Parallel iteration over lists in makefile or CMake file

Question

Is there a way to loop over multiple lists in parallel in a makefile or CMake file?

I would like to do something like the following in CMake, except AFAICT this syntax isn't supported:

set(a_values a0 a1 a2)
set(b_values b0 b1 b2)
foreach(a in a_values b in b_values)
  do_something_with(a b)
endforeach(a b)

This would execute:

do_something_with(a0 b0)
do_something_with(a1 b1)
do_something_with(a2 b2)

I would accept an answer in either CMake or Make, though CMake would be preferred. Thanks!

Answer 1

Here you go:

set(list1 1 2 3 4 5)
set(list2 6 7 8 9 0)

list(LENGTH list1 len1)
math(EXPR len2 "${len1} - 1")

foreach(val RANGE ${len2})
  list(GET list1 ${val} val1)
  list(GET list2 ${val} val2)
  message(STATUS "${val1}  ${val2}")
endforeach()

Answer 2

As of CMake 3.17, the foreach() loop supports a ZIP_LISTS option to iterate through two (or more) lists simultaneously:

set(a_values a0 a1 a2)
set(b_values b0 b1 b2)
foreach(a b IN ZIP_LISTS a_values b_values)
    message("${a} ${b}")
endforeach()

This prints:

a0 b0
a1 b1
a2 b2

Answer 3

In make you can use the GNUmake table toolkit to achieve this by handling the two lists as 1-column tables:

include gmtt/gmtt.mk

# declare the lists as tables with one column
list_a := 1 a0 a1 a2 a3 a4 a5
list_b := 1 b0 b1 b2

# note: list_b is shorter and will be filled up with a default value
joined_list := $(call join-tbl,$(list_a),$(list_b), /*nil*/)

$(info $(joined_list))

# Apply a function (simply output "<tuple(,)>") on each table row, i.e. tuple
$(info $(call map-tbl,$(joined_list),<tuple($$1,$$2)>))

Output:

2 a0 b0 a1 b1 a2 b2 a3 /*nil*/ a4 /*nil*/ a5 /*nil*/
<tuple(a0,b0)><tuple(a1,b1)><tuple(a2,b2)><tuple(a3,/*nil*/)><tuple(a4,/*nil*/)><tuple(a5,/*nil*/)>