Undocumented Ext JS Grouping Sort Property

July 22, 2010 at 04:28 (extjs, javascript)

Background: Ext JS is an open source JavaScript library used for web design and geared towards interactive apps using technology such as Ajax. I am using version 3.2.1.

Today after an hour or two of frustration, I discovered an omission in the official online documentation of Ext JS. Under Ext.data.GroupingStore, the very useful property groupDir is mentioned nowhere, as of the time of this post. Without this property, it is exceedingly difficult (in my experience) to specify a simple sorting preference on the grouping field for this type of grid.

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MediaWiki – Multi-Line <pre></pre> within List

July 14, 2010 at 21:03 (mediawiki, tutorial)

Background: MediaWiki is open source software used to create wikis; it was developed for and is currently used by Wikipedia. I am using version 1.15.4.

I’ve spent a lot of time searching without success for a way to insert a multi-line pre-formatted block of text within an ordered or unordered list in MediaWiki using wiki markup * and # syntax. Today I finally figured out this can be done using either &#10; (Line Feed) or &#13; (Carriage Return). I haven’t tested across platforms or browsers, but here is an example:

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Clustered k-Subsets

July 11, 2010 at 04:39 (algorithm, java, math.CO)

This post is mostly copied and pasted from my posts in this thread on MHF.

Problem statement: Let \displaystyle S_n = \{1,\ \dots\ ,n\} and let \displaystyle f(n,k,x) be the number of k-subsets of \displaystyle S_n such that when the elements are ordered from least to greatest, the absolute difference of any two adjacent elements is less than \displaystyle x. Find a formula or algorithm to determine \displaystyle f(n,k,x) for arbitrary \displaystyle n,k,x \in \mathbb{Z}, n\ge 0.

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Continued Fractions of Square Roots – Steps

July 4, 2010 at 19:31 (algorithm, java, math.NT, tutorial)

Everyone knows what continued fractions are, right? Continued fractions have interesting properties and can be used to obtain best rational approximations for real numbers, among other things. Here is an example of a finite continued fraction:

8.309 = 8+\cfrac{1}{3+\cfrac{1}{4+\cfrac{1}{4+\cfrac{1}{3+\cfrac{1}{2+\cfrac{1}{2}}}}}}

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