return statement vs no return statement?

Question

def nat_fn1(k):
  if k == 0:
    return 1
  elif k%2 == 0:
    return 2
  else:
    return nat_fn1 (k-1)

I first tried nat_fn1(5) and I got an answer of 2. This was expected. Then I tried the same code without return, I got a value of None, this was expected too. So applying the same concept, I tried this code :

def process_strings(s,t):
   if s == "":
      if not(t== ""):
         print(t)
   else:
      print("{0}{1}".format(s[0],t[0]))
      process_strings(s[1:], t[1:])

I tried process_strings("cat","dog"), expecting None as there is no return in the recursion. However, I still got an answer of :

cd
ao
tg

I understand how it got these values. But there's no return statement. Why is this working?

Your just printing values, the function isn't returning them. — Spencer Wieczorek, Jun 18 '17 at 03:07
I don't quite understand. Sorry I'm new to Python. So what's happening when I do recursive without return? Does it still recurse or stop before recursing? — Compsci, Jun 18 '17 at 03:15
The function runs regardless of its return value (even None). That means that the printing is executed and the line calling the same function is also executed. — zondo, Jun 18 '17 at 03:21
Zondo, using the same logic, I tried nat_fn1 (5) to the first program without the return. so this program recurses, can runs again as nat_fn1(4), thus 4 % 2 == 0 should return 2. However, it doesnt. Why? — Compsci, Jun 18 '17 at 03:26
Each function will be called. The one that results in 1 or 2 will not do anything with that information (I.e. no return) so you won't *see* the result, but it is calculated. You can verify that by printing it ;) — zondo, Jun 18 '17 at 03:32
Here's a tip. `print(process_strings("cat","dog"))`... That does print None as well as the other output. Meanwhile, `print(nat_fn1(5))` only outputs 2. You say you "apply the same concept", but you really didn't because your other function doesn't properly return anything. It's also unclear if you are wanting to return a list or a string — OneCricketeer, Jun 18 '17 at 04:52

zhenguoli · Answer 1 · 2017-06-18T03:13:36.600

1

What you get is not the return value, but the output of print statement.
If you run your code interactively in interactive prompt, it will echo the return value.

But if you run it in a file, it won't do that automatically. If you don't print the return value exactly, it will ignore the return value and output nothing.

edited Jun 18 '17 at 03:13

answered Jun 18 '17 at 03:07

zhenguoli

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I know that but why is this program recursing to begin with without the return statement? To be getting 3 different printed outputs, format(s[0],t[0]) , s and t has to be different each time, proving that this program recurses. So i tried nat_fn2(5) to the first piece of code i posted without the return statement. I was expecting an answer of 2 since this program recurses and the elif statement with k % 2 has a return statement. Yet I got nothing returned. I'm completely lost right now – Compsci Jun 18 '17 at 03:19
`process_strings(s[1:], t[1:])` is recursive. Though it doesn't return value, it does some work. That is, it prints the string. So it recursively prints strings. @Compsci – zhenguoli Jun 18 '17 at 03:24
is there any way I can chat with you? I'm still a bit confused at this point – Compsci Jun 18 '17 at 03:24
chat in the chat room, you can create one @Compsci – zhenguoli Jun 18 '17 at 03:26
How would I access this chat room? – Compsci Jun 18 '17 at 03:28
@compsci: That requires 20 reputation. You'll need something external. – zondo Jun 18 '17 at 03:30
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@Compsci, How can I contact with you in that? – zhenguoli Jun 18 '17 at 03:33
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@Compsci, yes, it is. – zhenguoli Jun 18 '17 at 03:40
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return statement vs no return statement?

1 Answers1