Variable Scope (The GNU Awk User’s Guide)

9.2.3.2 Controlling Variable Scope

Unlike in many languages, there is no way to make a variable local to a { … } block in awk, but you can make a variable local to a function. It is good practice to do so whenever a variable is needed only in that function.

To make a variable local to a function, simply declare the variable as an argument after the actual function arguments (see section Function Definition Syntax). Look at the following example, where variable i is a global variable used by both functions foo() and bar():

function bar()
{
    for (i = 0; i < 3; i++)
        print "bar's i=" i
}

function foo(j)
{
    i = j + 1
    print "foo's i=" i
    bar()
    print "foo's i=" i
}

BEGIN {
      i = 10
      print "top's i=" i
      foo(0)
      print "top's i=" i
}

Running this script produces the following, because the i in functions foo() and bar() and at the top level refer to the same variable instance:

top's i=10
foo's i=1
bar's i=0
bar's i=1
bar's i=2
foo's i=3
top's i=3

If you want i to be local to both foo() and bar(), do as follows (the extra space before i is a coding convention to indicate that i is a local variable, not an argument):

function bar(    i)
{
    for (i = 0; i < 3; i++)
        print "bar's i=" i
}

function foo(j,    i)
{
    i = j + 1
    print "foo's i=" i
    bar()
    print "foo's i=" i
}

BEGIN {
      i = 10
      print "top's i=" i
      foo(0)
      print "top's i=" i
}

Running the corrected script produces the following:

top's i=10
foo's i=1
bar's i=0
bar's i=1
bar's i=2
foo's i=1
top's i=10

Besides scalar values (strings and numbers), you may also have local arrays. By using a parameter name as an array, awk treats it as an array, and it is local to the function. In addition, recursive calls create new arrays. Consider this example:

function some_func(p1,      a)
{
    if (p1++ > 3)
        return


    a[p1] = p1

    some_func(p1)

    printf("At level %d, index %d %s found in a\n",
         p1, (p1 - 1), (p1 - 1) in a ? "is" : "is not")
    printf("At level %d, index %d %s found in a\n",
         p1, p1, p1 in a ? "is" : "is not")
    print ""
}

BEGIN {
    some_func(1)
}

When run, this program produces the following output:

At level 4, index 3 is not found in a
At level 4, index 4 is found in a

At level 3, index 2 is not found in a
At level 3, index 3 is found in a

At level 2, index 1 is not found in a
At level 2, index 2 is found in a