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That's working as documented. From section 3.4 of the Lua Reference

Both function calls and vararg expressions can result in multiple values. If a function call is used as a statement (see §3.3.6), then its return list is adjusted to zero elements, thus discarding all returned values. If an _expression_ is used as the last (or the only) element of a list of expressions, then no adjustment is made (unless the _expression_ is enclosed in parentheses). In all other contexts, Lua adjusts the result list to one element, either discarding all values except the first one or adding a single nil if there are no values.

Just below that in the list of examples:

a,b,c = f(), x     -- f() is adjusted to 1 result (c gets nil)

So in your string.format code, the last %d really wants a number, but it got a nil so it threw an error.


On 11/25/22 5:56 PM, bil til wrote:
PS: The following also might be interesting:
function Test() return 3, 5 end
s= string.format( "%d %d", Test(), 4)
3 4

... so it looks like the '...' of string.format somehow keeps only the
first return value of Test, except if Test function is the last
parameter... . Is this normal?

(I thought, that for such "further parameter stripping" it would be
required to specify this explicitely by brackets like in s=
string.format( "%d %d", (Test()), 4)? (this gives the same as above,
in this case it is clear to me...).

Am Fr., 25. Nov. 2022 um 23:46 Uhr schrieb bil til <>:
If I do the following in Lua 5.4:
function Test() return 3, 5 end
s= string.format( "%d %d %d", 4, Test())
4 3 5
... this is as expected...

function Test() return 3, 5 end
s= string.format( "%d %d %d", Test(), 4)
stdin:3: bad argument #4 to 'format' (no value)
stack traceback:
        [C]: in function 'string.format'
        stdin:3: in main chunk
        [C]: in ?
... this somehow does NOT work as I would expect... (I would expect
that s now is '3 5 4')

(if I debug with breakpoint in str_format, then the top of stack ( int
top = lua_gettop(L);) is only 3 in this last case... in first case it
is 4 as expected.... .

Is this some error of Lua, or do I miss here something?