How to append lists in makefile using loops - list

Question

I am trying to create a list of targets that depend on one file only. The list I want to create is very long and I may have to add even more elements to it, so I would like to use loops to create that target list. The targets differ mainly by their paths.
I think I just need to find out how to append or add to list in makefile, so I can create the target list I want (TARGETS) in a loop.
Here is what I have so far:
.PHONY: all dircreate dircreate_sub
# Create shortcuts to directories ##############################################
DAT4 = data/4-Year/
DAT2 = data/2-Year/
DEPVARS = a b
# Create directories ###########################################################
dircreate:
mkdir -p \
data/ \
data/4-Year/ \
data/2-Year/
dircreate_sub:
for d in $(DEPVARS); do \
mkdir -p data/4-Year/$$d ; \
mkdir -p data/2-Year/$$d ; \
done;
TARGETS = \
for d in $(DEPVARS); do \
$(DAT4)$$d/train_index.RDS \
$(DAT2)$$d/train_index.RDS \
$(DAT4)$$d/test_index.RDS \
$(DAT2)$$d/test_index.RDS; \
done;
$(TARGETS): \
dataprep.R \
funcs.R \
../core/data/analysis.data.RDS
Rscript $<
all: dircreate dircreate_sub $(TARGETS)

Answer 1

Probably you want something like:
TARGETS := $(foreach d,$(DEPVARS),\
$(DAT4)$d/train_index.RDS \
$(DAT2)$d/train_index.RDS \
$(DAT4)$d/test_index.RDS \
$(DAT2)$d/test_index.RDS)
Note I used := instead of = for efficiency.

Answer 2

You're going to want to use the foreach makefile function:
You could do something like this:
TARGETS := $(foreach depvar,$(DEPVARS),$(DAT4)$$d/train_index.RDS)
TARGETS += $(foreach depvar,$(DEPVARS),$(DAT2)$$d/train_index.RDS)
TARGETS += $(foreach depvar,$(DEPVARS),$(DAT4)$$d/test_index.RDS)
TARGETS += $(foreach depvar,$(DEPVARS),$(DAT2)$$d/test_index.RDS)
or something like this:
TARGETS := $(foreach dat,$(DAT4) $(DAT2),$\
$(foreach filename,train_index.RDS test_index.RDS,$\
$(foreach depvar,$(DEPVARS),$(dat)$(depvar)/$(filename))))
Note: I used the $\ trick to allow going over multiple lines without adding spaces (see here)
if you wanted to do something more complicated, you can always use a shell script to do everything.
TARGETS := $(shell somescript a b c)