How to stop a recursion in GHpython

Grasshopper
,
1

Hi All
I am trying a simple example on GHpython (please see attached) and got stuck at the end of the recursive function by its meaning.
I know it is a kind of a way to stop the recursion but cannot understand the meaning behind the code.

I would be so grateful of someone can explain these lines to me in simple English so I can understand it , thank you so much in advance!!

yutaka

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reccusion stop.gh (18.4 KB)

2

See attached, get the gist of it (it’s C# - BTW: I hate P) and keep going.

Recursion_ForAmateurs_V1.gh (11.4 KB)

Note: You can add numerous things/parameters (like a wind direction vector, random min/max generations … blah, blah)

3

Thanks Peter! waht a fantastic script! Will certainly learn from it.

just to clear my foggy brain,

Can you or someone tell me what the code I have marked down mean ? Thanks!!

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image1212Ă—799 42.4 KB

4

Well … there’s no adios amigos question around: something (an if clause) to tell the Method that game is over. (see public vars Loops. Loop in the C# above).

Screen Shot 010
Screen Shot 0101125Ă—580 99.3 KB

Note the usage of the found bool: since lines “srink”" per Loop … is possible to end with tiny segments that are not a very good thing. So a 2nd if is required.

Lesson of the way: in Recursion PRIOR anything else mastermind some safe exit.

Some tips:

  1. In real life a Recursion in most of cases terminates PRIOR the max Loops are reached. Obviously I’m talking with some serious piece of code in mind - not just a few lines,
  2. Learn what Stack and Heap are:
    Achieving stack and heap safety in recursive functions | by Bertjan Broeksema | bigdatarepublic | Medium
  3. Use public vars (static or not) in order to avoid the obvious.
  4. Use public collection(s) (say DataTrees or - better - some other collection) for your results.
  5. Be modest with your Loops var (use a secondary [ better safe than sorry] exit clause when testing the thing).
  6. Always try to achieve tail recursion.
  7. If computer hangs > go ride a proper Ducati (and forget all that).
5

When you first call the recursion() function you need to pass it an initial line and an integer number for gen, say 10. Now, the function will call itself recursively as long as gen is greater than 0 and before that decrement gen by one.
I think line 25 should read return recursion(newline, gen) though.

1 Like
6

Find 5 minutes more for that entry level thingy: Added Wind and Gravity. Spot the obvious: Method SHOULD terminate (in real-life) PRIOR max Loops … if you want some sort of rational results (so mastermind proper exit clauses).

Recursion_ForAmateurs_V1A.gh (122.2 KB)

7

Thanks DA

I understand if gen> 0 will run the recursion
How does the decrement by one ends the recursion?

Thank you so much!

8

Yes, each time the function is called it, decrements gen by 1, then if gen is still greater than 0, it calls itself with the decremented gen value. Otherwise, if gen is equal to 0, it simply stops and None gets returned, since you’re not returning anything else at the end of the function.

9

Many thanks does it mean that the recursion starts to count down ?
Let’s say I initiate gen of 5 in total :
Then it starts to work like this ?

Iteration

5 -1 = 4

4 -1 = 3

…

1 -1 =0 ( stop)

10

Yes, exactly.

11

Exactly right. When it reaches 0 the final recursion is not called so the function exits.
More on recursion with Python here:

1 Like
12

Thanks for the recommendation Graham will certainly take a look!

Just wonder what happens if I didn’t use the
gen=gen -1

In that case for example, would a total of 5 gens loop on forever like this:

5 gens. (Loop 1)

10 gens ( Loop 2)
…
100 gens (Loop 20)

Etc?

13

From a real-world perspective, the original code is so simple that it shouldn’t use a recursion.

14

If you don’t decrement gen, than your function will call itself infinitely, but the program will probably stop at the default recursion limit that is set in Python by your OS.

1 Like
15

IronPython doesn’t, by default, have a limit on recursion. So it’s likely Rhino is going to crash when recursion goes infinitely.

1 Like
16

I see well understood now, many thanks everyone for helping out !
Will keep going !