Gnuplot macros, do for, and arrays - arrays

Question

I am using gnuplot 5.2.4.
I am doing a for-loop, to analyse in each cycle a different column (or combination of columns) of a file. The column to be used is written in an array and is passed to the commands as a macro.
The code is written in a .plt file that I load in gnuplot.
It looks like this
(here for example I'm using the 1.dat file in the \gnuplot\demo)
:
set macros
array label[2]
label[1]="First_Cycle"
label[2]="Second_Cycle"
array index[2]
index[1]="(column(2))"
index[2]="(column(2)-column(1))"
fileDat = "1.dat"
do for [k = 1 : 2] {
fileExport = sprintf("%s.gif",label[k])
print fileExport
set term gif size 1920,1280 enhanced
set output fileExport
indexk = index[k]
print k," ",index[k]," ", indexk
stats fileDat u #indexk name "DV" #noout
plot fileDat u #indexk ti label[k]
}
indexk is printed correctly at each cycle, however, I obtain the following warning:
warning: indexk is not a string variable
and the commands stats and plot analise always the column in index[1] at each cycle.
However, if I comment the do for loop and increase the k by hand, the code works correctly, no warning, no problem,
like this:
set macros
array label[2]
label[1]="First_Cycle"
label[2]="Second_Cycle"
array index[2]
index[1]="(column(2))"
index[2]="(column(2)-column(1))"
fileDat = "1.dat"
k=2
#do for [k = 1 : 2] {
fileExport = sprintf("%s.gif",label[k])
print fileExport
set term gif size 1920,1280 enhanced
set output fileExport
indexk = index[k]
print k," ",index[k]," ", indexk
stats fileDat u #indexk name "DV" #noout
plot fileDat u #indexk ti label[k]
#}

Answer 1

You can think of gnuplot macros (# + string variable name) as roughly equivalent to C language preprocessor directives. They describe a substitution that is performed only once, the first time the line of code is encountered. So trying to change one inside a loop will not work.
In some contexts the gnuplot directive "evaluate( )" is equivalent to dynamic macro substitution, but in the case shown you need an expression rather than a statement. So something like:
Col(k) = (k == 1) ? column(2) : column(2) - column(1)
do for [k=1:2] {
...
stats fileDat using Col(k) name "DV"
plot fileDat using Col(k) ti label[k]
}