Robust way to pick value from X-Y table in TCL - tcl

Question

I would like to know the most robust method for extracting Y Value for a given X Value from column of X-Y data.
I am currently performing this operation with the following code, but is very unreliable/flakey as it keeps falling over with error of can't read or no variable var_01
Please advice.
Iterate based on Column Z
for {set i 0} {$i < [llength $Col_z]} {incr i} {
set Xdata [lindex $Col_x $i]
set Ydata [lindex $Col_y $i]
lappend var $Ydata
if { $Xdata >= 0.9 && $Xdata <= 1.1 } {
set a [lindex $var $i]
lappend var_01 $a
} else {lappend var_01 0
#set var_01 0}
}

Answer 1

It's very hard to work out what you want to do, but maybe it helps to simplify the code a bit:
foreach z $Col_z x $Col_x y $Col_y {
if {$z eq {}} {
break
}
if {$x >= 0.9 && $x <= 1.1} {
lappend var_01 $y
} else {
lappend var_01 0
}
}
Edit according to comment: is this better?
set var_01 {}
foreach z $Col_z x $Col_x y $Col_y {
if {$z eq {}} {
break
}
if {$x >= 0.9 && $x <= 1.1} {
lappend var_01 $y
}
}
Note that var_01 might be empty if no value of x is within the range.
Documentation:
&& (operator),
<= (operator),
>= (operator),
break,
eq (operator),
foreach,
if,
lappend,
set

Answer 2

A very convenient way to represent tables in tcl is by simple array. Here is an example:
array set xy {}
foreach i {1 2 3} {
foreach j {10 20 30} {
set xy($i,$j) [expr $i + $j]
}
}
Now xy is an array whose keys look like table indexes. Here:
% array names xy
3,10 2,20 1,30 3,20 2,30 3,30 1,10 2,10 1,20
Or more clear:
% foreach k [array names xy] {puts $k}
3,10
2,20
1,30
3,20
2,30
3,30
1,10
2,10
1,20
Here is how to access them:
% puts $xy(3,10)
13
The 3,10 inside the parenthesis is a string! The array returns the value associated with that string, which was associated in the above loop. (Therefore there must not be space after the comma).
It's easy to access the values if the indexes are given in variables:
% set x 3
3
% set y 10
10
% puts $x,$y
3,10
The last command is equivalent to explicit quotation marks:
% puts "$x,$y"
3,10
And here is how we access the array element at that key:
% puts $xy($x,$y)
13
And if the key doesn't exist:
% puts $xy(4,10)
can't read "xy(4,10)": no such element in array
Let's conclude with printing the keys and values of the array:
% foreach k [array names xy] {puts "$k: $xy($k)"}
3,10: 13
2,20: 22
1,30: 31
3,20: 23
2,30: 32
3,30: 33
1,10: 11
2,10: 12
1,20: 21
ADDED
Now suppose you have the y and z values, how do you find the x?
set y 20
set z 23
Using the special, powerful tcl property of everything is a string:
Here we find all keys and values matching the key pattern *,20:
set results [array get xy *,$y]
Let's see:
puts $results
% 2,20 22 3,20 23 1,20 21
We got a list of 3 pairs, each contains the key and value.
Now let's extract the key/value that corresponds to outr $z. We will use the powerful regexp tcl command, seeing $results now as a string instead of a list:
regexp "(\\d+),($y) ($z)" $results whole x1 y1 z1
And now x1, y1, z1 hold all the information we want:
puts "$x1 $y1 $z1"
% 3 20 23