int Tcl_GetIndexFromObj(interp, objPtr, tablePtr, msg, flags, indexPtr)
int Tcl_GetIndexFromObjStruct(interp, objPtr, structTablePtr, offset, msg, flags, indexPtr)
This procedure provides an efficient way for looking up keywords, switch names, option names, and similar things where the value of an object must be one of a predefined set of values. ObjPtr is compared against each of the strings in tablePtr to find a match. A match occurs if objPtr's string value is identical to one of the strings in tablePtr, or if it is a non-empty unique abbreviation for exactly one of the strings in tablePtr and the TCL_EXACT flag was not specified; in either case the index of the matching entry is stored at *indexPtr and TCL_OK is returned.
If there is no matching entry, TCL_ERROR is returned and an error message is left in interp's result if interp isn't NULL. Msg is included in the error message to indicate what was being looked up. For example, if msg is option the error message will have a form like bad option "firt": must be first, second, or third.
If Tcl_GetIndexFromObj completes successfully it modifies the internal representation of objPtr to hold the address of the table and the index of the matching entry. If Tcl_GetIndexFromObj is invoked again with the same objPtr and tablePtr arguments (e.g. during a reinvocation of a Tcl command), it returns the matching index immediately without having to redo the lookup operation. Note: Tcl_GetIndexFromObj assumes that the entries in tablePtr are static: they must not change between invocations. If the value of objPtr is the empty string, Tcl_GetIndexFromObj will treat it as a non-matching value and return TCL_ERROR.
Tcl_GetIndexFromObjStruct works just like Tcl_GetIndexFromObj, except that instead of treating tablePtr as an array of string pointers, it treats it as the first in a series of string ptrs that are spaced apart by offset bytes. This is particularly useful when processing things like Tk_ConfigurationSpec, whose string keys are in the same place in each of several array elements.