From Newsgroup: comp.lang.awk

On 07.11.2023 12:48, Ed Morton wrote:

On 11/6/2023 5:14 AM, Janis Papanagnou wrote:

switch (some_variable) [
case 42: ...
case "string": ...
case /pattern/: ...

Nitpick - it's

And I expected the nitpick on my typo using '=' (instead of '==') in
the 'if' comparison. ;-)

case /regexp/:

rather than:

case /pattern/

The word "pattern" is ambiguous and misused all over awk documentation.

You are right that the historic (and, sadly, surviving) documentation
speaks about 'pattern' and '/regexp'/'.

You can make some argument for it in:

pattern { action }

since that includes `BEGIN`, integers, etc. I'd argue that should be:

condition { action }

Decades ago I was the first one suggesting the term 'condition' here so
you certainly don't need to teach me. (Myself I was using that term in
my Awk courses even since the 1990's.)

but in the "case" statement above what goes inside `/.../` is simply and always a regexp.

Yes. Sorry for my sloppiness here. Thanks for the nitpick.

[...]

One point you may want to consider is the trim() function; the
two substitutions can be combined in one

sub (/^[[:space:]]+(.*)[[:space:]]+$/, "&", str)

(but test that in your Awk versions before using it; "&" is an
old feature but off the top of my head I'm not sure whether the
subexpression with parenthesis /...(...).../ is generally
supported in other Awks).

You can write that, but it's not a capture group that can be
backreferenced from the replacement and if it was "&" wouldn't refer to
the string that matched ".*" anyway, it'd refer to the string that
matched the whole regexp.

Using my Awk it does what advertised. I merely didn't find it clearly documented whether the '&' is generally guaranteed to refer to the
grouping.

You could use a capture group in GNU awk for gensub():

I deliberately abstained from gensub() here since the OP avoids GNU
Awk specifics.

str = gensub (/^[[:space:]]+(.*)[[:space:]]+$/, "\\1", 1, str)

and in most awks you could do:

gsub (/^[[:space:]]+|[[:space:]]+$/, "", str)

Yes, this is a sensible alternative.

but there are some out there that will fail to do both substitutions for
that case (tawk and nawk maybe?) and so you need:

Really? But why? - Alternatives in regexp is certainly an old feature
(at least since nawk in the 1980's).

sub(/^[[:space:]]+/, "", str)
sub(/[[:space:]]+$/, "", str)
[...]

Janis

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From Newsgroup: comp.lang.awk

On 07.11.2023 14:02, Janis Papanagnou wrote:

One point you may want to consider is the trim() function; the
two substitutions can be combined in one

sub (/^[[:space:]]+(.*)[[:space:]]+$/, "&", str)

(but test that in your Awk versions before using it; "&" is an
old feature but off the top of my head I'm not sure whether the
subexpression with parenthesis /...(...).../ is generally
supported in other Awks).

You can write that, but it's not a capture group that can be
backreferenced from the replacement and if it was "&" wouldn't refer to
the string that matched ".*" anyway, it'd refer to the string that
matched the whole regexp.

Using my Awk it does what advertised. I merely didn't find it clearly documented whether the '&' is generally guaranteed to refer to the
grouping.

I stand corrected, I had a wrong test case! - "&" does _not_ work
on grouping, and as you said (and as it is specified), it generally
defines the whole match. (It wouldn't make sense otherwise.) - Thanks!

Janis

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From Newsgroup: comp.lang.awk

On 2023-11-06, Mike Sanders <porkchop@invalid.foo> wrote:

Ping Janis: Question... Are you interested in rewriting this
as a Gawk only implementation? Would be great for switch/case
statements IMO. If so, I'll add your version to the file at
my website.

The cppawk preprocessor supports a case macro which
compiles to the switch statement for Gawk, or to portable
Awk code.

The macro is documented in its own man page:

https://www.kylheku.com/cgit/cppawk/tree/cppawk-case.1

case is safer than switch because it doesn't have implicit
"fallthrough".

Each case must end with one of: cbreak, cfall or cret: break,
fallthrough or return.
--
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @Kazinator@mstdn.ca
NOTE: If you use Google Groups, I don't see you, unless you're whitelisted.
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From Newsgroup: comp.lang.awk

Janis Papanagnou <janis_papanagnou+ng@hotmail.com> wrote:

[*] https://www.gnu.org/software/gawk/manual/gawk.html#Switch-Statement

Will read/study (as always thanks Janis!)
--
:wq
Mike Sanders

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From Newsgroup: comp.lang.awk

Kaz Kylheku <864-117-4973@kylheku.com> wrote:

The cppawk preprocessor supports a case macro which
compiles to the switch statement for Gawk, or to portable
Awk code.

The macro is documented in its own man page:

https://www.kylheku.com/cgit/cppawk/tree/cppawk-case.1

case is safer than switch because it doesn't have implicit
"fallthrough".

Each case must end with one of: cbreak, cfall or cret: break,
fallthrough or return.

Hey-hey Kaz.

That's really nifty in fact. I might try my hand at a cppawk
project just to familiarizes myself with its workings.

Thanks & mucho appreciate the head's up kind sir, must study
more about it all.
--
:wq
Mike Sanders

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From Newsgroup: comp.lang.awk

On 11/7/2023 7:02 AM, Janis Papanagnou wrote:

On 07.11.2023 12:48, Ed Morton wrote:

<snip>

and in most awks you could do:

gsub (/^[[:space:]]+|[[:space:]]+$/, "", str)

Yes, this is a sensible alternative.

but there are some out there that will fail to do both substitutions for
that case (tawk and nawk maybe?) and so you need:

Really? But why? - Alternatives in regexp is certainly an old feature
(at least since nawk in the 1980's).

In my opinion it's just a bug. It was demonstrated to me when I posted
an answer on Stack Overflow several years ago that I can't find right
now. I know it's not gawk and I'm about 99% sure it's neither BSD awk
nor /usr/xpg[46]/bin/awk so the only non-oawk awks I can imagine would
have this problem are nawk, tawk (I'm about 80% sure I remember tawk is
one that DOES have the problem), and/or busybox awk, none of which I
have access to, so if anyone has and could test them by running:

$ echo ' foo ' | awk '{gsub(/^ +| +$/,""); print "<" $0 ">"}'
<foo>

and let us know which don't produce that output, that'd be great.

Ed.

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From Newsgroup: comp.lang.awk

On 11/8/2023 4:22 AM, Ed Morton wrote:

On 11/7/2023 7:02 AM, Janis Papanagnou wrote:

On 07.11.2023 12:48, Ed Morton wrote:

<snip>

and in most awks you could do:

    gsub (/^[[:space:]]+|[[:space:]]+$/, "", str)

Yes, this is a sensible alternative.

but there are some out there that will fail to do both substitutions for >>> that case (tawk and nawk maybe?) and so you need:

Really? But why? - Alternatives in regexp is certainly an old feature
(at least since nawk in the 1980's).

In my opinion it's just a bug. It was demonstrated to me when I posted
an answer on Stack Overflow several years ago that I can't find right
now. I know it's not gawk and I'm about 99% sure it's neither BSD awk
nor /usr/xpg[46]/bin/awk so the only non-oawk awks I can imagine would
have this problem are nawk, tawk (I'm about 80% sure I remember tawk is
one that DOES have the problem), and/or busybox awk, none of which I
have access to, so if anyone has and could test them by running:

$ echo ' foo ' | awk '{gsub(/^ +| +$/,""); print "<" $0 ">"}'    <foo>

and let us know which don't produce that output, that'd be great.

    Ed.

I found a different answer I had posted (also years ago) that mentions
this bug and there I say it's tawk and mawk1 where it occurs. Still
can't find the original where I was told about it (and it was in a
discussion in comments that's probably been removed by now) but that
should be good enough for others to reproduce it.

The specific case was removing quotes from around a field in a CSV:

$ printf '"foo"' | awk '{gsub(/^"+|"+$/,""); print "<" $0 ">"}'
<foo>

but I doubt if that detail matters.

Regards,

Ed.
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