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Recursive function with no apparent recursive relationship

B B
Posted 11 years ago

Hello,

I would like to understand a code which is a recursive function with this kind of structure f[x, y] := {f[x], f[y]}. This case is present in the XMLNote function used in the help for "Transforming XML".

I have already spent time on the help to look for more about recursive functions. But, I have seen only recursive functions such as Pise[0] = Pise[1] = 1; Pise[n_] := Pise[n - 1] + Pise[n - 2] . In this case, I understand how the indentation is made that is to say how the term of index n is calculated from the terms of index n-1 and n-2.

But, for this kind of recursive function f[x, y] := {f[x], f[y]}, i don't understand how the indentation is made.

May you complete me this example/ or furnish me a small example so that I can see how this kind of recursion works ?

Thanks for your help
POSTED BY: B B
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Alternatively one could make the statement:

Attributes[f] = Listable;

This works in the case that the lists contain more than two elements.

Henrik
POSTED BY: Henrik Schachner
B B
B B
Posted 11 years ago

Perfectly clear. Thank you
POSTED BY: B B
Kuba Podkalicki
Kuba Podkalicki
Posted 11 years ago

Since {1, 2} is one argument, a list with two elements, you have to add another definition like f[{x_, y_}] := {f[x], f[y]}.

Or you can do this in one line if you want:

f[{x_, y_} | PatternSequence[x_, y_]] := {f[x], f[y]}
f[{1, 2}, {2, 3}]

{{f[1], f[2]}, {f[2], f[3]}}
POSTED BY: Kuba Podkalicki
B B
B B
Posted 11 years ago
POSTED BY: B B
Kuba Podkalicki
Kuba Podkalicki
Posted 11 years ago

The function you've provided is not refering to itself basing on values of arguments that were provided but basing on the pattern that those arguments match*.

p.s. you probably wanted to write:

f[x_, y_]:= {f[x], f[y]}

So f refers to itself only when you provide two arguments. Then it is returning itself scanned over those arguments. And since for f acting on one argument are no associated values the recursion stops.

Is that clear?
POSTED BY: Kuba Podkalicki
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