Hacker News Reader

Comment by dnet | Hacker News Reader

Removing leading zeroes doesn't require Python. One easy solution would be sed:

    $ echo -e '0001\n0010\n0002' | sed 's/^0*//'
    1
    10
    2

View on HN

Yeah plus seq can generate sequences with leading zeroes (something like seq -f %04.f 1 20).

So instead of scripting, he could have generated a sorted list of numbers from the files he had. Created a file with the sequence of numbers for the range and diffed/commed the whole thing. Voilà...

  seq -f %04g 0 3
  echo {0000..0003} # if bash

Did not know that. Thanks! That's much easier.

The seq provided in the GNU toolset has a -w flag to turn on "equal width" mode, so one can also get zero padded numbers out (from GNU seq) by turning on that mode and zero padding the input:

    $ seq -w 0001 0003
    0001
    0002
    0003
    $

Nice, this works automatically like this:

    $ seq -w 98 102
    098
    099
    100
    101
    102

Yes. Or if you're going to whip out Python, might as well make it all in Python.

Very much my thinking, especially when there are no significant commands you need to shell out to:

    import sys, pathlib
    basedir = pathlib.Path(sys.argv[1])
    for i in range(1, 501):
        if not (basedir / f'{i:04}_A.csv').is_file():
            print(i)

Nice one!

Just for fun some bash:

    for k in $(seq -f %04.f 1 501); do 
        f="${k}_A.csv"
        ! [[ -f "$f" ]] && echo $f || :
    done

or more succinctly,

    for k in {0001..0501}_A.csv; do
        ! [[ -f "$f" ]] && echo $f || :
    done

and if you have GNU parallel installed:

    parallel -kj1 '! [[ -f "{}" ]] && echo {} || :' ::: {0001..0501}_A.csv

    $ echo -e '0001\n0010\n0002\n0' | bc
    1
    10
    2
    0

This is better relative to the `sed` solution as it handles the `0000` case well.

If one of the items is actually zero, this would delete it entirely, which probably isn't the desired result.

Yeah, no need for Python. Even the following seems to work fine:

$ printf "%d\n" 003

Well, that works up to a point. Fore some, that point might be considered a bit too close to zero...

    $ printf "%d\n" 009
    sh: 1: printf: 009: not completely converted
    0

0-prefix octal notation continues to be a mistake.

I love when programs mysteriously fail when passed an IP address like 192.168.001.009.

Thankfully modern tools have been moving away from supporting that notation and are less likely to explode all over unsuspecting users.

Yes, you are right. This seems to work better:

$ printf "%1.0f\n" 009

$ printf "%1.0f\n" 00125990

125990

  $ printf %d 010
  8

Given the situation in the article, you might as well do this:

  ls ????_A.csv | grep -o '[1-9][0-9]*'

Oh nice. sed still scares me a bit.

It’s useful but highly cryptic in my usage.

I’m not terribly experienced with Unix tools but I reckon that it might be best to just use Perl instead. Then you just have to worry about PCRE instead of PCRE in addition to old-style regexps.

Then again, Perl is even scarier.

perl one-liners are pretty powerful and can replace awk/sed/cut/tr/etc. That being said, then you have to remember which command-line options you should give perl (was it -lane to do X, or was it -pie, or something else entirely?).

But yeah, the perl rabbit hole goes as deep as you want to, and in some sense it makes it more difficult to say "screw it, I'm redoing this in a real language" as complexity rises. And then you end up with a thousand lines of line noise.

I haven’t coded Perl beyond one-liners. What attracts me to it are its regular expressions. So much of Unix scripting seems to involve regexps. So I figure Perl+utilities is the better option compared to utilities+Bash.

I wouldn’t wanna use it for more than short scripts. Perl 6 might be fun, but it doesn’t seem to have a large enough community.

If you want to improve your coding skills you should read “Higher Order Perl”. (made available for free online by the author)

If you want to improve your Perl code read “Modern Perl”. (There is more than one version, and I know the first version was made freely available online)

Perl is a better language for large codebases than most people give it credit for. That said it allows more creativity when it comes to your code. So you can make awful code just as easily as beautiful code. Perl6 makes the beautiful code easier to write and shorter, while making awful code a bit harder to write.

> There is more than one version, and I know the first version was made freely available online

Good news! Every version is freely available online. Here's the most recent:

http://modernperlbooks.com/books/modern_perl_2016/index.html

Well Perl is a real language, so of course it makes it more difficult to say “screw it, I'm redoing this in a real language”.

That said the Perl that you write in a one-liner should be different than the Perl that you write when the complexity rises.

If you have more than a few hundred lines, you probably need to move some of it out into a module. There should also be tests for that module. It might even make sense to structure it like a CPAN module so that you can use the existing tools to test and install it.

I have to admit the last time I wrote anything larger than short scripts in perl was in the perl 4 period (on HP-UX to boot). I eagerly awaited the perl 5 version of the Camel book, but in the end I jumped ship to python before getting seriously into perl 5.

Definitely check out perl one liner patterns. Perl is less scary and more powerful and usually almost as short as sed commands. Perl can often replace pipelines that use both sed and awk. Perl one liners need judicious flags though, common patterns look like perl -ne, perl -pie, perl -lane ... these do very different things. Once you know them, it’s like a minor superpower.

Reply on news.ycombinator.com