TheBackShed.com

Forum Index : Microcontroller and PC projects : MMBasic, Subroutine Recursion

Author

Message

apam
Newbie

Joined: 14/05/2021
Location: Australia
Posts: 6

Posted: 01:06am 22 May 2021

Copy link to clipboard

Print this post

Hi

I'm looking at code someone else has written...

They have things like:

Sub ThisSub
print "Doing Stuff"
ThisSub
End Sub

The intention there is so it will go back to the start of the sub after doing stuff. Is that going to cause infinite recursion and exhaust the stack?

I'm new in MMBasic, so not sure if it behaves differently to something like C

Andrew

vegipete

Guru

Joined: 29/01/2013
Location: Canada
Posts: 1132

Posted: 01:15am 22 May 2021

Copy link to clipboard

Print this post

On a CMM2, if you run this:

ThisSub 1
end

sub ThisSub n
Print "Doing Stuff",n
ThisSub n+1
end sub

you get:

> run
Doing Stuff 1
Doing Stuff 2
Doing Stuff 3
Doing Stuff 4
Doing Stuff 5
[many lines chopped out]
Doing Stuff 99
Doing Stuff 100
Error in line 5: Too many SUB and FUN

>

Visit Vegipete's *Mite Library for cool programs.

JohnS
Guru

Joined: 18/11/2011
Location: United Kingdom
Posts: 4044

Posted: 05:55am 22 May 2021

Copy link to clipboard

Print this post

It's infinite recursion.

Some compilers cope (tail recursion optimising) but that's just hiding the fact.

John

CaptainBoing

Guru

Joined: 07/09/2016
Location: United Kingdom
Posts: 2170

Posted: 05:56am 22 May 2021

Copy link to clipboard

Print this post

Welcome

apam said Hi

I'm looking at code someone else has written...

<snip>

The intention there is so it will go back to the start of the sub after doing stuff. Is that going to cause infinite recursion and exhaust the stack?

Yes. if you invoke the Sub name that will start a new "instance" of it, stack, local variables and all. Soon run out of resources doing that. Could probably do something clever with a Static variable (on the right platform) to keep track of it all but why break your back?

If the intention is to simply go back to the top of the code inside then structuring with a Do/Loop etc. is probably the best method as they are easy to jump out of and look cute - I doubt the user wants to stay in the Sub forever. You could be a free-thinker and use a GoTo

Edited 2021-05-22 16:02 by CaptainBoing

MustardMan

Senior Member

Joined: 30/08/2019
Location: Australia
Posts: 175

Posted: 06:04am 22 May 2021

Copy link to clipboard

Print this post

I'm guessing that the sample snippet has some terminating condition inside it which has not been shown, otherwise it does not make programmatic sense.

Recursion is a stack-heavy (and compute heavy) way of performing things, but sometimes it is the best way. One example that springs to mind is filling an odd-shaped polygon by specifying a point inside and then recursively stepping further and further out to an edge, and then dropping back to the previous level, and repeat until filled.

Cheers,

apam
Newbie

Joined: 14/05/2021
Location: Australia
Posts: 6

Posted: 09:06am 22 May 2021

Copy link to clipboard

Print this post

Thanks all,

I modified the code to use loops.

I just didn't want to assume it was an error given that I am new to MMBasic.

Print this page

To reply to this topic, you need to log in.