Closure over numeric for-loop variable in Lua

Question

I have a question about Lua, closures and local variables.

From my understanding, the loop variable in a numerical for-loop is implicitly local so I was expecting the following code to behave identical.

local a = 1
t1 = function()
  return a
end
a = 2
t2 = function()
  return a
end
print( t1() )
print( t2() )

table = {}
for i=1,2 do 
  table[i] = function() 
    return i
  end
end
print( table[1]() )
print( table[2]() )

To my surprise, it did not. Why does the code print

and not

?

shingo · Accepted Answer · 2025-02-26 08:34:34Z

1

Early Lua manual has an equivalent code for the for statement:

for v = e1, e2, e3 do block end

-- is equivalent to the code: 

do
  local var, limit, step = tonumber(e1), tonumber(e2), tonumber(e3)
  if not (var and limit and step) then error() end
  while (step > 0 and var <= limit) or (step <= 0 and var >= limit) do
    local v = var
    block
    var = var + step
  end
end

It can be seen that the loop variable v is declared at the beginning of each iteration, in other words, the loop variable for each iteration is independent.

answered Feb 26 at 8:34

shingo

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Comments

Ivo · Accepted Answer · 2025-02-26 08:17:04Z

0

Each i is local to their iteration but so is each of the function declarations. They refer to their corresponding i. Even if the function didn't return i but a constant, like for example

table = {}
for i=1,2 do 
  table[i] = function() 
    return 5
  end
end
print( table[1] )
print( table[2] )
print( table[1] == table[2] ) -- false

you will see that the prints will be different. (I removed the () part in the print so you can see the memory address to the functions)

answered Feb 26 at 8:17

Ivo

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