Amiel Moodley Amiel Moodley - 7 months ago 33
Python Question

How do I find a prime number using recursion in Python

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.