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

Question

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()).

There is no array-type in Lua, only tables which might have consecutive elements starting from index 1.

The generic for-loop, in contrast to the numeric for-loop, expects three values:

1. A callable
2. A context-value it passes on
3. An initial index-value

It calls the callable with context-value and index-value, storing all the returned values in the provided new variables. The first one is additionally saved as the new index-value.

Now some representative examples of callables for the loop:

1. ipairs(t) returns a function, the table t, and the starting-point 0.
   The function is the moral equivalent to:

   function ipairs_next(t, i)
       i = i + 1
       var v = t[i]
       if v ~= nil then
           return i, v
       end
   end
   

   Thus, all numeric entries starting at 1 until the first missing one are shown.

2. pairs(t) either delegates to t's metatable, specifically to __pairs(t), or returns the function next, the table t, and the starting-point nil. next accepts a table and an index, and returns the next index and the associated value, if it exists.

   Thus, all elements are shown in some arbitrary order.

3. There are no limits to how creative one can be with the function, and that is what vanilla Lua expects.
   See "Bizzare "attempt to call a table value" in Lua" for an example of a user-written callable, and how some dialects react if the first value is not actually a callable.