I was trying to make a program which would check a number for its greatest prime factor. I was almost done when this error message came up.
list index out of range.
What does this mean and what is wrong with my code?
Here is my code.
for i in range(3, n):
if n % i == 0:
x = 144
n = 2
not_a_factor = 
z = 
prime = 
not_a_prime = 
while n < x:
if x%n == 0:
n = n + 1
for i in z:
if is_prime(z[i]) == True:
You're just a bit off. for-loops in Python iterate an object and return it's entities, not a pointer/ index.
So just use the thing you get from each iteration of 'z'
(Side note: might want to check out this post, it'll help you make your is_prime function more performant)
def is_prime(n): for i in range(3, n): if n % i == 0: return False return True def Problem3(): x = 144 n = 2 not_a_factor =  z =  prime =  not_a_prime =  while n < x: if x%n == 0: z.append(n) else: not_a_factor.append(n) n =+ 1 # Python version of n++ for i in z: # Python for-loop is more like a say "for each", no need for the indexing if is_prime(i): # no need for '=='; Python will 'truthify' your object prime.append(i) else: not_a_prime.append(i) print(prime) Problem3()
list index out of range- what does this mean?"
list index out of range refers to an
IndexError. Basically, this means that you are attempting to refer to an index in a list that doesn't exist.
Using your code as an example: you generate a list,
z, containing the factors of the number 144. You then iterate through each element in this list (
for i in z:). This means that for the:
iis the 1st element in
z, which is 2;
iis the 2nd element in
z, which is 3;
Then, you attempt
if isprime(z[i]) == True:. So, as written, your program works like this:
if isprime(z) == True:;
if isprime(z) == True:;
if isprime(z) == True:
At this point, your code prompts an
IndexError, because there are only 13 elements in
One way to get the result that you want is to iterate through
range(len(z)) instead of each element of
z. So, adjust the line
for i in z to
for i in range(len(z)).
prime is a list, and you want to return the greatest prime factor, change
These two changes will give you the result you are looking for.
Overall, your program could be written much more efficiently. If you want a simple algorithm to determine the greatest prime factor of a number, here is one possibility:
def greatest_prime_factor(n): greatest_prime = 1 for i in range(n + 1): # iterate through range(n). We skip index 0. if i == 0: continue # determine if the number i is a factor of n. if n % i == 0: # determine if the number i is prime. for i_ in range(2,i): if i % i_ == 0: break else: # update greatest_prime. greatest_prime = max(greatest_prime, i) return greatest_prime print (greatest_prime_factor(144))
This algorithm saves a lot of memory space when compared with your original program by not initializing lists to store numbers that are primes, that aren't primes, etc. If you want to store those values, that's up to you; there are just far more efficient possibilities for what you appear to want to achieve.
Check this link for some more info on algorithmic efficiency and how to think about time and space complexity.