Sarnold -- 2005/12/13
Typically, when you do some list processing, you can use the foreach command which is efficient, but a construct like:
for {set i 0} {$i<[llength $mylist]} {incr i} {...}is slow because it has to call llength at each iteration. I admit it is not new. So a construct like :
for {set i 0;set len [llength $mylist]} {$i<$len} {incr i} {...}would be faster, but less readable as there is a new variable that does not mean anything in the rest of the program.
In Python, you can get more speed by the range() function:
for i in range(l.length):
# some codeI submit a new command called foriter that mimics Python's functionality:
foriter loopvar ?start? end ?increment? body loopvar - the name of a variable (existing or not) that holds the counter start - defaults to 0 increment - defaults to 1
Then,
foriter i start end increment body
is equivalent to:
for {set i $start} {$i<$end} {incr i $increment} $bodySome benchmarks show that it is faster than for in many cases.
Here is an implementation in C:
critcl::ccode {
#include <stdio.h>
}
package provide Foriter 0.1
critcl::ccommand foriter {dummy interp objc objv} {
int result;
/* foriter loop : from start to end-1 increment by <increment> do <body> */
Tcl_Obj *obj_body = NULL;
/* the int values of the loop range */
int start = 0;
int end;
int increment = 1;
/* the name of the variable to set the counter */
char *counter_name = NULL;
if (objc<4 || objc>6) {
Tcl_WrongNumArgs(interp, 1, objv, "foriter varname ?start? end ?increment? body");
return TCL_ERROR;
}
if (objc == 4) {
result = Tcl_GetIntFromObj(interp, objv[2], &end);
if (result != TCL_OK) {
return result;
}
obj_body = objv[3];
} else {
/* the user provided 'start' as 2nd argument,
and the 'end' of the range is then the 3rd argument */
result = Tcl_GetIntFromObj(interp, objv[2], &start);
if (result != TCL_OK) {
return result;
}
result = Tcl_GetIntFromObj(interp, objv[3], &end);
if (result != TCL_OK) {
return result;
}
if (objc == 5) {
obj_body = objv[4];
} else {
obj_body = objv[5];
}
}
if (objc == 6) {
result = Tcl_GetIntFromObj(interp, objv[4], &increment);
if (result != TCL_OK) {
return result;
}
}
Tcl_LinkVar(interp, counter_name=Tcl_GetString(objv[1]), &start, TCL_LINK_INT);
if ((end-start) * increment < 0) {
/* the iterating range goes the other way
than incrementation does */
Tcl_SetObjResult(interp, Tcl_NewStringObj(
"invalid range : an endless loop would occur", -1));
return TCL_ERROR;
}
/* please note that a negative increment could be used
(I wonder if one shall do so ?)
now, the loop begins */
for ( ; (increment > 0)? (start < end): (start > end) ; ) {
/* we are into the loop */
/* printf("."); */
result = Tcl_EvalObjEx(interp, obj_body, 0);
if ((result != TCL_OK) && (result != TCL_CONTINUE)) {
if (result == TCL_ERROR) {
Tcl_SetObjResult(interp, Tcl_NewStringObj(
"error in foriter body", -1));
/* when an error occurs, we quit the loop
and clean up things like References */
break;
}
/* when the user breaks the evaluation
we have to break out of the loop */
break;
}
/* updating the integer value */
start += increment;
Tcl_UpdateLinkedVar(interp, counter_name);
}
Tcl_UnlinkVar(interp, counter_name);
if (result == TCL_ERROR) {
return result;
}
Tcl_ResetResult(interp);
return TCL_OK;
}