Please explain awk command

Hi,

a few days ago Ed Morton gave hints selecting lines from a file.
Can anyone explain me the following:

e) Print the N records after some pattern:

awk 'c&&c--;/pattern/{c=N}' file

I checked it and it worked fine, but I don't know exactly how it works.
For example what's the meaning of c&&c-- or for the first three loops
what are the assignments for c or why needn't I a print-statement to print
the output lines?

This is my input file:

MARKER
row1
row2
row3
row4
MARKER
row5
row6
row7
MARKER
....

pattern=MARKER
N=2

Output:
row1
row2
row5
row6

Thanks
Heinz

Re: Please explain awk command

Heinz Müller wrote:
> Hi,
>
> a few days ago Ed Morton gave hints selecting lines from a file.
> Can anyone explain me the following:
>
> e) Print the N records after some pattern:
>
> awk 'c&&c--;/pattern/{c=N}' file
>
> I checked it and it worked fine, but I don't know exactly how it works.
> For example what's the meaning of c&&c-- or for the first three loops
> what are the assignments for c or why needn't I a print-statement to print
> the output lines?
>
....

You can write

c&&c--;

as

c!=0&&c--!=0

or

c!=0{c--;print}

When the expression is true and {} is omitted,
print is the default action.


--
Michael Tosch @ hp : com

Re: Please explain awk command

On Mon, 27 Aug 2007 19:47:08 +0200, Heinz Müller
wrote:
>
>
> Hi,
>
> a few days ago Ed Morton gave hints selecting lines from a file.
> Can anyone explain me the following:
>
> e) Print the N records after some pattern:
>
> awk 'c&&c--;/pattern/{c=N}' file
>
> I checked it and it worked fine, but I don't know exactly how it works.
> For example what's the meaning of c&&c-- or for the first three loops
> what are the assignments for c or why needn't I a print-statement to print
> the output lines?
>
"c&&c--" if c != 0, subtract 1 from c
The semicolon, outside braces, terminates a pattern-action pair. A null
action is equivalent to {print}.


--
"Show business is just like high school, except you get paid."
-- Martin Mull

Re: Please explain awk command

"Bill Marcum" schrieb im Newsbeitrag
news:r8ibq4-ihq.ln1@don.localnet...
>>
> "c&&c--" if c != 0, subtract 1 from c
> The semicolon, outside braces, terminates a pattern-action pair. A null
> action is equivalent to {print}.

So,

one can write:

awk 'c&&c--{ print $0 };
/pattern/{c=N}' file

instead?

Heinz

Re: Please explain awk command

Heinz Müller wrote:
> "Bill Marcum" schrieb im Newsbeitrag
> news:r8ibq4-ihq.ln1@don.localnet...
>> "c&&c--" if c != 0, subtract 1 from c
>> The semicolon, outside braces, terminates a pattern-action pair. A null
>> action is equivalent to {print}.
>
> So,
>
> one can write:
>
> awk 'c&&c--{ print $0 };
> /pattern/{c=N}' file
>
> instead?
>
> Heinz
>
>
>

Yes, and with an explicit {action}
you can even use an explicit if():

awk '{if(c&&c--){print $0}}
{if(/pattern/){c=N}}'

--
Michael Tosch @ hp : com

Re: Please explain awk command

Heinz Müller wrote:
> "Bill Marcum" schrieb im Newsbeitrag
> news:r8ibq4-ihq.ln1@don.localnet...
>
>>"c&&c--" if c != 0, subtract 1 from c
>>The semicolon, outside braces, terminates a pattern-action pair. A null
>>action is equivalent to {print}.
>
>
> So,
>
> one can write:
>
> awk 'c&&c--{ print $0 };
> /pattern/{c=N}' file
>
> instead?

Not quite. It'd be (semicolon removed):

awk 'c&&c--{ print $0 }
/pattern/{c=N}' file

if you wanted to redundantly specify the default action.

Ed.