Dynamically defined logical expressions (tests/cuts)¶
15 April 2004
Overview¶
A useful feature of Podd is the ability to define arithmetic and logical expressions dynamically (on the fly) without the need to recompile any code. Underlying both types of expressions are "global variables" that are represented by the THaVar
class and collected in the global variable list THaVarList
. Each analysis module (detector, apparatus, physics module) normally adds internal variables of interest to the global variable list gHaVars
as part of its initialization and removes those variables from the list in its destructor. For an example, see THaScintillator
. In this way, analysis results are conveniently available for use in expressions and by THaOutput
.
Arithmetic expression parsing is implemented via the THaFormula
class, while tests/cuts are special cases of THaFormula@s which evaluate to either 1 or 0 (true or false) and are represented by the "@THaCut
":http://hallaweb.jlab.org/podd/html/THaCut.html class. In the C++ analyzer, THaFormulas
are primarily used in the THaOutput
system, while THaCuts
are used for two purposes: (1) in THaOutput
as conditions on histograms; and (2) to control the flow of the data analysis in the standard analysis algorithm. The latter is supported by a special global list of tests, THaCutList
, THaCutList
supports the concept of "blocks" of cuts which can be evaluated as a unit. Blocks are evaluated at the end of each stage of the analysis (Decode/Reconstruct/Physics).
The interactive interface to the analyzer, THaInterface
, automatically creates an instance of THaVarList
and THaCutList
upon startup. These are called the "global variable list" and "global cut list", respectively. They are accessible via the global program variables gHaVars
and gHaCuts
from anywhere in the analyzer if you include THaGlobals.h
.
Examples¶
A short tutorial on using the test/cut classes along with the global variable system follows. As mentioned, the relevant classes are:
THaCutList
-- Define and evaluate cuts. Automatically manages cuts with the help of internal lists.THaVarList
-- Manage "global" symbolic variables.THaCut
-- Definition of a single cut. This is a low-level class which should normally only be used internally byTHaCutList
.THaVar
-- Definition of a single symbolic variable. This is a low-level class which should normally only be used internally byTHaVarList
.
Here is a sample session to demonstrate the use of these classes:
analyzer [0] double xvar=10 // Define a variable xvar analyzer [1] gHaVars->Define("x",xvar) // Add xvar to global var list, name it "x" analyzer [2] gHaVars->PrintFull() // Show all global vars defined including their current values OBJ: THaVar x x (Double_t)[1] 10 analyzer [3] gHaCuts->Define("cut1","x>0") // Define a cut named "cut1" that is true if x>0. analyzer [4] gHaCuts->Define("cut2","abs(x)>5") // dto., but |x|>5 analyzer [5] gHaCuts->Print() // List all defined cuts Name Def T Block Called Passed ------------------------------------------------- cut1 x>0 0 Default 0 0 (0.0%) cut2 abs(x)>5 0 Default 0 0 (0.0%) analyzer [6] gHaCuts->Eval() // Evaluate all defined cuts analyzer [7] gHaCuts->Print() Name Def T Block Called Passed ------------------------------------------------- cut1 x>0 1 Default 1 1 (100%) cut2 abs(x)>5 1 Default 1 1 (100%) (Note the current value ("T") of each of the cuts. Both are true since both conditions are true for x=10). analyzer [8] xvar=2 // Give xvar a new value analyzer [9] gHaCuts->Eval() // Evaluate the cuts again analyzer [10] gHaCuts->Print() Name Def T Block Called Passed ------------------------------------------------- cut1 x>0 1 Default 2 2 (100%) cut2 abs(x)>5 0 Default 2 1 (50%) (Note that cut2 is now false since |x| is less than 5.) analyzer [11] gHaCuts->Result("cut2") // Retrieve result of cut2 // NB: This does not re-evaluate the cut (Int_t)0 analyzer [11] gHaCuts->Result("cut1") (Int_t)1
Tests may refer to other tests already defined in gHaCuts. When evaluating cuts containing other tests, the referenced tests are not re-evaluated, but the result of their last evaluation is used (as in the call to
THaCutList::Result()
above). This ensures that the test statistics (i.e. number of calls/number of passes) remain consistent when evaluating all the tests for an event. Here's an example, continuing from above:analyzer [12] gHaCuts->Define("cut3","cut1&&!cut2") // Define cut based on two previously-defined cuts analyzer [13] gHaCuts->Eval() // Evaluate defined cuts again analyzer [14] gHaCuts->Print() Name Def T Block Called Passed ---------------------------------------------------- cut1 x>0 1 Default 3 3 (100%) cut2 abs(x)>5 0 Default 3 1 (33.3%) cut3 cut1&&!cut2 1 Default 1 1 (100%)
Updated by Ole Hansen almost 7 years ago · 2 revisions