Python: find the largest prime number of X

Question

Hey awesome python people,

Warning: I am new to coding. Ok, now that you have been warned...

I am trying to write a python file that finds the largest prime number of a variable I declare inside the code.

Here is my thought process:

Step 1:Find Factors of X
Step 2:Put factors of X in an array a
Step 3:Analyze last element of array a
Step 4:Check if last element of array a is Prime
Step 5: if last element of array a is prime print "found the largest prime" along with the number itself, else, analyze second to last element in array a, and so on until at a[1]
Step 6: if no prime numbers in array, print "no primes found"

The issue is somewhere in the last else statement, when dealing with x=28 and its factors in an array: [1, 2, 4, 7, 14] my code thinks 7 is not a prime...

My steps are listed in-line:

#find factors, put them in an array a

#1.Find factors of X
#2.Put factors of X in an array a

x=28
i=1
a=[]
length = 0

while i<x:      
    if x%i == 0:    #checks to see if X is divisible by anything between 1 and X-1
        a.append(i)  #adds factor to array a
    i = i+1         

print "your factors are: ", "\n", a
print "\n"


#3. Analyze the last element in array a
# Before loop below, a = [1, 2, 4, 7, 14] and length = 5

length = 0
length = len(a)
n=a[length-1]-1  
print "checking for primes in your array now...", "\n"

while len(a) > 2:
    if a[length-1]%n != 0:       
        n=n-1
        if n == 1:
            print "PRIME TIME"
            break
    else:
        print a[length-1], "is not a prime"  #added
        del a[-1]
        length = len(a) 
        print "length = ",length
        if length == 2:
            print "NO Primes"

A few questions:

How would you re-assign variables like x, i a[], and n to make the code more readable
In the second loop, after the first iteration, when analyzing 7 why does the code not recognize that it is a prime number?

Thanks so much for any constructive feedback!!

I think this question is a better fit for code review. http://codereview.stackexchange.com/ — shuttle87, Jun 27 '14 at 22:20
Here's a general programming hint - name your variables as expressively as possible, that way you'll know exactly what they are and what should be happening to them. — MattDMo, Jun 27 '14 at 22:20
@DietrichEpp and why shouldn't it be in this case? Quick, glance at the code and tell me what `n` is supposed to be. No, you don't need to name `for i in range(42):` as `my_iterator`, but you get what I'm trying to get across to a newbie... — MattDMo, Jun 27 '14 at 22:25
@MattDMo: I said it's not **always** good advice. For example, single letters are great for loop variables. — Dietrich Epp, Jun 27 '14 at 22:28
@MattDMo: For example, `x` and `i` are perfectly reasonable variable names in the above code. — Dietrich Epp, Jun 27 '14 at 22:30

score 2 · Answer 1 · edited May 23 '17 at 11:49

Ok, I would do it like this:

def primes(n):
    primfac = []
    d = 2
    while d*d <= n:
        while (n % d) == 0:
            primfac.append(d)  
            n /= d
        d += 1
    if n > 1:
       primfac.append(n)
    return primfac

print "Factors: ", primes(28)
print "The largest is: ", max(primes(28))

First use the primes function which I took from here. It returns an array containing all prime factors. Then, simply aply the max-function, which gives you the largest element of the array.

The output is as follows:

Factors:  [2, 2, 7]
The largest is:  7

I hope this helps.

Python: find the largest prime number of X

1 Answers1