Euclidean algorithm (GCD) with multiple numbers?

So I'm writing a program in Python to get the GCD of any amount of numbers.

def GCD(numbers):

    if numbers[-1] == 0:
        return numbers[0]


    # i'm stuck here, this is wrong
    for i in range(len(numbers)-1):
        print GCD([numbers[i+1], numbers[i] % numbers[i+1]])


print GCD(30, 40, 36)

The function takes a list of numbers. This should print 2. However, I don't understand how to use the the algorithm recursively so it can handle multiple numbers. Can someone explain?

updated, still not working:

def GCD(numbers):

    if numbers[-1] == 0:
        return numbers[0]

    gcd = 0

    for i in range(len(numbers)):
        gcd = GCD([numbers[i+1], numbers[i] % numbers[i+1]])
        gcdtemp = GCD([gcd, numbers[i+2]])
        gcd = gcdtemp

    return gcd

Ok, solved it

def GCD(a, b):

    if b == 0:
        return a
    else:
        return GCD(b, a % b)

and then use reduce, like

reduce(GCD, (30, 40, 36))

Solution 1:

Since GCD is associative, GCD(a,b,c,d) is the same as GCD(GCD(GCD(a,b),c),d). In this case, Python's reduce function would be a good candidate for reducing the cases for which len(numbers) > 2 to a simple 2-number comparison. The code would look something like this:

if len(numbers) > 2:
    return reduce(lambda x,y: GCD([x,y]), numbers)

Reduce applies the given function to each element in the list, so that something like

gcd = reduce(lambda x,y:GCD([x,y]),[a,b,c,d])

is the same as doing

gcd = GCD(a,b)
gcd = GCD(gcd,c)
gcd = GCD(gcd,d)

Now the only thing left is to code for when len(numbers) <= 2. Passing only two arguments to GCD in reduce ensures that your function recurses at most once (since len(numbers) > 2 only in the original call), which has the additional benefit of never overflowing the stack.

Solution 2:

You can use reduce:

>>> from fractions import gcd
>>> reduce(gcd,(30,40,60))
10

which is equivalent to;

>>> lis = (30,40,60,70)
>>> res = gcd(*lis[:2])  #get the gcd of first two numbers
>>> for x in lis[2:]:    #now iterate over the list starting from the 3rd element
...    res = gcd(res,x)

>>> res
10

help on reduce:

>>> reduce?
Type:       builtin_function_or_method
reduce(function, sequence[, initial]) -> value

Apply a function of two arguments cumulatively to the items of a sequence,
from left to right, so as to reduce the sequence to a single value.
For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) calculates
((((1+2)+3)+4)+5).  If initial is present, it is placed before the items
of the sequence in the calculation, and serves as a default when the
sequence is empty.

Solution 3:

A solution to finding out the LCM of more than two numbers in PYTHON is as follow:

#finding LCM (Least Common Multiple) of a series of numbers

def GCD(a, b):
    #Gives greatest common divisor using Euclid's Algorithm.
    while b:      
        a, b = b, a % b
    return a

def LCM(a, b):
    #gives lowest common multiple of two numbers
    return a * b // GCD(a, b)

def LCMM(*args):
    #gives LCM of a list of numbers passed as argument 
    return reduce(LCM, args)

Here I've added +1 in the last argument of range() function because the function itself starts from zero (0) to n-1. Click the hyperlink to know more about range() function :

print ("LCM of numbers (1 to 5) : " + str(LCMM(*range(1, 5+1))))
print ("LCM of numbers (1 to 10) : " + str(LCMM(*range(1, 10+1))))
print (reduce(LCMM,(1,2,3,4,5)))

those who are new to python can read more about reduce() function by the given link.

Solution 4:

The GCD operator is commutative and associative. This means that

gcd(a,b,c) = gcd(gcd(a,b),c) = gcd(a,gcd(b,c))

So once you know how to do it for 2 numbers, you can do it for any number

To do it for two numbers, you simply need to implement Euclid's formula, which is simply:

// Ensure a >= b >= 1, flip a and b if necessary
while b > 0
  t = a % b
  a = b
  b = t
end
return a

Define that function as, say euclid(a,b). Then, you can define gcd(nums) as:

if (len(nums) == 1)
  return nums[1]
else
  return euclid(nums[1], gcd(nums[:2]))

This uses the associative property of gcd() to compute the answer

Solution 5:

Python 3.9 introduced multiple arguments version of math.gcd, so you can use:

import math
math.gcd(30, 40, 36)

3.5 <= Python <= 3.8.x:

import functools
import math
functools.reduce(math.gcd, (30, 40, 36))

3 <= Python < 3.5:

import fractions
import functools
functools.reduce(fractions.gcd, (30, 40, 36))