in pairs gives strange ordering

Hello,
I am a bit confused as to the output i get when running this code:

table={
["a"]="1",
["b"]="2",
["c"]="3",
["d"]="4",
["e"]="5"}
for k,v in pairs(table) do
print(v)
end

I would expect the output to be a list going 1 2 3 4 5. But instead i get a list like: 1 3 2 5 4.
I was hoping someone could explain why it does this and also how to prevent it.
Thanks

it does this because lua tables are hash maps, which are not ordered. They are only ordered when used as arrays, with ascending, numeric indices. The only way around this in general is to use arrays instead of hash maps.

GopherAtl, on 20 June 2013 - 05:43 PM said:

The only way around this in general is to use arrays instead of hash maps.

How do i do that?

arrays have numbers as keys, starting with 1 and going up. They can be iterated with iPairs, or just by doing "for i=1,#myArray"

ex…

myArray={"a","b","c","d","e",}
for i=1,#myArray do
print(myArray[i])
end

So there is no way to have custom keys and still work through them in order

not directly, no. You could build a separate sorted list of values before printing, of course.

like so:

myTable={"foo"=1,"bar"=5,"splat"=27}

--//make an array of the values
valueArray={}
for k,v in pairs(myTable) do
  table.insert(valueArray,v)
end

--//Sort it
table.sort(valueArray)

--print
for i=1,#valueArray do
  print(valueArray[i])
end

As GopherAtl stated it is not directly possible to get the default pairs to work in that way.

However, we can also write a pairs function to do it for us

local function orderedPairs( _t )
  --# make a copy of our table, ONLY the keys
  local keys = {}
  for k,_ in pairs( _t ) do
	keys[#keys+1] = k
  end

  --# sort the keys, now they will be returned in order
  table.sort(keys)

  --# the starting index
  local i = 0

  --# return the function for the generic for loop
  return function()
	--# increment the index
	i = i + 1
	--# return the key and the value from the original table
	return keys[i], _t[keys[i]]
  end
end

And then we use orderedPairs instead of pairs in our loop.

local t = {
  ["a"]="1",
  ["b"]="2",
  ["c"]="3",
  ["d"]="4",
  ["e"]="5"
}

for k,v in orderedPairs(t) do
  print(k,':',v)
end

Doing it this way also solves another problem with pairs, and that is you cannot remove a value as it then fails to get the next key/value pair. Code that breaks

local t = {
  ["a"]="1",
  ["b"]="2",
  ["c"]="3",
  ["d"]="4",
  ["e"]="5"
}

for k,v in pairs(t) do
  t[k] = nil
end

will error with "invalid key to 'next'"

however when we do it with the new pairs

local t = {
  ["a"]="1",
  ["b"]="2",
  ["c"]="3",
  ["d"]="4",
  ["e"]="5"
}

for k,v in orderedPairs(t) do
  t[k] = nil
end

we can successfully nil values in our table.