[clug-progsig] Recursion (was Javascript how to write to a <textarea>)

[Shawn]

The simplest explanation of recursion is a function that calls itself, and has 
and ending condition of some sort.

The key is the ending condition - otherwise you essentially end up in an 
endless loop, but sucking up stack space for each function call.

Um... I'm not comfortable enough with C/C++ to create code in it, but 
JavaScript is a similar syntax (if you stick with the basics), so I'll use 
that to show a sample:

---------------------
function AddTotals(CurrentIndex, Totals)
{
	//End Condition
	if (CurrentIndex >= 10)
	{
		return parseFloat(Totals) + 10;
	}
	else {
		CurrentIndex = CurrentIndex + 1;
		Totals = CurrentIndex + parseFloat(AddTotals(CurrentIndex, Totals));
	}

//This could be a printf() function - the purpose is just so we can see what
//is happening
document.write(parseFloat(CurrentIndex) +" : " + parseFloat(Totals) + "<br>");
	
	return Totals;
}

alert(AddTotals(0,0));
---------------------

The alert() function at the end of the snippet triggers our recursive 
function, and passes in our starting values of 0 and 0.

The first time through the loop , CurrentIndex is less than 10, so we'll 
increase it by 1, and then the value of the new call to the AddTotals() 
function, to our function.

Critical points to take note of:
- all variables declared in the function have local scope - changing a global 
variable in this manner can lead to some very unexpected (and unpleasent) 
results, if not carefully managed.
- we have a very clear end condition - When CurrentIndex is >= 10, we don't 
call the function again.
- we are updating the values used in our ending condition, so we know we'll 
eventually hit it.  I guess you can rely on external values for your 
condition, but that's probably asking for trouble.

The syntax might be a little different in C/C++, but the concepts, and 
critical points still apply.  I've yet to see a language where they don't 
apply.  Just be careful you're not relying on a pointer that could 
potentially be updated outside the recursive function.

Hope this helps.  (btw, the code above works - I tested it first.. <grins>)

Shawn



On Saturday 12 February 2005 00:25, Rob S wrote:
> Doug Boyd wrote:
> >Hi guys,
> >
> >I'm working on a Javascript program to solve the classic Hanoi Towers
> > problem. The recursive function and algorithm works fine, but I'm unable
> > to write the 'textstring' variable to the <textarea>.
>
> I was wondering if someone could walk me through an explanation of
> recursion in the context of the C programming language? This is probably
> best saved for a meeting session, but seeing as the next meeting's a few
> weeks away, I was wondering if someone could try explaining it to me
> over email?
>
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