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| [[CMake_User_Contributed_Macros|Back]]
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| Lists are an important component of most CMakeLists.txt files. Lists are built automatically from arguments to commands and macros. Thus, we often build lists in variables by calling SET with more than one variable argument. Surprisingly, there are very few CMake commands to handle lists. Here is a compilation of a few basic list manipulation commands.
| | This page has moved [https://gitlab.kitware.com/cmake/community/wikis/contrib/macros/ListOperations here]. |
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| '''Update:''' Since CMake 2.4 most of the issues discussed here can be handled using the native CMake LIST command. However, these were very useful macros for handling lists in previous versions and they still are if you would like to add LIST support to your CMakeLists.txt files and for some reason you can't update your CMake installation. As such, I have maintained the original content as is, and provided a section at the bottom that makes use of the new CMake LIST command to perform the same functionality as the CAR, CDR, LIST_LENGTH, and LIST_INDEX macros below. The LIST_CONTAINS macro is still a useful macro to have, which the current LIST command doesn't support.
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| == CAR and CDR ==
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| Our first operations we take from lisp, a programming language built entirely around lists. Two fundamental functions of lisp are car, which returns the first element of a list, and cdr, which returns a list minus the first element. Here we give implementations of car and cdr CMake macros. Although they are not as efficient as their counterparts in lisp, they can still be surprisingly useful.
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| <pre>
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| MACRO(CAR var)
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| SET(${var} ${ARGV1})
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| ENDMACRO(CAR)
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| MACRO(CDR var junk)
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| SET(${var} ${ARGN})
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| ENDMACRO(CDR)
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| </pre>
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| Here is a simple example of their use.
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| <pre>
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| SET(MYLIST hello world foo bar)
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| CAR(car ${MYLIST})
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| CDR(cdr ${MYLIST})
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| MESSAGE("car: ${car}")
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| MESSAGE("cdr: ${cdr}")
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| </pre>
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| The output of the command above is
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| <pre>
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| car: hello
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| cdr: world;foo;bar
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| </pre>
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| Note that the CAR and CDR macros (as well as the rest of the macros in this document) take advantage of CMake's calling convention of expanding lists into arguments. If you quote the list argument, you get a very different result. If you change the previous example to
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| <pre>
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| CAR(car "${MYLIST}")
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| CDR(cdr "${MYLIST}")
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| MESSAGE("car: ${car}")
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| MESSAGE("cdr: ${cdr}")
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| </pre>
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| you get
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| <pre>
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| car: hello;world;foo;bar
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| cdr:
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| </pre>
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| == LIST_LENGTH ==
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| Here is a macro to get the length of a list.
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| <pre>
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| MACRO(LIST_LENGTH var)
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| SET(entries)
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| FOREACH(e ${ARGN})
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| SET(entries "${entries}.")
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| ENDFOREACH(e)
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| STRING(LENGTH ${entries} ${var})
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| ENDMACRO(LIST_LENGTH)
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| </pre>
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| If you use the LIST_LENGTH macro as follows
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| <pre>
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| SET(MYLIST hello world foo bar)
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| LIST_LENGTH(length ${MYLIST})
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| MESSAGE("length: ${length}")
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| </pre>
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| you get this output:
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| <pre>
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| length: 4
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| </pre>
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| == LIST_INDEX ==
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| Sometimes you want to get a particular entry of a list. This macro lets you grab the <math>n^\mathrm{th}</math> entry in a list (indexed from 1).
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| <pre>
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| MACRO(LIST_INDEX var index)
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| SET(list . ${ARGN})
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| FOREACH(i RANGE 1 ${index})
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| CDR(list ${list})
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| ENDFOREACH(i)
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| CAR(${var} ${list})
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| ENDMACRO(LIST_INDEX)
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| </pre>
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| If you use LIST_INDEX with the following commands
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| <pre>
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| SET(MYLIST hello world foo bar)
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| LIST_INDEX(first 1 ${MYLIST})
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| LIST_INDEX(second 2 ${MYLIST})
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| LIST_INDEX(third 3 ${MYLIST})
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| LIST_INDEX(fourth 4 ${MYLIST})
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| MESSAGE("first: ${first}")
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| MESSAGE("second: ${second}")
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| MESSAGE("third: ${third}")
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| MESSAGE("fourth: ${fourth}")
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| </pre>
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| you get this output:
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| <pre>
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| first: hello
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| second: world
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| third: foo
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| fourth: bar
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| </pre>
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| == LIST_CONTAINS ==
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| Sometimes you just want to know if a list contains a particular entry.
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| <pre>
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| MACRO(LIST_CONTAINS var value)
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| SET(${var})
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| FOREACH (value2 ${ARGN})
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| IF (${value} STREQUAL ${value2})
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| SET(${var} TRUE)
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| ENDIF (${value} STREQUAL ${value2})
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| ENDFOREACH (value2)
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| ENDMACRO(LIST_CONTAINS)
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| </pre>
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| Here are some simple examples of using LIST_CONTAINS.
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| <pre>
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| SET(MYLIST hello world foo bar)
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| LIST_CONTAINS(contains foo ${MYLIST})
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| IF (contains)
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| MESSAGE("MYLIST contains foo")
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| ENDIF (contains)
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| LIST_CONTAINS(contains baz ${MYLIST})
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| IF (NOT contains)
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| MESSAGE("MYLIST does not contain baz")
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| ENDIF (NOT contains)
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| </pre>
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| The result of the previous examples is:
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| <pre>
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| MYLIST contains foo
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| MYLIST does not contain baz
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| </pre>
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| == Using CMake LIST Command ==
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| The following code is based on the macros presented above:
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| <pre>
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| SET(MYLIST hello world foo bar)
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| CAR(car ${MYLIST})
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| CDR(cdr ${MYLIST})
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| LIST_LENGTH(length ${MYLIST})
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| LIST_INDEX(first 1 ${MYLIST})
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| LIST_INDEX(second 2 ${MYLIST})
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| LIST_INDEX(third 3 ${MYLIST})
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| LIST_INDEX(fourth 4 ${MYLIST})
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| </pre>
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| The previous code could have been implemented using the CMake LIST command (CMake 2.4+) in the following way:
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| <pre>
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| SET(MYLIST hello world foo bar)
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| LIST(GET MYLIST 0 car)
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| SET(cdr ${MYLIST})
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| LIST(REMOVE_AT cdr 0)
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| LIST(LENGTH MYLIST length)
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| LIST(GET MYLIST 0 first)
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| LIST(GET MYLIST 1 second)
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| LIST(GET MYLIST 2 third)
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| LIST(GET MYLIST 3 fourth)
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| </pre>
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| The output of the variables in both cases is:
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| <pre>
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| MESSAGE("car: ${car}")
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| MESSAGE("cdr: ${cdr}")
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| MESSAGE("length: ${length}")
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| MESSAGE("first: ${first}")
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| MESSAGE("second: ${second}")
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| MESSAGE("third: ${third}")
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| MESSAGE("fourth: ${fourth}")
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| --
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| car: hello
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| cdr: world;foo;bar
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| length: 4
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| first: hello
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| second: world
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| third: foo
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| fourth: bar
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| </pre>
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| Note that the CMake LIST command also provides additional flexibility, such as APPEND, INSERT, SORT, REVERSE, etc. Type
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| <pre>
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| cmake --help-command LIST
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| </pre>
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| for the current usage information.
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| -----
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| [[CMake_User_Contributed_Macros|Back]]
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| {{CMake/Template/Footer}}
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| [[Category:CMakeMacro]]
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