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foreach
Function
The foreach
function is very different from other functions. It
causes one piece of text to be used repeatedly, each time with a different
substitution performed on it. It resembles the for
command in the
shell sh
and the foreach
command in the C-shell csh
.
The syntax of the foreach
function is:
$(foreach var,list,text) |
The first two arguments, var and list, are expanded before anything else is done; note that the last argument, text, is not expanded at the same time. Then for each word of the expanded value of list, the variable named by the expanded value of var is set to that word, and text is expanded. Presumably text contains references to that variable, so its expansion will be different each time.
The result is that text is expanded as many times as there are
whitespace-separated words in list. The multiple expansions of
text are concatenated, with spaces between them, to make the result
of foreach
.
This simple example sets the variable `files' to the list of all files in the directories in the list `dirs':
dirs := a b c d files := $(foreach dir,$(dirs),$(wildcard $(dir)/*)) |
Here text is `$(wildcard $(dir)/*)'. The first repetition
finds the value `a' for dir
, so it produces the same result
as `$(wildcard a/*)'; the second repetition produces the result
of `$(wildcard b/*)'; and the third, that of `$(wildcard c/*)'.
This example has the same result (except for setting `dirs') as the following example:
files := $(wildcard a/* b/* c/* d/*) |
When text is complicated, you can improve readability by giving it a name, with an additional variable:
find_files = $(wildcard $(dir)/*) dirs := a b c d files := $(foreach dir,$(dirs),$(find_files)) |
Here we use the variable find_files
this way. We use plain `='
to define a recursively-expanding variable, so that its value contains an
actual function call to be reexpanded under the control of foreach
;
a simply-expanded variable would not do, since wildcard
would be
called only once at the time of defining find_files
.
The foreach
function has no permanent effect on the variable
var; its value and flavor after the foreach
function call are
the same as they were beforehand. The other values which are taken from
list are in effect only temporarily, during the execution of
foreach
. The variable var is a simply-expanded variable
during the execution of foreach
. If var was undefined
before the foreach
function call, it is undefined after the call.
See section The Two Flavors of Variables.
You must take care when using complex variable expressions that result in variable names because many strange things are valid variable names, but are probably not what you intended. For example,
files := $(foreach Esta escrito en espanol!,b c ch,$(find_files)) |
might be useful if the value of find_files
references the variable
whose name is `Esta escrito en espanol!' (es un nombre bastante largo,
no?), but it is more likely to be a mistake.
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