Ipairs

[LuaWeaver]

Can someone explain ipairs for me? I know the following:

ipairs uses the length of the table selected with 1 as the first value and the # of things in the table and the second.

ipairs uses the number in the table in some form. So, instead of for i=1, #h do print(h) end you can do something else with ipairs to make it simpler.


Sorry, but thank you. I believe these are true about ipairs. If not, feel free to correct me .

[BlackBulletIV]

As far as I know

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for i, v in ipairs(t) do
  -- code here
end

is basically the same as

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local i = 1

while true do
  local v = t[i]
  if not v then break end
  -- code here
  i = i + 1
end

or

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for i = 1, #t do
  local v = t[i]
  if not v then break end
  -- code here
end

The only difference is that ipairs is slower, and more convenient.

[bmelts]

Well, let's look at the syntax.

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for i,v in ipairs(t) do
    -- ...
end

i and v are special variables (they can be called anything, but i/v is standard) that ipairs provides you with which refer to an element of the table you're looking through (in this case, t). i is the index of the element, and v is the element itself. So something like

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t = {"hi", "bye", 42}
for i,v in ipairs(t) do
    print(i..": "..v)
end

would output

Code: Select all

1: hi
2: bye
3: 42

v, then, is equivalent to t. It's just a convenient shorthand because generally when you're iterating through a table you want to do things with the table's contents. Thus, you could rewrite the code in your post as:

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for i,v in ipairs(h) do
    print(v)
end

Note that ipairs will iterate over every consecutive numeric index in the table, from 1 to wherever. So if your table's like this:

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t = {}
t[1] = "foo"
t[2] = "bar"
t[3] = "baz"

then running the table printing code above would output

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1: foo
2: bar
3: baz

But if the table looked like

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t = {}
t[1] = "foo"
t[2] = "bar"
t[4] = "honk"

then the output would look like

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1: foo
2: bar

because 4 does not come after 2. If you then added

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t[3] = "baz"

4 would be part of the chain again, since 3 comes after 2 and 4 comes after 3. Thus, the output would be

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1: foo
2: bar
3: baz
4: honk

[T-Bone]

I've never understood why you would want to use ipairs.

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t={"a","b","c"}
for i=1,#t do
    print(i..": "..t[i])
end

feels much cleaner in my opinion

[BlackBulletIV]

I've never understood why you would want to use ipairs.

Code: Select all

t={"a","b","c"}
for i=1,#t do
    print(i..": "..t[i])
end

feels much cleaner in my opinion

For the convenience of not having to index the table every time you need the value. Usually table names aren't as short as t. You could of assign the value into a local variable, but I don't think you would call that cleaner.

[T-Bone]

I've never understood why you would want to use ipairs.

Code: Select all

t={"a","b","c"}
for i=1,#t do
    print(i..": "..t[i])
end

feels much cleaner in my opinion

For the convenience of not having to index the table every time you need the value. Usually table names aren't as short as t. You could of assign the value into a local variable, but I don't think you would call that cleaner.

Are these two methods equally fast by the way?

[Astusvis]

I usually use the normal

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for i,v in pairs(t)

[slime]

I usually use the normal

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for i,v in pairs(t)

pairs and ipairs have slightly different uses.

[LuaWeaver]

pairs and ipairs have slightly different uses.

Explain this please, I get ipairs, may as well get in pairs now that it's mentioned, but thanks for you're help!

[slime]

pairs and ipairs have slightly different uses.

Explain this please, I get ipairs, may as well get in pairs now that it's mentioned, but thanks for you're help!

ipairs will only loop through the array part of a table (t[1], t[2], ...., t[100], etc), but it does it in the correct order. pairs, on the other hand, loops through all key/value pairs of a table, but the order is not guaranteed.

For example, if you had this table:

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tbl = {
	"this is an array value",
	"this is another array value",
	notarrayvalue = true,
	thegame = "hi",
	"third array value",
}

then doing this:

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for i, v in ipairs(tbl) do print(i, v) end

will output this:

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1	this is an array value
2	this is another array value
3	third array value

However when you use pairs to iterate over the table, you will get all the keys and values, but in no guaranteed order. This:

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for k,v in pairs(tbl) do print(k,v) end

will output something like this:

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1	this is an array value
2	this is another array value
3	third array value
notarrayvalue	true
thegame	hi