Detect optional function argument (scalar)

argumentsawkfunction

Consider this script:

function alfa(bravo, charlie) {
  if (charlie)
    return "charlie good"
  else {
    return "charlie bad"
  }
}
BEGIN {
  print alfa(1, 1)
  print alfa(1, 0)
  print alfa(1, "")
  print alfa(1)
}

Result:

charlie good
charlie bad
charlie bad
charlie bad

Does Awk have a way to tell when an argument has not been provided?

Best Answer

Yes, you can do this:

function alfa(bravo, charlie) {  
  if (charlie) {
    return "charlie good"
  }
  if (charlie == 0 && charlie == "") {
    return "charlie not provided"
  }
  if (!charlie && charlie != 0) {
    return "charlie null"
  }
  if (!charlie && charlie != "") {
    return "charlie 0"
  }
}

Result:

charlie good
charlie 0
charlie null
charlie not provided

Related Solutions

Gawk: Passing arrays to functions

Function parameters are local to the function.

awk '
    function foo(x,y) {y=x*x; print "y in function: "y} 
    BEGIN {foo(2); print "y out of function: " y}
'

y in function: 4
y out of function:

If you pass fewer values to a function than there are parameters, the extra parameters are just empty. You might sometimes see functions defined like

function foo(a, b, c            d, e, f) {...

where the parameters after the whitespace are local variables and are not intended to take a value at invocation.

No reason why this can't work for local arrays:

awk '
    function bar(x) {
        split("hello world", x)
        print "in: " x[1]
    }
    BEGIN {
        x[1]="world"
        bar()
        print "out: " x[1]}
'

in: hello
out: world

BEGIN and END with the awk command

The BEGIN isn't superfluous. If you don't specify BEGIN then the print would be executed for every line of input.

Quoting from the manual:

A BEGIN rule is executed once only, before the first input record is read. Likewise, an END rule is executed once only, after all the input is read.

$ seq 5 | awk 'BEGIN{print "Hello"}/4/{print}'   # Hello printed once
Hello
4
$ seq 5 | awk '{print "Hello"}/4/{print}'        # Hello printed for each line of input
Hello
Hello
Hello
Hello
4
Hello
$

Best Answer

Related Solutions

Gawk: Passing arrays to functions

BEGIN and END with the awk command

Related Question