Question

hi guys
in lec8 (6.00, 2011), function fact(n)
if i modify it a little(2 different ways) i am getting error whereas i don`t think removing this statement should affect the code..please can you tell me my mistake(also little details about error). Here is the code:
http://pastebin.com/FXpk8NqW

Answer 1

for recursive functions you need to have the option of having a recursive call *and* a base case in each definition. neither of your definitions have base cases like

if n==1 or n==0:
return 1
^^^^^^
these are your base cases

Answer 2

can you explain a little more about base case....
and isn't the assert doing the same....and in the function fact(n) i have removed only if condition i.e if the number being negetive or less than 1 then it should return the same no.

Answer 3

You need to think a little more mechanically about each step:
First of all, assert only limits the types of inputs allowed; it has nothing to do with a base case.
The base case is what all possible inputs will tend to as n decreases on each recursive call.
Since every recursive call is to n*fact(n - 1), this tells us that our input n is decreasing on each call.
We will get *no final result* unless this terminates, which without a base case, it does not.

Answer 4

def fact1(n):
assert n >= 0 #makes sure that input is positive and an integer
return n*fact1(n-1) #calls back to fact1 with an input of one less

problem: you have nothing in the code that tells it to terminate. There is no endpoint to this function; it will just call on fact(n-1) even after n gets negative. You need a base case to fix this.

Answer 5

def fact(n):
assert n >= 0
if n > 1:
return n*fact(n - 1)

okay, this one is a little better, but you have in no way specified what happens after n==0

all we have are recursive calls to the function again, with no final output. Your only return statement involves another function, so you will get no numerical output.

Answer 6

it seems to me that you think that the assert operator will give you some return once we get to a point where the input is negative, but it will not.

you need another return statement with the base case(s) is the moral of the story.

Answer 7

i appreciate your efforts to explain but i am little confused
how about this code

def fact(n):
if n > 1:
return n*fact(n - 1)

(if we will run this code i expect to get fact will input 1 less so at some point we will get x=0 and it will terminate

but

can you explain the error i get while running fact(n)
TypeError: unsupported operand type(s) for *: 'int' and 'NoneType'

Answer 8

your only return statement is
return n*fact(n - 1)
but when n==2 this is
2*fact(1)
but what is fact(1) ? the way you defined the function it's nothing since the function is not defined for n<=1. Therefor Python sees

2*None
^ ^
int None

which is a static semantic error

Answer 9

try doing

type(fact(1))

and you will see it is the None type, because your function is not defined for n<=1

Answer 10

@TuringTest
finally i got the concept
thank you

Answer 11

welcome!