CODEWITHSUNDEEP
rubyarrays
itzmurd4
itzmurd4

Reputation: 663

Array of prime numbers

I just started getting my hands dirty on ruby and was wondering if someone can help me with this array of primes example. I got the problem off of project Euler, and I want ruby to print an array of prime numbers. However, every time I run the program, it outputs just "0". Can someone shed some light here. Thank you in advance.

def prime
    x = 13195
    count = 0
    a = [ ]
    while count < x
        if count % x == 0
            a.push(count)
            a.sort
        end
        count += 1
    end
    puts a
end

Upvotes: 0

Views: 400

Answers (2)

Martin Konecny
Martin Konecny

Reputation: 59651

Assuming you want to test if 13195 is a prime, and you want a to keep a list of which numbers divide into 13195

You need to start count at 2, since every number including a prime is divisible by 1. You also need to use x % count instead of count % x. x % count divides x by count and gives you the remainder (which are correctly checking against 0 for). 

def prime
    x = 13195
    count = 2
    a = [ ]
    while count < x
        if x % count == 0
            a.push(count)
        end
        count += 1
    end
    a
end

arr = prime 
p arr #this will print out a list of numbers which fit into 13195
arr.size == 0 #true if number is a prime, false otherwise

Note that there is a lot of optimizations you can do in this algorithm to check if a number is a prime - namely your for loop condition can be:

sqrt = Math.sqrt(x)
while count < sqrt

you only need to check up to the square root of your number to see if it's a prime

Upvotes: 1

user448810
user448810

Reputation: 17866

Your array is 0 because count is always less than x, so dividing count by x, as the modulus operator does, always returns 0.

Your variable count is poorly named; I would call it f for factor. And it should be initialized to 2, not 0. Your modulus operator is reversed; it chouls be x % f instead of the other way around. And your while loop should stop as soon as f * f > x, since at that point the remaining x must be prime.

Upvotes: 0

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