A2P(1) Perl Programmers Reference Guide A2P(1)NAMEa2p - Awk to Perl translator
SYNOPSISa2p [options] filename
DESCRIPTION
A2p takes an awk script specified on the command line (or
from standard input) and produces a comparable perl script
on the standard output.
Options
Options include:
-D<number>
sets debugging flags.
-F<character>
tells a2p that this awk script is always invoked with
this -F switch.
-n<fieldlist>
specifies the names of the input fields if input does
not have to be split into an array. If you were
translating an awk script that processes the password
file, you might say:
a2p-7 -nlogin.password.uid.gid.gcos.shell.home
Any delimiter can be used to separate the field
names.
-<number>
causes a2p to assume that input will always have that
many fields.
-o tells a2p to use old awk behavior. The only current
differences are:
Old awk always has a line loop, even if there
are no line actions, whereas new awk does not.
In old awk, sprintf is extremely greedy about
its arguments. For example, given the statement
print sprintf(some_args), extra_args;
old awk considers extra_args to be arguments to
"sprintf"; new awk considers them arguments to
"print".
"Considerations"
A2p cannot do as good a job translating as a human
would, but it usually does pretty well. There are
some areas where you may want to examine the perl
script produced and tweak it some. Here are some of
them, in no particular order.
There is an awk idiom of putting int() around a
string expression to force numeric interpretation,
even though the argument is always integer anyway.
This is generally unneeded in perl, but a2p can't
tell if the argument is always going to be integer,
so it leaves it in. You may wish to remove it.
Perl differentiates numeric comparison from string
comparison. Awk has one operator for both that
decides at run time which comparison to do. A2p does
not try to do a complete job of awk emulation at this
point. Instead it guesses which one you want. It's
almost always right, but it can be spoofed. All such
guesses are marked with the comment ""#???"". You
should go through and check them. You might want to
run at least once with the -w switch to perl, which
will warn you if you use == where you should have
used eq.
Perl does not attempt to emulate the behavior of awk
in which nonexistent array elements spring into exis
tence simply by being referenced. If somehow you are
relying on this mechanism to create null entries for
a subsequent for...in, they won't be there in perl.
If a2p makes a split line that assigns to a list of
variables that looks like (Fld1, Fld2, Fld3...) you
may want to rerun a2p using the -n option mentioned
above. This will let you name the fields throughout
the script. If it splits to an array instead, the
script is probably referring to the number of fields
somewhere.
The exit statement in awk doesn't necessarily exit;
it goes to the END block if there is one. Awk
scripts that do contortions within the END block to
bypass the block under such circumstances can be sim
plified by removing the conditional in the END block
and just exiting directly from the perl script.
Perl has two kinds of array, numerically-indexed and
associative. Perl associative arrays are called
"hashes". Awk arrays are usually translated to
hashes, but if you happen to know that the index is
always going to be numeric you could change the {...}
to [...]. Iteration over a hash is done using the
keys() function, but iteration over an array is NOT.
You might need to modify any loop that iterates over
such an array.
Awk starts by assuming OFMT has the value %.6g. Perl
starts by assuming its equivalent, $#, to have the
value %.20g. You'll want to set $# explicitly if you
use the default value of OFMT.
Near the top of the line loop will be the split oper
ation that is implicit in the awk script. There are
times when you can move this down past some condi
tionals that test the entire record so that the split
is not done as often.
For aesthetic reasons you may wish to change the
array base $[ from 1 back to perl's default of 0, but
remember to change all array subscripts AND all sub_
str() and index() operations to match.
Cute comments that say "# Here is a workaround
because awk is dumb" are passed through unmodified.
Awk scripts are often embedded in a shell script that
pipes stuff into and out of awk. Often the shell
script wrapper can be incorporated into the perl
script, since perl can start up pipes into and out of
itself, and can do other things that awk can't do by
itself.
Scripts that refer to the special variables RSTART
and RLENGTH can often be simplified by referring to
the variables $`, $& and $', as long as they are
within the scope of the pattern match that sets them.
The produced perl script may have subroutines defined
to deal with awk's semantics regarding getline and
print. Since a2p usually picks correctness over
efficiency. it is almost always possible to rewrite
such code to be more efficient by discarding the
semantic sugar.
For efficiency, you may wish to remove the keyword
from any return statement that is the last statement
executed in a subroutine. A2p catches the most com
mon case, but doesn't analyze embedded blocks for
subtler cases.
ARGV[0] translates to $ARGV0, but ARGV[n] translates
to $ARGV[$n]. A loop that tries to iterate over
ARGV[0] won't find it.
ENVIRONMENT
A2p uses no environment variables.
AUTHOR
Larry Wall <larry@wall.org>
FILESSEE ALSO
perl The perl compiler/interpreter
s2p sed to perl translator
DIAGNOSTICSBUGS
It would be possible to emulate awk's behavior in select
ing string versus numeric operations at run time by
inspection of the operands, but it would be gross and
inefficient. Besides, a2p almost always guesses right.
Storage for the awk syntax tree is currently static, and
can run out.
2002-06-25 perl v5.6.1 A2P(1)
