Reading lua table with word index gives random order

Question

Following is a lua code to read a table with word indexes. reading this into another table and printing it in output gives random order everytime it is run.

earthquakes = {
        date8 = "1992/01/17",
        date7 = "1971/02/09",
        date6 = "2010/04/04",
        date5 = "1987/10/19"
}
sf = string.format
earthquake_num ={}

for k, v in pairs(earthquakes) do
        table.insert(earthquake_num, {key=k,value=v})
end

for i, v in pairs (earthquake_num) do
print(sf(" row %d key = %s", i, v.value))
end

OUTPUT : everytime in different order

score 3 · Answer 1 · edited May 23 '17 at 11:52

3

This is special feature of Lua 5.2.1 :-)
But what for this feature was introduced?
Anyway, you should not rely on ordering generated by pairs function.

EDIT :
This feature was introduced to fight hash collision attacks on web servers that are using Lua.
Randomized hash algorithm prevents easy generating of strings with equal hashes.
Ordering of table keys generated by pairs function depends on hashes of strings for string-type keys, so string keys are happened to be mixed up on every program run.

edited May 23 '17 at 11:52

Community

1
1

answered Apr 08 '13 at 07:09

Egor Skriptunoff

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2

It was probably introduced to explicitly *break* any code that relied on a specific order. Thus, people who accidentally wrote such code will get incorrect behavior and fix it. – Nicol Bolas Apr 08 '13 at 07:34
2

@NicolBolas, that wasn't the reason but it does have this side effect, which is good. – lhf Apr 08 '13 at 11:28

score 3 · Answer 2 · answered Apr 08 '13 at 09:53

From Lua PiL on iterators:

The pairs function, which iterates over all elements in a table, is similar, except that the iterator function is the next function, which is a primitive function in Lua:
function pairs (t)
  return next, t, nil
end
The call next(t, k), where k is a key of the table t, returns a next key in the table, in an arbitrary order. (It returns also the value associated with that key, as a second return value.) The call next(t, nil) returns a first pair. When there are no more pairs, next returns nil.

And the enumeration for next states:

next (table [, index])

The order in which the indices are enumerated is not specified, even for numeric indices. (To traverse a table in numeric order, use a numerical for or the ipairs function.)

score 0 · Accepted Answer · edited May 23 '17 at 12:15

As Egor says the pairs iterator returns table values in an arbitrary order. To sort data and return it in a sequenced format you need to use ipairs for example

earthquakes = {
        date8 = "1992/01/17",
        date7 = "1971/02/09",
        date6 = "2010/04/04",
        date5 = "1987/10/19"
}
sf = string.format
earthquake_num ={}

for k, v in pairs(earthquakes) do
        table.insert(earthquake_num, {key=k,value=v})
end
table.sort(earthquake_num,function(a, b) return a.value < b.value end)
for i, v in ipairs (earthquake_num) do
print(sf(" row %d key = %s", i, v.value))
end

see lua: iterate through all pairs in table for more information.

Reading lua table with word index gives random order

3 Answers3

`next (table [, index])`

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