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Showing posts from April 28, 2013

Palindromes again?

Actually, it's more Haskell than palindromes.

I've been looking over other people's solutions to the "Fair and Square" problem, and they are pretty impressive. As a still-inexperienced Haskeller, I am taking inspiration from then rather than feeling discouraged.

In particular, jagd's solution reminded me that I was not thinking Haskell. Haskell is lazy; values aren't calculated until you require them. That lets you define infinite data structures! You only get into trouble if you do something that requires the whole thing.

For example, [1..] is the infinite list of all positive integers. You can safely use it, as long as you only use a finite amount of it. For example, if you try

uptoSqrt n = [x | x <- [1..], x * x <= n]

and then ask for

uptoSqrt 25

you will get back 1, 2, 3, 4, and 5, and then the function call will hang. You and I  realize that \x -> x * x is monotonically increasing for positive numbers, but Haskell doesn't, and obligingly t…

Fun with palindromes, and the joys of Haskell

An item in Hacker News caught my eye recently. (Ouch!) It linked to a post, "Ruby is Too Slow for Programming Competitions", by a gentleman named Clif Reeder. A summary: Mr. Reeder chose Ruby to try on a Google Code Jam qualification round problem, and found that his solution ran too slowly to meet the Code Jam time constraints (on the official runs, you have eight minutes from downloading the input to upload your output). A version written in the (compiled) Go language ran much faster.

Commenters on the HN item pointed out
Mr. Reeder didn't think through the problem fully; there were more possible optimizations. In fact, there's a list of 387 people who used Ruby to successfully handle either the small sample input (which has a four-minute time limit) or the large or really large inputs (with the eight-minute time limit).Like C, the size of int in Go may vary, so the Go version might or might not be able to handle the larger inputs correctly. There was also a long di…

Night of the Living Blog

A good friend nudged me to start putting up stuff I am working/have worked on, so Aging Nerd Notes is shuddering back to activity after what, eight years? I'm amazed Google has kept it around so long.