Python List Indexing Error

Question

def getPrimeList(check):
    storedprimes = []
    i = 2
    while i <= check:
        if isPrime(check):
            storedprimes = storedprimes + [i]
        i = i + 1
    return storedprimes
def getPrimeFact(check):
    primelist = getPrimeList(check)
    prime_fact = []
    i = 0
    while check !=1:
        if check%primelist[i]==0:
            prime_fact=prime_fact+[primelist[i]]
            check = check/primelist[i]
        i = i + 1
        if i == len(primelist):
            i = 0
    return prime_fact
def getGCF(checks):
    a=0
    listofprimefacts=[]
    while a<len(checks):
        listofprimefacts=listofprimefacts+[getPrimeFact(checks[a])]
        a=a+1
    b=0
    storedprimes=[]
    while b<len(primefactlist):
        c=0
        while c<len(listofprimefacts[b]):
            if listofprimefacts[b][c] not in storedprimes:
                storedprimes=storedprimes+[listofprimefacts[b][c]]
            c=c+1
        b=b+1
    prime_exp=[]
    d=0
    while d<len(storedprimes):
        prime_exp=prime_exp+[0]
        d=d+1

    e=0
    while e<len(storedprimes):
        f=0
        while f<len(listofprimefacts):
            if f==0:
                prime_exp[e]=listofprimefacts[f].count(storedprimes[e])
            elif prime_exp[e]-(listofprimefacts[f].count(storedprimes[e]))>0:
                prime_exp[e]=listofprimefacts[f].count(storedprimes[e])                
            f=f+1
        e=e+1
    g=0
    GCF=1
    while g<len(primelist):
        GCF=GCF*(storedprime[g]**prime_exp[g])
        g=g+1
    return GCF

I am creating a program that will use these functions for the purpose of calculating fractions; however, after testing my GCF function in the shell I keep getting a list indexing error. I have no idea, where the error is coming from considering im 99% sure there is no problems with my indexes, usually i would not post such a "fixable" problem in SO but this time i just have no idea what the problem is, thanks again.

Oh and heres the exact error

File "<pyshell#1>", line 1, in <module>
    getGCF(checks)
  File "E:\CompProgramming\MidtermFuncts.py", line 31, in getGCF
    listofprimefacts=listofprimefacts+[getPrimeFact(checks[a])]
  File "E:\CompProgramming\MidtermFuncts.py", line 20, in getPrimeFact
    if check%primelist[i]==0:
IndexError: list index out of range

Blender your name and sarcastic yet serious style of right makes me feel like ive met you before. Hello again stranger:D — Alvaro
– Alvaro, Commented Feb 1, 2013 at 1:06
Is this a learning exercise, or are you trying to solve a problem? I'm pretty sure there are pre-existing Python solutions that should solve your fractions needs, but if you are learning then carry on. — steveha
– steveha, Commented Feb 1, 2013 at 1:22

Blender · Accepted Answer · 2013-02-01 01:24:26Z

You might want to re-think how you attack this problem. In its current form, your code is really hard to work with.

Here's how I'd do it:

def is_prime(n):
    for i in range(2, int(n ** 0.5) + 1):
        if n % i == 0:
            return False

    return True

def prime_factors(number):
    factors = []

    for i in range(2, number / 2):
        if number % i == 0 and is_prime(i):
            factors.append(i)

    return factors

def gcf(numbers):
    common_factors = prime_factors(numbers[0])

    for number in numbers[1:]:
        new_factors = prime_factors(number)
        common_factors = [factor for factor in common_factors if factor in new_factors]

    return max(common_factors)

This line right here:

common_factors = [factor for factor in common_factors if factor in new_factors]

Is a list comprehension. You can unroll it into another for loop:

temp = []

for factor in common_factors:
    if factor in new_factors:
        temp.append(factor)

common_factors = list(temp)  # Pass by value, not by reference

Martijn Pieters · Accepted Answer · 2013-02-01 01:58:27Z

0

You mixed up i and check in your getPrimeList() function; you test if check is a prime, not i; here is the correct function:

def getPrimeList(check):
    storedprimes = []
    i = 2
    while i <= check:
        if isPrime(i):  # *not* `check`! 
            storedprimes = storedprimes + [i]
        i = i + 1
    return storedprimes

This, primelist will be set to an empty list (as getPrimeList(check) returns an empty list) and your primelist[i] (for any i) will fail with an index error.

Another way for primelist to be empty is when isPrime() never returns True; you don't show us that function to verify it.

Your next error is in getGCF(); you define a listofprimefacts variable (a list) first, but later refer to a non-existing primefactlist variable, leading to a NameError. The next name error is going to be primelist further in that function.

You really want to re-read the Python tutorial; you are missing out on many python idioms in your code; specifically on how to create loops over sequences (hint: for check in checks: is easier to use than a while loop with an index variable) and how to append items to a list.

My personal toolkit defines this:

from math import sqrt

def prime_factors(num, start=2):
    """Return all prime factors (ordered) of num in a list"""
    candidates = xrange(start, int(sqrt(num)) + 1)
    factor = next((x for x in candidates if (num % x == 0)), None)
    return ([factor] + prime_factors(num / factor, factor) if factor else [num])

which doesn't need a isPrime() test.

edited Feb 1, 2013 at 1:58

answered Feb 1, 2013 at 1:05

Martijn Pieters

1.1m326 gold badges4.2k silver badges3.4k bronze badges

2 Comments

Alvaro Over a year ago

The thing is im using python as a gateway to learning other langauges that is why the python convetional 'for' is not in any of my code, i am aware of what it is although i refrain myself from using it, also for testing purposes i open up the shell define checks ex:checks= 125,525,325 then i simply call getGCF(checks)

Jon Clements Over a year ago

@Alvaro If you're deliberately ignoring language features, I'm not sure what you should expect. The for idiom is in loads of languages, either through some kind of iterator or it's spelt foreach or for each or some other syntantical construct - not sure you're doing yourself any good "learning" with this approach :)

Collectives™ on Stack Overflow

Python List Indexing Error

2 Answers 2

Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related