def std_dev(list)

  # Calculate neccesary values
  list_squared = list.map {|item| item*item }
  n = list.size

  #Calculate the std deviation
  right = (Float (sum(list)**2))/n

  ((Float (sum(list_squared)) - right) / (n-1)) ** 0.5

end

def correlation(x, y)

  # Calculate the necessary values
  n = x.size

  sum_x = sum(x)
  sum_y = sum(y)

  x_squared = x.map {|item| item*item }
  y_squared = y.map {|item| item*item }

  sum_x_squared = sum(x_squared)
  sum_y_squared = sum(y_squared)

  xy = []
  x.each_with_index do |value, key|
    xy << value * y[key]
  end

  sum_xy = sum(xy)

  # Calculate the correlation value
  left = n * sum_xy - sum_x * sum_y
  right = ((n * sum_x_squared - sum_x**2) * (n * sum_y_squared - sum_y**2)) ** 0.5

  left / right
end

def sum(list)
  list.inject( nil ) { |sum,x| sum ? sum+x : x };
end

After going through a bunch of other yDecoder implementations, I still couldn’t figure out what I was doing wrong.  Then I happened across a bright red message on the yEnc site :

Frequent programming problem (28.03.2003)

I was contacted more than a dozen time by developers who are having problems with the decoding or encoding. They all had the same problem: They did not realize that the NNTP-protocol requires to double a dot in the first colum when a line is sent - and to detect a double dot (and remove one of them) when receiving a line.
So if you are directly posting to or reading from a socket (internet conenction socket), then dont forget this, please

The NNTP protocol repeats the . character if it occurs at the beginning of the line because  the . is used as a terminating character. So the moral of the story is always read your RFC’s.

UPDATE: After some more research and trouble shooting I’ve found that you can’t add instance variables through categories, it looks as though I’d be better off writing a subclass of NSString for this.

I’m currently writing the decoder for yDec and decided I wanted to read files a line at a time.  Unfortunately, I wasn’t able to find a simple way of doing this in objective-c so I wrote a category for the NSString class that allows you to read from a NSString a line at a time.

Header:

//
//  getLine.h
//  swagr
//
//  Created by Trevor Berg on 8/7/08.
//  Copyright 2008 Trevor Berg. All rights reserved.
//

#import <Cocoa/Cocoa.h>

@interface NSString (getLine)
- (void)setFilePointer:(int)index;
- (int)getFilePointer;
- (NSString*) getLine;
- (void)resetLine;
@end

Implementation:

//
//  getLine.m
//  swagr
//
//  Created by Trevor Berg on 8/7/08.
//  Copyright 2008 Trevor Berg. All rights reserved.
//

#import "getLine.h"

@implementation NSString (getLine)

static NSMutableDictionary *ivarHolder = nil;

- (NSString*) getLine{

	// chop off the first part of the string so that
	// you actually get the next line instead of the first line
	NSString* mySelf = [self substringFromIndex:[self getFilePointer]];

	// find the index of the end of the line
	int first = [mySelf rangeOfString:@"\n"].location;

	// make sure it found a line
	if (first != NSNotFound) {
		NSString* line = [mySelf substringToIndex:(first-1)];

		// move the index to the start of the next line
		[self setFilePointer:(first + 1 + [self getFilePointer])];

		// return the line
		return line;
	}
	else {
		// Didn’t find another line so return nil
		return nil;
	}
}

- (void) resetLine {

	// reset the index to the beginning of the file
	[self setFilePointer:0];
}

- (void)setFilePointer:(int)index {
	if ( ivarHolder == nil )
        ivarHolder = [[NSMutableDictionary alloc] init];

	[ivarHolder setObject:[NSNumber numberWithInt:index] forKey:self];
}

- (int)getFilePointer {
	if(ivarHolder == nil) {
		return 0;
	}
	else {
		NSNumber* index = [ivarHolder objectForKey:self];
		return [index intValue];
	}
}
@end

Obviously, this isn’t the most polished solution so feel free to email me any changes to trevorb(at)gmail.com .

I just got back from my first trip to San Francisco, and I have to say I really liked the place.  I flew down to check out Dropbox. If you don’t know about Dropbox, they provide data synchronization, backup, version control, and web sharing free of charge ( up to 2 gb).  I have a few invites if anyone needs some. It’s clear they have a very good thing going.

https://github.com/dlfnation/swagr/tree

I’ve also purchased Aaron Hillegass’ Cocoa Programming for Mac OS X (Third Edition) to help guide me through the process.

I spent a few hours the last couple of days working on a fun little small project called BinaryURL. It does pretty much the opposite of tinyurl, converts a url to a binary string.

check it out

I was looking up a computer science term on Wikipedia the other week and as is wont to happen, one thing led to another and I was about 5 or 6 articles deep on a trail of discovery and research to build up my knowledge.

As I realized I was randomizing myself and getting way off the original track, I decided to start assembling the links in one location for my own reference if not some other geek without the funds for their own degree.

Yes, there’s much more that I don’t have below, I wound up throttling back quite a bit after a while to leave it as is…for now.

» View the list

Answer the following:

147 50 61 65 51 75 77 73 147 50 156 51 75 147 50 156 55 61 51 136 62 55 61 56 71 73 147 50 60 51 75 60

The Shoulda Rails plugin makes it easy to write elegant, understandable, and maintainable tests. Shoulda consists of test macros, assertions, and helpers added on to the Test::Unit framework. It’s fully compatible with your existing tests, and requires no retooling to use.   

Helpers – context and should give you rSpec like test blocks. In addition, you get nested contexts and a much more readable syntax.

Macros – Generate hundreds of lines of Controller and ActiveRecord tests with these powerful macros. They get you started quickly, and can help you ensure that your application is conforming to best practices.

Assertions – Many common rails testing idioms have been distilled into a set of useful assertions.

The presentation is a little over an hour long, but I think it’s definitely worth a watch.

BDD with Shoulda - Tammer Saleh at MountainWest RubyConf 2008