'''Recursion'''  occurs when a [routine] calls itself.

** Examples **



** Description **

[Richard Suchenwirth] 2005-04-08

Recursion is popular in [functional programming]. In Tcl, we're a bit handicapped by the [[interp recursionlimit]] which is at ~398 on [Windows], even if set higher.


Here's an example for a recursive [integer range generator], so that [[iota1 5]] == {1 2 3 4 5} :

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proc iota1 n {expr {$n == 1? 1: [concat [iota1 [- $n 1]] $n]}}
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[rdt] For completeness, shouldn't your definition of - (from [func]) be here also? - [RS]: Oops, of course - just another one-liner :)

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proc - {a {b ""}} {expr {$b eq ""? -$a: $a-$b}}
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To illustrate the recursionlimit problem (which is directly related to the C stack):

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% interp recursionlimit {} 10000
10000
% proc Llength list {expr {$list eq ""? 0: 1 + [Llength [lrange $list 1 end]]}}
% Llength [iota1 398]
398
% Llength [iota1 399]
too many nested evaluations (infinite loop?)
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Of course it's silly to reimplement `[llength]` this wasteful way, as [list%|%lists] first and foremost know how long they are, but in [Lisp] this implementation might make more sense :)

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[Lars H]: On the [bifurcation] page there is a Tcl command with which one can do "in-place recursion" (even branching recursions), i.e., recursion without using up space on the [C] stack.

[NEM]: See [tail call optimization] for other ways of achieving recursion in constant stack space.

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