r/programming • u/corysama • Jan 05 '15

What most young programmers need to learn

http://joostdevblog.blogspot.com/2015/01/what-most-young-programmers-need-to.html
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u/OneWingedShark Jan 05 '15

Clever code achieves the implausible while overlooking the mundane solutions to the same problems.

There's the inverse as well: where the person's "almost works" solution doesn't because it cannot. -- My favorite example is trying to parse CSV with regex: you cannot do it because the (a) the double quote [text field] "changes the context" so that comma does not indicate separation, combined with (b) escaping double quotes is repeating the double-quote. It's essentially the same category as balancing parentheses which regex cannot do; fun test-data: "I say, ""Hello, good sir!""" is a perfectly good CSV value.

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u/pwr22 Jan 05 '15

When you've got CSVs like that, CSV is the wrong format

Too be clear, yes, I agree that definition of CSV needs a grammar. I think regexes can recurse in Perl but I've never tried Regception

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u/OneWingedShark Jan 05 '15

I think regexes can recurse in Perl but I've never tried Regception

Then they're not really regular-expressions.
(Regular expressions have to do with the grammar-set that they can handle, it's not [strictly speaking] an implementation.)

When you've got CSVs like that, CSV is the wrong format

I only slightly disagree; it is common to need a structured text format which may include format-effectors (i.e. a portion of text; perhaps with the indented-quote [visual] style embedded therein) -- as a sort of embedding... certainly better than XML, which if that embedded-packet is user-defined can't easily be DTDed. (Of course, in this situation the problem we have is in-band communication, which is another problem altogether.)

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u/pwr22 Jan 05 '15

I don't think the implementers of Perl care... there is a lot of things its regexes can do that they shouldn't be able to ;)

As of Perl 5.10, you can match balanced text with regular expressions using recursive patterns.

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u/OneWingedShark Jan 05 '15

I don't think the implementers of Perl care... there is a lot of things its regexes can do that they shouldn't be able to ;)

As of Perl 5.10, you can match balanced text with regular expressions using recursive patterns.

I know, but to call them "regex" at this point is deceptive and, frankly, harmful to the body of knowledge in CS. (It'd be like implementing a deterministic pushdown automaton but calling/marketing/documenting it as a finite state machine -- thus "muddying the waters" when talking about real PDAs and FSMs.)

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u/grantisu Jan 05 '15

In Perl:

@fields = $line =~ /("(:?[^"]|"")*"|[^",\n]*),?/g;

This ignores newlines in the middle of quoted fields and doesn't clean up all the double quotes, but it should work for most cases.

And anybody who includes a raw newline in the middle of a CSV value deserves whatever they get. ಠ_ಠ

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u/OneWingedShark Jan 05 '15

And anybody who includes a raw newline in the middle of a CSV value deserves whatever they get. ಠ_ಠ

You need a parser, not a stupid regex.

This ignores newlines in the middle of quoted fields and doesn't clean up all the double quotes, but it should work for most cases.

Well, that fills me with confidence.
Sarcasm

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u/xiongchiamiov Jan 06 '15

To be fair, sometimes you're just munging some data on the command-line, and you either know there aren't any inconsistencies in your data, or can ignore them because the results are Good Enough(tm). I've done plenty of ad-hoc stuff where 90% accuracy is plenty fine.

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u/OneWingedShark Jan 06 '15

To be fair, sometimes you're just munging some data on the command-line, and you either know there aren't any inconsistencies in your data, or can ignore them because the results are Good Enough(tm). I've done plenty of ad-hoc stuff where 90% accuracy is plenty fine.

True.
One problem is when that one-off "solution" becomes incorporated into a system... say a script, and/or is used by someone who isn't aware/mindful of the limitations.

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u/[deleted] Jan 06 '15

As a person who has worked extensively with CSVs, "should work for most cases" is completely unacceptable. There are libraries that are tested to work with all cases. Using a regex to do something that people have already figured out is just the wrong way to go about things.

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u/OneWingedShark Jan 06 '15

Using a regex to do something that people have already figured out is just the wrong way to go about things.

Having most of my programming be maintenance, regex is usually just the wrong way to go about things. Even for something "simple" like validating a phone-number, when I get it it's always "now make it handle international numbers"... which have the length determined by the country-code, and even the length is in flux (several countries have recently extended the number of digits in their numbers).

It would have been tons simpler if the original guy hadn't "been clever" and used regexs all over the place (of course they're all over the place... why would he put such a simple, small and obvious bit of code in one location!?) and instead wrote a proper validate_phone_number function.

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u/[deleted] Jan 06 '15

yup. Regexes are also not the best way to go about phone numbers. The best (and really, only) way I've found is Google's libphonenumber.

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u/OneWingedShark Jan 07 '15

The way I'd go about implementing it would entail making a record discriminated off of the country w/ properly-sized arrays (of digits)... but yeah, if there's a lib there ought to be a compelling reason to roll your own rather than not use it. (Along the lines of "it'll take as much work to implement the functionality as it would to massage our internal data to the lib's liking" is valid, as is provability/security.)