Thanks a lot.

It complied sucessfully.

The function name is DayTrueHigh, same as the output name.

When creating this new function, I choose the return type as numeric and function storage as auto-detect. Does this make a difference?

var: DayTrueHigh(0);

if CloseD(1) > HighD(0) then
DayTrueHigh = CloseD(1)
else
DayTrueHigh = HighD(0);

Thanks geektrader.
But this compiled with error:

"syntax error, unexpected 'function return value', expecting 'identificator'
errLine 1, errColumn 5, errLineEnd 1, errColumnEnd 5"

I am creating a new function with the following codes. if CloseD[1] > HighD then DayTrueHigh = CloseD[1] else DayTrueHigh = HighD; But when compiled, it shows "Invalid number of parameters. 1 parameter(s) expected errLine 1, errColumn 3, errLineEnd 1, errColumnEnd 3" How could I solve this? Thank yo...