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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: Recursion or loop, design questions
Date: Sat, 18 Jul 2015 15:44:31 -0700
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Malcolm McLean <malcolm.mclean5@btinternet.com> writes:
> On Saturday, July 18, 2015 at 11:01:12 PM UTC+1, Keith Thompson wrote:
>> Malcolm McLean <malcolm.mclean5@btinternet.com> writes:
>> > On Saturday, July 18, 2015 at 9:31:44 PM UTC+1, Ben Bacarisse wrote:
>> >> Malcolm McLean <malcolm.mclean5@btinternet.com> writes:
>> >> 
>> >> > However using a pointer is kind of cluntsy. So functional programming 
>> >> > language will let you do something along the lines of
>> >> >
>> >> > int countnodes(root)
>> >> > {
>> >> >    if(root is null) return 0;
>> >> >    return sum(1, countnodes(root->left), countnodes(root->right) );
>> >> > } 
>> >> >
>> >> > In C, that's not tail recursive, because countnodes must be called
>> >> > before sum.
>> >> 
>> >> That's not a tail call in any language.
>> >> 
>> >
>> > From Ben:
>> > [defining tail recursion]
>> > In a non-void function its a return statement whose expression is a 
>> > function call.  In a void function it's an expression statement that 
>> > consists of nothing but a function call immediately preceding a return 
>> > statement. 
>> 
>> By that definition, the call to sum() is a tail call, but it's not tail
>> recursive.  The (recursive) calls to countnodes() are not tail calls.
>> 
> In C, it isn't.

What isn't what?

>                 Because the two recursive calls must be made before 
> the call to sum(). But if we devise a language which calls sum,
> the calls the arguments, then that objection disappears, it's
> a case of two mutually recursive functions.

You're suggesting a language in which a function is called before its
arguments are evaluated?  I fail to see how that could make sense.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"