Lua Least Common Multiple Program Not Enough Memory

Question

This is a Lua program I wrote to find the least common multiple of two numbers. When I run it, it asks for two numbers as intended, but when it tries to run them through the function, it runs out of memory.

function lcm(a,b)
    aList={}
    bList={}
    c=0
        if a<b then
            repeat
                c=c+1
                aList[c]=a*c
                bList[c]=b*c
                aL=table.getn(aList)
            until aL==b
            else
            if a>b then
                repeat
                    c=c+1
                    aList[c]=a*c
                    bList[c]=b*c
                    bL=table.getn(bList)
                until bL==a
            end
        end
    e=1
    repeat
        d=1
        repeat
            if aList[e]==bList[d] then
                f=aList[e]
                return f
            end
        d=d+1
        until d==table.getn(aList)
    e=e+1
    until e==table.getn(bList)
end

n1=io.read()
n2=io.read()
ans=lcm(n1,n2)
print(ans)

Using `repeat ... until` to iterate over lists is very non-idiomatic — Eric, Jul 21 '13 at 19:32
Also, you've got an off by one error somewhere - `lcm(3, 5)` returns nil. As does `lcm(3, 3)` — Eric, Jul 21 '13 at 19:44
[Here's](https://gist.github.com/eric-wieser/9923fe959cf2d50e4f8a) the same (incorrect) algorithm, written idiomatically — Eric, Jul 21 '13 at 19:48

score 4 · Accepted Answer · answered Jul 21 '13 at 19:37

The bug arises because of the io.read call. From PiL

The read function reads strings from the current input file.

Thus, your values n1 and n2 are passed as strings inside the function and hence, never causing the until aL == b or until bL == a to be satisfied as one of them is a string another a number. To solve this problem, you can one of the following:

Pass "*number" as control argument:

n1 = io.read( "*number" )
n2 = io.read( "*number" )

Cast a and b to numbers:

function lcm(a,b)
    a, b = tonumber(a), tonumber(b)

@user2604809 You can thank him by clicking that green checkmark — greatwolf, Jul 21 '13 at 23:25

Lua Least Common Multiple Program Not Enough Memory

1 Answers1