unpack function issue in Lua

Question

I have the following unpack() function:

function unpack(t, i)  
    i = i or 1  
    if t[i] then  
        return t[i], unpack(t, i + 1)  
    end  
end  

I now use it in the following test code:

t = {"one", "two", "three"}  
print (unpack(t))  
print (type(unpack(t)))  
print (string.find(unpack(t), "one"))  
print (string.find(unpack(t), "two"))

which outputs:

one two three  
string  
1   3  
nil  

What puzzles me is the last line, why is the result nil ?

Answer 1

If a function returns multiple values, unless it's used as the last parameter, only the first value is taken.

In your example, string.find(unpack(t), "one") and string.find(unpack(t), "two") , "two" and "three" are thrown away, they are equivalent to:

string.find("one", "one")  --3

and

string.find("one", "two")  --nil

Answer 2

Lua Pil has this to say under 5.1 - Multiple Results :

Lua always adjusts the number of results from a function to the circumstances of the call. When we call a function as a statement, Lua discards all of its results. When we use a call as an expression, Lua keeps only the first result. We get all results only when the call is the last (or the only) expression in a list of expressions . These lists appear in four constructions in Lua: multiple assignment, arguments to function calls, table constructors, and return statements.

It gives the following example to help illustrate:

function foo0 () end                  -- returns no results
function foo1 () return 'a' end       -- returns 1 result
function foo2 () return 'a','b' end   -- returns 2 results

x, y = foo2(), 20      -- x='a', y=20
x, y = foo0(), 20, 30  -- x='nil', y=20, 30 is discarded

Yu Hao's answer shows how this applies specifically to your example.

Answer 3

The function unpack returns multiply parameters, string.find takes the first parameters only (the rest are discarded).

This function will unpack and concatenate all strings, so the function output will be a single parameter.

function _unpack(t,char)
    return table.concat(t,(char or " "));
end
t = {"one", "two", "three"}  
print (_unpack(t))  
print (type(_unpack(t)))  
print (string.find(_unpack(t), "one"))  
print (string.find(_unpack(t), "two"))

output

one two three 
string 
1 3 
5 7