s84 - 11 days ago 9x
Ruby Question

# All factors of a given number

For example, I have 4800 and I would like to see all the factors of this number.

`````` # num = the number you want factors of

def factors_of(num)
(1..num).collect { |n| [n, num/n] if ((num/n) * n) == num}.compact
end
``````

divisors_of(4800)
=> [[1, 4800], [2, 2400], [3, 1600], [4, 1200], [5, 960], [6, 800], [8, 600], [10, 480], [12, 400], [15, 320], [16, 300], [20, 240], [24, 200], [25, 192], [30, 160], [32, 150], [40, 120], [48, 100], [50, 96], [60, 80], [64, 75], [75, 64], [80, 60], [96, 50], [100, 48], [120, 40], [150, 32], [160, 30], [192, 25], [200, 24], [240, 20], [300, 16], [320, 15], [400, 12], [480, 10], [600, 8], [800, 6], [960, 5], [1200, 4], [1600, 3], [2400, 2], [4800, 1]]

How would you do this in ruby or any language?

In Ruby, the `prime` library gives you the factorization:

``````require 'prime'
4800.prime_division #=> [[2, 6], [3, 1], [5, 2]]
``````

To get that list of yours, you take the cartesian product of the possible powers:

``````require 'prime'
def factors_of(number)
primes, powers = number.prime_division.transpose
exponents = powers.map{|i| (0..i).to_a}
divisors = exponents.shift.product(*exponents).map do |powers|
primes.zip(powers).map{|prime, power| prime ** power}.inject(:*)
end
divisors.sort.map{|div| [div, number / div]}
end

p factors_of(4800) # => [[1, 4800], [2, 2400], ..., [4800, 1]]
``````

Note: In Ruby 1.8.7, you must `require 'mathn'` instead of `require 'prime'`. In Ruby 1.8.6, `require 'backports/1.8.7/enumerable/inject'` or modify the `inject` above...

Source (Stackoverflow)