AWK - execute string as command? - awk

Question

This command prints:
$ echo "123456789" | awk '{ print substr ($1,1,4) }'
1234
Is it possible to execute a string as command? For example, this command:
echo "123456789" | awk '{a="substr"; print a ($1,1,4) }'
Result:
$ echo "123456789" | awk '{a="substr"; print a ($1,1,4) }'
awk: {a="substr"; print a ($1,1,4) }
awk: ^ syntax error
EDIT:
$ cat tst.awk
function my_substr(x,y,z) { return substr(x,y,z) }
{ a="my_substr"; print #a($1,1,4) }
bolek#bolek-desktop:~/Pulpit$ echo "123456789" | gawk -f tst.awk
gawk: tst.awk:3: { a="my_substr"; print #a($1,1,4) }
gawk: tst.awk:3: ^ nieprawidłowy znak '#' w wyrażeniu
bolek#bolek-desktop:~/Pulpit$

Answer 1

I don't think it is possible to do that in awk directly, but you can get a similar effect by using the shell. Recall that the awk program is given as a string, and strings are concatenated in the shell just by writing them next to one another. Thus, you can do this:
a=substr
echo "123456789" | awk '{ print '"$a"'($1, 1, 4) }'
resulting in 1234.

Answer 2

You can call user-defined functions via variables in GNU awk using indirect function calls, see http://www.gnu.org/software/gawk/manual/gawk.html#Indirect-Calls
$ cat tst.awk
function foo() { print "foo() called" }
function bar() { print "bar() called" }
BEGIN {
the_func = "foo"
#the_func()
the_func = "bar"
#the_func()
}
$ gawk -f tst.awk
foo() called
bar() called
Unfortunately due to internal implementation issues, if you want to call builtin functions that way then you need to write a wrapper for each:
$ cat tst.awk
function my_substr(x,y,z) { return substr(x,y,z) }
{ a="my_substr"; print #a($1,1,4) }
$ echo "123456789" | gawk -f tst.awk
1234