Thursday, April 21, 2011

Sprint Fast, Squished Frogs and Overloading. (Ruby Program pt. 12?)

I've decided to take an online Ruby course. I started last Saturday. I've put aside the FBI-website reading project to the side a bit, since I need to get more core basics before I tackle that one. One of the first exercises on the class has to do with the "sprintf" documentation and this bit of code:

puts "%05d" % 123

This looks like the modulo operator from before (see my previous blog) so it left me totally confused. I tried to get more explanations but all seemed to follow the equational approach: see the formula, see how it works, understand. This approach must work for some people but I totally don't "get" it that way. That step three, just doesn't happen. I look at the equation nod stupidly and fail to grasp it.

When I look at spritf, I think Sprint Fast and imagine runners gunning it and don't get what the percentage sign is doing there.

Finally I happened upon Why's explanation in his book, which soooo needs an index. His explanation is fantastic and near the end of the book, as while this is in the first lesson on the online class it is actually fairly complex.

Why starts off with a long explanation of rush hour traffic on the city of Wixl and how many white-collared animals (yes its an animal city) pay frogs to hold the seats for them in the bus. Our two intrepid protagonists can't find a seat because of the frogs.

Think of the percentage sign as squished frogs...

So the frogs hold the seats for different kinds of clients. The %s places full strings, the %d (and %i) places integers and the %f places floating point numbers. Suddenly the code above starts to make some sense. It's something %d. But that "something" is in the middle.

%"something"d

Turns out there are two things you tend to put in the middle there (why the middle?) are the order of elements or the width. In this case it's the width. Whatever plain digits are put in between the squished frog and the identifier are how many spaces the incoming element will take. Those cheeky frogs take more that one seat!

The whole thing is actually fairly complicated as a look at the sprintf documentation reveals.

After reading that explanation, I started to wonder if sprintf came from another language (it does! It comes from C, those bastards!) and looked it up on my Learning Python book and got the best explanation of why it uses a percentage sign rather than say .format (which is what I would have used).

Python overloads the % binary operator to work on strings (the % operator also means remaider-of-division modulus for numbers). When applied to strings; it serves the same role as C's sprintf function.

Well Ruby does the same thing, and now I know what overloading an operator means. I also figured that sprintf is not sprint-f but s-print-f for string-print-format. Why they didn't call it spf is beyond me, or maybe they wanted to reserve that for sunscreen... Who knows? But now I have cool mnemonic when I see the percentage sign, I see a squished frog that had been holding a seat.

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