Lua - Syntax error in if-statement

Question

Just tried to execute a small Lua script, but unfortunately I'm doing something wrong. I've no more ideas what the fault might be.

function checkPrime( n )
    for i = 2, n-1, 1 do
        if n % i == 0 then
            return false
        end
    end
    return true
end

The interpreter says:

lua: /home/sebastian/luatest/test.lua:3: `then' expected near `%'

I think it's not a big thing and perhaps it's quite clear what is wrong. But somehow I cannot see it at the moment.

On an unrelated note, it is perfectly enough to check roots up to math.floor(math.sqrt(n)) instad of n-1, when you want to check if a number is prime or not. — David Hanak
– David Hanak, Commented Jan 20, 2009 at 21:22
That's right, but I use the script only to measure execution times of different scripting languages. — okoman
– okoman, Commented Jan 22, 2009 at 14:34

David Hanak · Accepted Answer · 2009-01-20 20:48:13Z

6

There is probably some version problem, check your version of lua. The usage of '%' as an infix operator for modulo can only be used in Lua 5.1, in 5.0 it is not supported yet. Try using math.mod instead:

if math.mod(n,i) == 0 then

Edit: Also note that in 5.1, math.mod still exists, but it has been renamed to math.fmod. For now, the old name still works, but support will probably be removed in future versions.

answered Jan 20, 2009 at 20:48

David Hanak

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Comments

Dave · Accepted Answer · 2009-01-20 20:36:42Z

-1

Have you tried wrapping "n% i == 0" in parentheses? Stupid question, but sometimes overlooked!

answered Jan 20, 2009 at 20:36

Dave

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3 Comments

okoman Over a year ago

In parentheses? But then it is a string. As I see it, LUA does not eval it or so...

David Hanak Over a year ago

No, lua does not require parentheses around the expression, since 'if' and 'then' perfectly delimit it.

RCIX Over a year ago

This is a perfect solution as it causes lua to process the entire expression as a single boolean (and thus avoids the cryptic error message). BTW, it doesn't turn into a string if you do that...

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