Strange results from unpack() ?
test = {"hello","there"}
print(unpack(test))-- Print 1
print("" .. unpack(test))-- Print 2
x = unpack(test)
print(x)-- Print 3
Print 1 outputs "hello there" as expected. Print 2 and Print 3 only output "hello". Why does unpack() only return the first thing in the table if you try to do anything with the data it's giving you?
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2 answers
This is because unpack returns multiple values. It doesn't concatenate them all together for a single use. If you wanted to do that, you could use table.concat (for an array, of course). This is a consistent feature in Lua.
local x = f()will set x to the first result off()
Lua supports multiple variable assignments, meaning a, b = 1, 2 is a valid variable assignment that results in a = 1; b = 2. However, if a is given more than one value, it cannot be multiple values at the same time. So Lua picks the first one it was given. For example...
local t = {1, 2, 3}
local a = unpack(t)
a is given the values 1, 2, 3 which doesn't make any sense. Therefore, Lua decides that a will be given the first value in the tuple-like arrangement, which is 1.
However, multiple values can be assigned to multiple variables at the same time. For example, if we did this...
local t = {1, 2, 3}
local a, b, c = unpack(t)
...then we would see that it assigned a = 1, b = 2, and c = 3. This is because it could map all of the given values from unpack(t) to a unique pointer. This is very reminiscent to how you define functions in mathematics. The definition of a function (in math) is as follows:
A relationship between x and y such that, for every x, there is a unique y-value
The behavior between mapping x's to y's in math is the exact same as mapping variables to values in Lua.
Remember that in multiple variable assignment, additional results are discarded.
x = unpack(test)
is the same as
x = "hello","there"
"there" is discarded and x is assigned to "hello". This is true for any function call, not only unpack. x was assigned the first return value of unpack, which is "hello". These are not strange results.