Checking user-entered expressions for validity

From: Tom Roberts <tjrob_at_fnal.gov>
Date: Sat, 13 Jan 2007 13:11:39 -0600

In my Root application the user can enter several arbitrary expressions using any C functions and operators, and the branch names of a selected TNtuple (all names are valid C identifiers). It then creates a function scan() that will scan the TNtuple using these expressions, writing it to the file ".scan.C", and does:

         Int_t err=TInterpreter::kNoError;
         gROOT->ProcessLineSync(".L .scan.C",&err);
         if(err != TInterpreter::kNoError) {
		new TGMsgBox(gClient->GetRoot(),window,"Error",
                                 "Invalid Expression(s)");
		return false;
	}
	scan(); // call the generated function

This works fine as long as the expressions are valid. Unfortunately, if any expression is invalid, Cint prints an error message to stderr and never returns from ProcessLineSync(). To the user this is disconcerting, as no sensible error message is presented (the user probably won't watch stderr, which may not be visible in a graphical environment).

How can I get Cint to return to the program when there is an error loading the file?

Alternatively, how can I test the individual expressions for validity? -- all my attempts have failed, either Cint does not return an error for an invalid expression (!), or it does not return from ProcessLineSync(), or it goes crazy: When I tried this Cint got into a strange state that could not write any files or do anything useful:

	Int_t err=-99;
	char *str = "{\nfloat x,y;\n.p x+y;\n}\n";
	gROOT->ProcessLineSync(str,&err);

Omitting the braces makes it merely not return when an invalid expression is given.

Do I need to follow Rube Goldberg and:

	set a timer for 500 ms in the future
	call ProcessLineSync()
	clear the timer if it returns success
	... display an error MsgBox if the timer fires

Note this is always done in response to a GUI button push, and the fact that ProcessLineSync() does not return does not seem to bother the application.

This is Root version 5.12/00.

Tom Roberts Received on Sat Jan 13 2007 - 20:11:50 CET