In Lua, can I easily select the Nth result without custom functions?

Question

Suppose I am inserting a string into a table as follows:

table.insert(tbl, mystring)

and that mystring is generated by replacing all occurrences of "a" with "b" in input:

mystring = string.gsub(input, "a", "b")

The obvious way to combine the two into one statement doesn't work, because gsub returns two results:

table.insert(tbl, string.gsub(input, "a", "b"))  -- error!
-- (second result of gsub is passed into table.insert)

which, I suppose, is the price paid for supporting multiple return values. The question is, is there a standard, built-in way to select just the first return value? When I found select I thought that was exactly what it did, but alas, it actually selects all results from N onwards, and so doesn't help in this scenario.

Now I know I can define my own select as follows:

function select1(n, ...)
  return arg[n]
end

table.insert(tbl, select1(1, string.gsub(input, "a", "b")))

but this doesn't look right, since I'd expect a built-in way of doing this.

So, am I missing some built-in construct? If not, do Lua developers tend to use a separate variable to extract the correct argument or write their own select1 functions?

Answer 1

You can surround the expression by parentheses:

table.insert(tbl, (string.gsub(input, "a", "b")))

This will select only the first result.

To get the nth result, you can use select and surround it by parentheses:

func1( (select(n, func2())) )

Answer 2

Putting an expression into parentheses like this:

table.insert(tbl, (string.gsub(input, "a", "b")))

will force one return value. Or you could catch them both like this:

str,cnt = string.gsub(input, "a", "b")
table.insert(tbl, str)

or even better, dummy catch it to save a variable:

str,_ = string.gsub(input, "a", "b")
table.insert(tbl, str)