4.21 Loop Statements

The loop statement allows for repeated execution of a block of code, controlled by some conditional expression. It has the following form:

loop [label]
     [for stmt1] [,while expr1] [,stmt2]
do
  stmt3
done [while expr2]

where stmt1, stmt2, and stmt3 are statement lists, expr1 and expr2 are expressions.

The control flow is as follows:

1. If stmt1 is specified, execute it.
2. Evaluate expr1. If it is zero, go to 6. Otherwise, continue.
3. Execute stmt3.
4. If stmt2 is supplied, execute it.
5. If expr2 is given, evaluate it. If it is zero, go to 6. Otherwise, go to 2.
6. End.

Thus, stmt3 is executed until either expr1 or expr2 yield a zero value.

The loop body – stmt3 – can contain special statements:

break [label]

Terminates the loop immediately. Control passes to ‘6’ (End) in the formal definition above. If label is supplied, the statement terminates the loop statement marked with that label. This allows to break from nested loops.

It is similar to break statement in C or shell.

next [label]

Initiates next iteration of the loop. Control passes to ‘4’ in the formal definition above. If label is supplied, the statement starts next iteration of the loop statement marked with that label. This allows to request next iteration of an upper-level loop from a nested loop statement.

The loop statement can be used to create iterative statements of arbitrary complexity. Let’s illustrate it in comparison with C.

The statement:

loop
do
  stmt-list
done

creates an infinite loop. The only way to exit from such a loop is to call break (or return, if used within a function), somewhere in stmt-list.

The following statement is equivalent to while (expr1) stmt-list in C:

loop while expr
do
  stmt-list
done

The C construct for (expr1; expr2; expr3) is written in MFL as follows:

loop for stmt1, while expr2, stmt2
do
  stmt3
done

For example, to repeat stmt3 10 times:

loop for set i 0, while i < 10, set i i + 1
do
  stmt3
done

Finally, the C ‘do’ loop is implemented as follows:

loop
do
  stmt-list
done while expr

As a real-life example of a loop statement, let’s consider the implementation of function ptr_validate, which takes a single argument ipstr, and checks its validity using the following algorithm:

Perform a DNS reverse-mapping for ipstr, looking up the corresponding PTR record in ‘in-addr.arpa’. For each record returned, look up its IP addresses (A records). If ipstr is among the returned IP addresses, return 1 (true), otherwise return 0 (false).

The implementation of this function in MFL is:

#pragma regex push +extended

func ptr_validate(string ipstr) returns number
do
  loop for string names dns_getname(ipstr) . " "
           number i index(names, " "),
       while i != -1,
       set names substr(names, i + 1)
       set i index(names, " ")
  do
    loop for string addrs dns_getaddr(substr(names, 0, i)) . " "
             number j index(addrs, " "),
         while j != -1,
         set addrs substr(addrs, j + 1)
         set j index(addrs, " ")
    do
      if ipstr == substr(addrs, 0, j)
        return 1
      fi
    done
  done
  return 0
done