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The Library Section and the Data Section look very similar and can, to some degree, be used interchangeably. Their implementation and intended usage are, however, quite different.
The Data Section items are all pre-calculated before any test or scan is run. The calculated values are stored in memory arrays alongside the loaded bar data. Their calculation is extremely fast since it can take advantage of multi-threading, and they can be retrieved from memory very efficiently when accessed since they're stored as fixed-size arrays.
Library Section items, in contrast, are calculated "as needed". So if a formula refers to the same library item twice, it will be calculated twice. Another way to think of Library items is as "formula snippets". It is as if the library item's formula was copied and pasted into the formula that refers to it in place of its name.
While clearly less efficient than the Data Section approach, the advantage here is that a library formula has full access to the context in which it is used. For example, you cannot refer to something like FillPrice or BarsHeld or S.Equity in a Data item. No test has been run yet so there is no context to evaluate these.
In a library item, however, if it is referenced from a strategy formula, then every syntax element available to the strategy is available to the library formula. In other words, library formulas always inherit the context of the formula that refers to them.
See the clenow_stocks_on_move.rts example script for an example of using the Library section.
Library Functions
Library formulas can optionally serve as general-purpose one-line functions.
A Library item can be referenced with up to 9 arguments, as if it were a function, e.g. my_library_item(high, low).
Library items that serve as functions can access the values passed to them by referencing the special built-in variables Arg1 - Arg9 (or Ref1 - Ref9 to receive an argument by reference).
Here is a contrived example to show how this works:
Library:
AvgOfThree: (Arg1 + Arg2 + Arg3) / 3
Data:
TypicalPrice: AvgOfThree(H, L, C)
As with any built-in function, the arguments passed to a library function can be any expression (including other library functions).
Arguments referenced as Arg1-Arg9 are passed "by value". In the above example, the current values of H, L, C would be passed to the AvgOfThree function; because each is a single number, the function cannot, for example, access Arg3[1] to get the prior close.
To receive an argument "by reference" instead -- re-evaluated in the caller's context each time it is used, so that Ref3[1] and the use of arguments in multi-bar functions both work as expected -- reference it as Ref1-Ref9 rather than Arg1-Arg9. See Arg1 - Arg9 / Ref1 - Ref9.
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