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    <pre><code>t = {[1]=1, [2]=2, [4]=4} -- has a length of 4 </code></pre> <p>Well there's your problem right there; that does not have a length of 4. You might <em>think</em> it does, and <code>#t</code> might return 4. But as far as the Lua API is concerned, the length of this table is <em>undefined</em>.</p> <p><a href="http://www.lua.org/manual/5.1/manual.html#2.5.5" rel="nofollow">Lua 5.1 states</a>:</p> <blockquote> <p>The length of a table t is defined to be any integer index n such that t[n] is not nil and t[n+1] is nil; moreover, if t<a href="http://www.lua.org/manual/5.1/manual.html#2.5.5" rel="nofollow">1</a> is nil, n can be zero. For a regular array, with non-nil values from 1 to a given n, its length is exactly that n, the index of its last value. If the array has "holes" (that is, nil values between other non-nil values), then #t can be any of the indices that directly precedes a nil value (that is, it may consider any such nil value as the end of the array).</p> </blockquote> <p><a href="http://www.lua.org/manual/5.2/manual.html#3.4.6" rel="nofollow">Lua 5.2 is rather more explicit</a>:</p> <blockquote> <p>the length of a table t is only defined if the table is a sequence, that is, the set of its positive numeric keys is equal to {1..n} for some integer n. In that case, n is its length. Note that a table like</p> </blockquote> <pre><code> {10, 20, nil, 40} </code></pre> <blockquote> <p>is not a sequence, because it has the key 4 but does not have the key 3. (So, there is no n such that the set {1..n} is equal to the set of positive numeric keys of that table.) Note, however, that non-numeric keys do not interfere with whether a table is a sequence. </p> </blockquote> <p>But in both cases, the length is <em>undefined</em>.</p>
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    1. POHow to produce ipairs (as opposed to pairs) behavior in the C API
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    1. USNicol Bolas
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    1. COYes, I understand the sometimes undefined nature of table lengths, but I do not understand how this helps answer my question: "How to produce the `ipairs` behavior in the C API?" Like I said, I _naively_ could use the reported length but I suspect there is something more sophisticated going on under the covers of ipairs.
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      1. USPaul
 

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