Amiel Moodley - 1 year ago 78

Python Question

I have to find out whether number(N) is a prime or not using recursion, no loops are allowed. I've tried converting the usual code that uses a for loop to a recursive one, but it's not behaving the same. This function is included in another function, which is part of another function. only parameters a and N should be used and passed

Here is my function.

`a=2`

def is_prime(a,N):

prime = True

if N <=1:

return

else:

if a >= N:

return

else:

if N == 2:

prime = True

print(N)

return

elif (N % a) == 0:

prime = False

return is_prime(a+1,N)

else:

prime = True

print(N)

return

I believe the bug is somewhere here.

`elif (N % a) == 0:`

prime = False

return is_prime(a+1,N)

else:

prime = True

print(N)

Here is the code I tried to convert.

`if num > 1:`

for i in range(2,num):

if (num % i) == 0:

print(num,"is not a prime number")

print(i,"times",num//i,"is",num)

break

else:

print(num,"is a prime number")

else:

print(num,"is not a prime number")

Answer

Your solution is close, with just a few changes needed to make it work.

```
def is_prime(a,N):
print(a, N)
if N <= 1:
return
else:
if a >= N:
print(N)
else:
if N == 2:
print(N)
elif (N % a) == 0:
return False
else:
return is_prime(a+1,N)
return False
```

You didn't give any examples of calling this function, but I assume it's always called with `a`

being 2, since any other value wouldn't make sense. So if you run the above function like so, you should get the right output:

```
print(is_prime(2, 7)) => True
print(is_prime(2, 4)) => False
print(is_prime(2, 37)) => True
```

I think you have a misunderstanding of how recursion works, you're assigning this `prime`

variable in the body of the function, but never doing anything with it. Maybe you're confusion comes from a misunderstanding of scopes in Python. That `prime`

variable will not be 'shared' across invocations, it will just create a new `prime`

every time.

EDIT: Didn't realize you wanted the function to just print out the prime if it's a prime, changed the code accordingly.

Source (Stackoverflow)