Re: "sed" question

On 12.03.2024 23:49, Ed Morton wrote:

On 3/9/2024 2:07 PM, Janis Papanagnou wrote:

On 09.03.2024 17:52, Ed Morton wrote:

>
About 20 or so years ago we had a discussion in this NG (which I'm not
going to search for now) and, shockingly, a consensus was reached that
we should encourage people to always write:
>
     '{$2="1-1"} 1'
>

>
I don't recall such a "consensus".

 
I do, I have no reason to lie about it, but I can't be bothered
searching through 20-year-old usenet archives for it (I did take a very
quick shot at it but I don't even know how to write a good search for it
- you can't just google "awk '1'" and I'm not even sure if it was in
comp.lang.awk or comp.unix.shell).

I didn't say anything about "lying"; why do you insinuate so?

But your memory may mislead you. (Or mine, or Kaz', of course.)

(And no, I don't do the search for you; since you have been the
one contending something here.)

Without a reference such a statement is just void (and not more
than a rhetorical move).

You should at least elaborate on the details and facts of that
"consensus" - but for the _specific OP context_ (not for made
up cases).

 

If you want to avoid cryptic code you'd rather write
>
       '{$2="1-1"; print}'
>
Don't you think?

 
 
If I'm writing a multi-line script I use an explicit `print` but it just
doesn't matter for a tiny one-line script like that.

Actually, for the given case, the yet better solution is what the
OP himself said (in CUS, where his question was initially posted):

  Grant Taylor on alt.comp.software.thunderbird suggested [...]:
  $ awk '{print $1, "1-1"}'

Since this suggestion doesn't overwrite fields and is conceptually
clear. It inherently also handles (possible?) cases where there's
more than two fields in the data (e.g. by spurious blanks).

Everyone using awk
needs to know the `1` idiom as it's so common and once you've seen it
once it's not hard to figure out what `{$2="1-1"} 1` does.

The point is that  $2="1-1"  as condition is also an Awk idiom.

 
By changing `condition` to `{condition}1` we just add 3 chars to remove
the guesswork from anyone reading it in future and protect against
unconsidered values so we don't just make it less cryptic but also less
fragile.

Your examples below are meaningless since you make up cases that have
nothing to do with the situation here, and especially in context of
my posting saying clearly: "In this specific case of regular data".

The more problematic issue is that  $2="1-1"  and also  {$2="1-1"}
both overwrite fields and thus a reorganization of the fields is
done which has - probably unexpected by a newbie coder - side effects.

But YMMV, of course.

Janis

 
For example, lets say someone wants to copy the $1 value into $3 and
print every line:
 
$ printf '1 2 3\n4 5 7\n' | awk '{$3=$1}1'
1 2 1
4 5 4
 
$ printf '1 2 3\n0 5 7\n' | awk '{$3=$1}1'
1 2 1
0 5 0
 
$ printf '1 2 3\n4 5 7\n' | awk '$3=$1'
1 2 1
4 5 4
 
$ printf '1 2 3\n0 5 7\n' | awk '$3=$1'
1 2 1
 
Note the 2nd line is undesirably (because I wrote the requirements)
missing from that last output.
 
It happens ALL the time that people don't consider all possible input
values so it's safer to just write the code that reflects your intent
and if you intend for every line to be printed then write code that will
print every line.
 
    Ed.
 

>
And of course add more measures in case the data is not as regular as
the sample data suggests. (See my other postings what may be defined
as data, line missing or spurious blanks in the data, comment lines
or empty lines that have to be preserved, etc.)
>

instead of:
>
     $2="1-1"
>

>
Janis
>