Why doesn't my function work?

Question

When I wrote this code in python-2.7 I got a message that runs again and again, can someone tell me what's wrong with it?

num = raw_input("enter your number : ")

def calc():
    if num == "1":
        print "ok"
    else:
        print "right"
    calc()

calc()

It prints ok or right over and over.

So what should I do if I want to go back to the initial state (asking for input) after each print?

The edit makes it much clearer. Can someone put it off the on-hold list? — Flying_Banana, Oct 28 '15 at 15:31
Maybe you'd like to put `num = raw_input("enter your number : ")` inside the function. — Remi Guan, Oct 28 '15 at 15:35
@Flying_Banana It's clear now, but seems like this question really is off-topic. — Remi Guan, Oct 28 '15 at 15:38
But as it stands in the current edit, there is desired behavior (ask for input again), current error (printing over and over without asking input), and sample code that produces this error, so it constitutes as a minimal, complete, and verifiable example by definition. — Flying_Banana, Oct 28 '15 at 15:40

Praxis Ashelin · Accepted Answer · 2015-10-28T16:04:51.823

2

It is repeating itself because you call calc() again inside the calc() function, making it an infinite loop.

def calc():
    if num == "1":
        print "ok"
    else:
        print "right"
    calc()  # <-- Why is this here?

So what should I do if I want to go back to the initial state (asking for input) after each print?

Then you need to ask for the input again inside the loop:

def calc():
    num = raw_input("enter your number : ")
    if num == "1":
        print "ok"
    else:
        print "right"
    calc()

calc()

edited Oct 28 '15 at 16:04

answered Oct 28 '15 at 15:58

Praxis Ashelin

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While this works in principle, it will actually recurse indefinitely, adding a new function call to the stack every time. While it'll take a while, it's a waste and the interpreter is going to crash when the maximum recursion depth is exceeded. – MisterMiyagi Oct 28 '15 at 21:40
I would almost agree with you, except code compiled with `gcc -O3` will perform tail recursion so there is no difference :) – Flying_Banana Oct 29 '15 at 14:04
@Fyling_Banana It's python, not C/++. CPython may be compiled with `-O3`, the code it executes isn't. See also http://stackoverflow.com/a/13592002/5349916. In fact, `def foo(): foo()` is enough to test this. – MisterMiyagi Oct 30 '15 at 10:08

MisterMiyagi · Answer 2 · 2015-10-29T08:50:12.660

If you keep calling the same function over and over again, you will reach a stack overflow sooner or later. In order to repeat something indefinitely, use a loop, e.g. for or while.

def calc():
  while True:
    num = raw_input("enter your number : ")
    if num == "1":
      print "ok"
    else:
      print "right"

calc()

You should also definitely consider adding a condition that exits your loop. For example, checking whether the input is "q" and then executing break:

  while True:
    num = raw_input("enter your number : ")
    if num == "q":
      break
    elif num == "1":
      print "ok"
    else:
      print "right"

Why doesn't my function work?

2 Answers2