Attempt to call a boolean value

Programming Lua
1

My Lua-Function to do the nthPrime exercise runs perfectly on my compiler. But when i try to get it verified in the exercism editor, I get this error message:

./nth-prime_spec.lua:11: attempt to call a boolean value (upvalue 'nth')

What is wrong?

This is my function:

local function nthPrime(n)
    if n < 1 or n%1 ~= 0 then
        return "error"
    end
    local testPrime = 1
    local currentNumber = 0
    local currentPrime = 0
    while true do
        testPrime = testPrime + 1
        for i=2, testPrime, 1 do
            if i == testPrime then
                currentNumber = currentNumber + 1
                currentPrime = testPrime
                break
            elseif testPrime%i == 0 then
                break
            end
        end
        if n == currentNumber then
            return currentPrime
        end
    end
end
1 Like
2

The stub looks like this:

return function(n)

end

Maybe the first line needs to match?

3 
return function(n)
  ...
end

outputs:

Your tests timed out. ...
4

You have to see how the test file uses your code

local nth = require('nth-prime')
...
    assert.equal(2, nth(1))

You need to ensure your module returns nthPrime

5

May you please explain a bit more? My function takes in a value ‘n’ and gives the nth-prime of it out. I don’t understand what the error message means. Where is it calling a boolean value? What exactly do I need to change to get the output accepted?

6

The test file expects your module to return a value. That value is a function (that the test file names nth). What do you need to do in nth-prime.lua to return a function?

Look carefully at the error message

./nth-prime_spec.lua:11: attempt to call a boolean value ...

The error is coming from line 11 of the test suite. That will answer “Where is it calling a boolean value?”

If your module does not return a value, then local nth = require('nth-prime') assigns true to nth.

7

Thank you for clarification, but my function returns a value. If I enter for n=1, I get 2 as function output. Isn’t this what the function should do?

8

Yes your function returns a value, but does your module?

If this is not your first Lua exercise, check some of your other solutions. What gets returned from those modules?

9

Your module needs to return a function and that function needs to return an int.

10

Thanks, finally I got it. Every time I tried to it properly by returning a function, I got a timed out error with this for loop:

for i=2, n, 1 do
    if n%i == 0 then
         return false
    end
end

After including the fact that the for loop only needs to run up to the square root of n, the timed out error vanished and everything was working:

for i=2, math.sqrt(n), 1 do
      if n%i == 0 then
           return false
      end
end

So, I guess the exercism Lua compiler is way slower than my compiler and that is why i wasn’t able to find out what’s wrong.

11

There’s another optimization to be made: the loop for i=2, math.sqrt(n), 1 has twice as many iterations as necessary. You don’t need to test i = 4, 6, 8, … because even numbers (except 2) cannot be prime.

if n%2 == 0 then return false end
for i=3, math.sqrt(n), 2 do
      if n%i == 0 then
           return false
      end
end
12

I thought about it, but isn’t this making it more complicated? The for loop starting with i=2 would check for an even number in the first run anyway.

13

I posted the code, you can decide how much more complicated it is. It does use 50% fewer loop iterations so it will be correspondingly faster

1 Like