Monthly Archives: March 2013

Scheme Idiom: Loop over an open file input port

Dear Scheme wizards, I have a confession to make: I can never remember how to write loops in Scheme using the named-let convention. I’m working on a problem from the British Informatics Olympiad which you can read about here, with my own ugly imperative Ruby solution here. (My apologies to real Ruby programmers, of course.)

I’ll no doubt be apologizing again after I share my Scheme solution. Until then, here is some code to read in the contents from a file. This works in MIT Scheme but should be portable to any R5RS Schemes.

Note that read will blow up if the file contains certain characters, like #\#. MIT Scheme provides additional procedures like read-line to solve this problem.

(with-input-from-file "/home/rml/Desktop/current/INPUT.TXT"
  (lambda ()
    (let loop ((source (read)) (sink '()))
        (if (eof-object? source)
            (reverse sink)
            (loop (read) (cons source sink))))))
;Value 36: (5 7 3 8 8 1 0 2 7 4 4 4 5 2 6 5)

Just for Fun: Estimating pi with Scheme

../img/double-star-from-pentagon-backlit.jpg

A while back I shared some Perl code for calculating the circumference of a circle without knowing 𝛑. Just for fun, and due to my longtime infatuation with all things Schemish, I’ve written a little pi approximator in Scheme. It uses the idea that we can approximate a circle using smaller and smaller triangles stacked on top of each other. (See previously for a better explanation with a picture.)

And now, the code!

;;;; pi.scm -- Estimate the value of 𝛑 using smaller and smaller
;;;; triangles.

;;; Call it like so: (pi-estimate n), where n is the number of
;;; iterations you'd like to go through. It doesn't take many to get
;;; pretty accurate.

(define reference-pi 3.1415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253421170679)

(define (circumference radius)
  (* 3.14159 2 radius))

(define (square x)
  (* x x))

(define (hypotenuse a b)
  (sqrt (+ (expt a 2) (expt b 2))))

(define (pi-iter radius a b count maxcount)
  (let* ((hyp (hypotenuse a b))
         (newbase (- radius (sqrt (- (square radius)
                                     (square (/ hyp 2)))))))
    (if (= count maxcount)
        (/
         (* 2 hyp (expt 2 (+ 1 count)))
         (* 2 radius))
        (pi-iter radius newbase (/ hyp 2) (+ count 1) maxcount))))

(define (pi-estimate iterations)
  (pi-iter 128 128 128 0 iterations))

(Origami image courtesy Melisande under Creative Commons license.)

Recommendations for Technical Writers

This document describes my recommendations for technical writers working in the software industry (at least the web-focused corner of it). Be forewarned: it’s opinionated. I don’t even live up to it, so you can think of it as an extended note to self.

Learn the UNIX command line

The entire modern internet computing infrastructure is built on open source, UNIX-like operating systems (mostly Linux). If you’re not familiar with them, you will have a more difficult time learning about how large-scale software systems running on networked computers work, otherwise known as: virtually all software systems of importance 1.

To get there, you’ll need to know how to interact with the command line interface (also known as a “terminal”). This is a very plain-looking text-only interface where you will type commands formatted in a strict language understood by the computer. You will learn (if you haven’t already) that the computer always does exactly what you’ve asked it to, but not necessarily what you wanted it to do.

There are a number of “shells” you can use on a UNIX-like OS. I recommend Bash because it’s ubiquitous.

Recommended Reading:

Learn REST API principles

Here’s a light-speed introduction to REST APIs. A large software system has lots of “objects”, which are generally just pieces of stored data about something. For example, where I work, there are objects that represent an advertising campaign or a user in the system.

It doesn’t really matter what data the objects represent, the point of an API is that it creates a single point of contact for external entities (usually programs) that want to interact with the data in the system. It ensures that only certain operations can be performed, and only by authorized parties.

Here’s where the REST part comes in: for any object represented in the system, you can (in theory) perform 4 basic operations on it:

  • Create: Make a new object of that type
  • Read: Look at what information the object contains
  • Update: Change the information the object contains
  • Delete: Delete the object from the system altogether

Unfortunately, I know about RESTful APIs only from practice, not study. There’s a copy of O’Reilly’s RESTful Web Services at work that I’ve been meaning to read to fill in the gaps. See? This really is a note to self.

Recommended Reading:

Learn to use a programmer’s text editor

You’re a professional who gets paid to work with text all day. Programmers are also professionals who get paid to work with text all day. You should be following their example by using a powerful plain text editor; it will allow you to automate away some of the tedious parts of editing.

Remember, word processors are written by programmers for others to use. Text editors are written by programmers for themselves to use. It’s not even a contest.

I recommend vim or Emacs. Pick one, it doesn’t matter.

Recommended Reading:

Learn a programming language with strong UNIX integration

As noted above, you are going to be documenting software running on networked computers doing … networky stuff. For example, you might be making API calls to a “cloud” of some kind 2. A lot of it will be tedious and potentially time-consuming, which means that you should automate it using a programming language. Once you are pretty familiar with a range of terminal commands, it won’t be too difficult to pick up a language with strong UNIX integration. Tastes vary; I am most familiar with Perl. I do not recommend wasting much time writing shell scripts, as there are too many weird edge cases for beginners to trip over.

Recommended Reading:

  • Learning Perl by Randal L. Schwartz, brian d foy, and Tom Phoenix
  • Modern Perl by chromatic

Footnotes:

1 Where “importance” is defined as: someone will pay you good money to document it. Personally, I’d love to get a paying job rewriting half of the Emacs manual. But I digress.

2 A Freudian slip by the tech industry perhaps? After all, “the cloud” brings to mind an ephemeral, floating entity that is mostly vaporous, and exists at the whim of forces you don’t really understand, and certainly can’t control. In addition, it’s highly likely to be blown away at any time.