finding difference between list elements in Tcl

Question

I am a beginner in using Tcl. I am using it as VMD (molecular visualization) software uses it as a scripting language.
I have a list of co-ordinates of atom positions for a protein like: {{1 2 3} {7 9 13}, ...} I have a separate list of the same length with different positions say: {{3 5 2} {7 3 8}, ...}.
VMD has an inbuilt vecsub function which can subtract {1 2 3} and {3 5 2} to give {-2 -3 1}. I have written a foreach loop to iterate on the entire list and calculate vecsub.
My code is as follows:\

set sel1 [atomselect 0 "protein"] # selecting protein1
set sel2 [atomselect 1 "protein"] # selecting protein2
# create a list of same length as protein to store values
# $sel1 get index returns length of protein
foreach i [$sel1 get index] {
    lappend x 0 
}
# veclength2 returns square of vector length
# $sel1 get {x y z} returns a position list as described earlier
foreach i [$sel1 get index] {
    lset x $i [expr [veclength2 [vecsub [lindex [$sel1 get {x y z}] $i] [lindex [$sel2 get {x y z}] $i]]]]
}

Is there another way to do this in Tcl? Similar to python array subtraction perhaps?

Answer 1

I would try this, but it's just a guess

set x [lmap p1 [$sel1 get {x y z}] p2 [$sel2 get {x y z}] {
    expr [veclength2 [vecsub $p1 $p2]]
}]

With this, there's no need to pre-declare $x