+ 2

Factorial

hi guys, this is the famous factorial function def factorial(x): if x == 1: return 1 else: return x * factorial(x-1) print(factorial(5)) but i dont understend this. could anybody describe this process in details? what exactly happend with variable? if return() return new meaning why i dont have 5*4*20*19....

24th Mar 2018, 4:19 PM
Adam Kandur
Adam Kandur - avatar
7 Answers
+ 8
def factorial(x): if x == 1: return 1 else: return x * factorial(x-1) print(factorial(5)) when value of x is 1 then return 1 else return x * factorial(x-1) so x=5 if condition false evaluate else part 5!=1 so 5* factorial(5-1)=5*factorial(4) now value of x is 4 so if condition false evaluate else part 4!=1 so 5*4*factorial(3) then value of x=3 so if condition false evaluate else part 3!=1 so 5*4*3* factorial(2) value of x=2 so if condition false evaluate else part 2!=1 so 5*4*3*2* factorial(1) value of x is 1 so it will return 1 because if condition true so evaluate if part and returns 1 so finally the answer will be 5*4*3*2*1=120
24th Mar 2018, 4:34 PM
GAWEN STEASY
GAWEN STEASY - avatar
+ 5
yeah it's example of recursive function where a function calls itself again and again till the condition gets true
24th Mar 2018, 4:37 PM
GAWEN STEASY
GAWEN STEASY - avatar
+ 5
1: def factorial(x): 2: if x == 1: 3: return 1 4: else: 5: return x * factorial(x-1) 6: 7: print(factorial(5)) Start executing line 7 by calling factorial with 5. Enter factorial line 1 with x of 5. Test condition on line 2 5 isn't 1 so skip to line 5. Start line 5 by calling factorial again passing 5-1 or 4. Enter factorial line 1 with x of 4. Test condition on line 2 4 isn't 1 so skip to line 5. Start line 5 by calling factorial again passing 4-1 or 3. Enter factorial line 1 with x of 3. Test condition on line 2 3 isn't 1 so skip to line 5. Start line 5 by calling factorial again passing 3-1 or 2. Enter factorial line 1 with x of 2. Test condition on line 2 2 isn't 1 so skip to line 5. Start line 5 by calling factorial again passing 2-1 or 1. Enter factorial line 1 with x of 1. Test condition on line 2 1 is 1 so skip to line 3. Return 1 to caller. Back to line 5 with x of 2 and factorial return of 1. Return 2*1 or 2 to caller. Back to line 5 with x of 3 and factorial return of 2. Return 3*2 or 6 to caller. Back to line 5 with x of 4 and factorial return of 6. Return 4*6 or 24 to caller. Back to line 5 with x of 5 and factorial return of 24. Return 5*24 or 120 to caller. Back to line 7 with factorial return of 120. Print the 120.
24th Mar 2018, 4:41 PM
John Wells
John Wells - avatar
+ 3
well when using recursion you have to work with the mindset of "I already solved the problem". Recursion can be explained from the end-start and not from the start-end as were used to. With factorial recursion the pattern is this: factorial(5) x=5 -> return 5*factorial(4)= factorial(5) x=4-> return 4*factorial(3)= factorial(4) x=3 -> return 3*factorial(2) = factorial(3) x=2-> return 2*factorial(1) = factorial(2) x=1 -> return 1 = factorial(1) Then afterwards the whole process is calculated. x=2- > return 2*factorial(1)=2 x=3 -> return 3*factorial(2)= 6 x=4 -> return 4*factorial(3) = 24 x=5 -> return 5*factorial(4) = 120 the functions then stops because the max variable which factorial was given to is 5.
24th Mar 2018, 4:37 PM
Tomer Sim
Tomer Sim - avatar
+ 1
this is recursive function to calculate factorial of number . recursive function means function can call itself . factorial(5) --> assigns x=5 5*factorial(5-1) i.e. factorial(4) 5*4*factorial(4-1) i.e factorial(3) 5*4*3*factorial(3-1) i.e factorial(2) 5*4*3*2*factorial(2-1) i.e factorial(1) 5*4*3*2*1= 120
24th Mar 2018, 4:39 PM
Manorama
Manorama - avatar
0
oh, it's like function repeat itself with new meanings?
24th Mar 2018, 4:36 PM
Adam Kandur
Adam Kandur - avatar
0
got it
24th Mar 2018, 4:36 PM
Adam Kandur
Adam Kandur - avatar