How to sum coordinates in TCL - tcl

Question

I have a data file (*.dat) containing x, y, z coordinates. As following:
{26.3612117767334 40.19668960571289 54.13957977294922}
{27.351043701171875 40.57518768310547 54.05387496948242}
{29.48208999633789 42.08218765258789 56.42238235473633}
For this file I need to do a math operation as follow:
Xi + (Xf-Xi/4) ; Yi + (Yf-Yi/4) ; Zi + (Zf-Zi/4)
where "i" is the initial position and "f" the final, meaning that Xi,Yi,Zi are the data on the first line and Xf,Yf,Zf the data on the second.
I need to do these calculation for all the lines in a loop and then stored in a separate file, but I do not have idea how to do it in TCL. Thanks in advance for your help.

Answer 1

Since the contents of your file can be treated as a bunch of tcl lists, one per line (so basically a list of lists), parsing it is dead simple.
Something like:
set f [open file.dat]
set coords [read -nonewline $f]
close $f
for {set i 0} {$i < [llength $coords] - 1} {incr i} {
lassign [lindex $coords $i] xi yi zi
lassign [lindex $coords $i+1] xf yf zf
set xn [expr {$xi + ($xf - $xi/4.0)}]
set yn [expr {$yi + ($yf - $yi/4.0)}]
set zn [expr {$zi + ($zf - $zi/4.0)}]
puts "{$xn $yn $zn}"
}
This skips treating the last line as an initial set of coordinates because there is no next set for it.

Answer 2

This is a good opportunity to write a mathfunc:
proc tcl::mathfunc::f {ai af} {
expr {$ai * 0.75 + $af}
}
proc transform {file} {
set fh [open $file]
# read the first line, aka the initial "previous line"
gets $fh line
scan $line {{%f %f %f}} xi yi zi
# process the rest of the file
while {[gets $fh line] != -1} {
scan $line {{%f %f %f}} xf yf zf
puts "{[expr {f($xi, $xf)}] [expr {f($yi, $yf)}] [expr {f($zi, $zf)}]}"
lassign [list $xf $yf $zf] xi yi zi
}
close $fh
}
transform file.dat
outputs
{47.121952533721924 70.72270488739014 94.65855979919434]}
{49.9953727722168 72.51357841491699 96.96278858184814]}

Answer 3

I present an alternate method that uses lrange to pick the overlapping ranges of sublists that participate (so we can then process them element-wise) and then lmap to apply the same transformation expression to each coordinate axis.
# Same read-in code as Shawn's answer; it's the easiest way
set f [open file.dat]
set coords [read -nonewline $f]
close $f
foreach Ci [lrange $coords 0 end-1] Cf [lrange $coords 1 end] {
# I often like to put expressions on their own line for clarity
puts [list [lmap _i $Ci _f $Cf {expr {
$_i + ($_f - $_i/4.0)
}}]]
}
(The wrapping list call in there puts braces around the result of lmap.)