procedure IfThenElse(dataName : Str255; sym1 : Symbol; compNum1 : real; sym2 : Symbol; compNum2 : real; fillNum : real; doElse : boolean; elseNum : real; checkNaN : boolean);

Conditional editing of the dataName.  Symbols can be > ,>= ,< , or <=.  For example: 

1.  If we were wanted to replace any number between 1 and 10 with 5...

IfThenElse("testdata",>=,1,<=,10,5,FALSE,0,FALSE);

2.  If  we wanted to replace all numbers below -999 and NaN's with -999.

IfThenElse("testdata",-,0,<,-999,-999,FALSE,0,TRUE);

Also see:  If