{i,j},
        If[x>5,
         i=x;
         j=x;
         i--;
         j--;
         myf[i];
         myf[j],
        <something should go here or function will return Null>
        ]
        ]

You could probably eliminate i and j by simply calling myf[x-1] - but
it is a bit difficult to figure out what this code was designed to do.

David Bailey
Salford Software


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