What is the difference of pairs() vs. ipairs() in Lua?

for-loop lua
user11071538 picture user11071538 · Mar 11, 2019 · Viewed 32.3k times · Source

In a for loop, what is the difference between looping with pairs() and ipairs()? This page uses both: Lua Docs

With ipairs():

a = {"one", "two", "three"}
for i, v in ipairs(a) do
  print(i, v)
end

Result:

1   one
2   two
3   three

With pairs():

a = {"one", "two", "three"}
for i, v in pairs(a) do
  print(i, v)
end

Result: 

1   one
2   two
3   three

You can test it here: Lua Demo

Answer

Alain Merigot picture Alain Merigot · Mar 11, 2019

pairs() and ipairs() are slightly different.

  • pairs() returns key-value pairs and is mostly used for associative tables. key order is unspecified.
  • ipairs() returns index-value pairs and is mostly used for numeric tables. Non numeric keys in an array are ignored, while the index order is deterministic (in numeric order).

This is illustrated by the following code fragment.

> u={}
> u[1]="a"
> u[3]="b"
> u[2]="c"
> u[4]="d"
> u["hello"]="world"
> for key,value in ipairs(u) do print(key,value) end
1   a
2   c
3   b
4   d
> for key,value in pairs(u) do print(key,value) end
1   a
hello   world
3   b
2   c
4   d
>

When you create an tables without keys (as in your question), it behaves as a numeric array and behaviour or pairs and ipairs is identical.

a = {"one", "two", "three"}

is equivalent to a[1]="one" a[2]="two" a[3]="three" and pairs() and ipairs() will be identical (except for the ordering that is not guaranteed in pairs()).

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