Learn A New Language
June 25, 2010
I chose Lua as my target language. I have studied Lua a few times in the past, and I like some of its ideas, but I’ve never written any Lua code, except perhaps for a few small toy exercises several years ago. I also chose the Sieve of Eratosthenes exercise. Here’s what I came up with:
function primes (n)
max_index = math.floor((n-1)/2)
v = {}
for i = 1, max_index do
v[i] = true
end
p = 3
while p*p <= n do
i = math.floor(p/2)
if v[i] then
for j = 2*i*(i+1), max_index, p do
v[j] = false
end
end
p = p + 2
end
ps = {2}
for i = 1, max_index do
if v[i] then
table.insert(ps, 2*i+1)
end
end
return ps
end
The algorithm is exactly the same as the Scheme version, except that I changed the positions of i and p in the central loop to avoid a break out of the loop. The biggest change was the “bit” vector, which is zero-based in Scheme but one-based in Lua, causing the calculations involving the i and j variables to change. Here is how the function looks in use:
> print(#(primes(15485863)))
1000000
The Lua version of the program is noticeably slower than the Scheme version; I didn’t do any formal timings, but it seems to take about three times as long to run, and the sample function-call times out at codepad.org. Even when I removed the code to accumulate the result and instead just counted the true bits, the Lua version timed out. I don’t know enough about Lua to know if the interpreter is generally slow or if I have done something bad in my function.
You can run the code at http://programmingpraxis.codepad.org/IWI7F58z.
Programming Praxis 
[…] Praxis – Learn A New Language By Remco Niemeijer The goal in today’s Programming Praxis exercise is to solve a previous exercise in a language you’re not […]
Steve Yegge’s Phone-Screen Coding Exercises using Ruby (see http://bonsaicode.wordpress.com/2010/06/25/programming-praxis-learn-a-new-language/ for a version with comments):
def reverse(s) s.chars.inject {|rev, x| x + rev } end def fib(n) (2..n).inject([0,1]) {|fs, i| [fs[1], fs[0] + fs[1]]} [[1, n].min] end def timestable (1..12).map {|r| puts (1..12).map {|c| "%4d" % (r * c)}.join} end def sum_from_file(file) File.open(file) {|f| f.readlines.map(&:to_i).inject(:+)} end def odd_numbers p (1..99).find_all(&:odd?) end def maximum(array) array.inject {|max, i| i > max ? i : max} end def to_rgb(r, g, b) sprintf("%02x%02x%02x", r, g, b) endI did the Texas Hold’Em problem in Ruby. For the rest of the code go to http://codingjunkie.net
require ‘deck’
require ‘card’
require ‘player’
card_deck = Deck.new
keep_running = true
while keep_running
results = {}
card_deck.shuffle
seven_cards = card_deck.deal
seven_cards.sort!
copy = Array.new(seven_cards)
puts “Press enter to deal cards..”
line = gets
if line.chop == “quit”
keep_running = false
else
for i in 0..5
for j in i..5
copy.delete_at(i)
copy.delete_at(j)
hand = Player.check_hand(copy,0,0,nil)
if results[hand]
copy = Player.compare_hands(copy, results[hand], 4)
end
results[hand] = copy
copy = Array.new(seven_cards)
end
copy = Array.new(seven_cards)
end
max_score = -1
winning_hand = nil
best_cards = nil
results.each do |hand, cards|
score = Player.ranks[hand]
score = score.nil? ? cards[4].value : score
if score > max_score
max_score = score
score = Player.ranks[hand]
best_cards = cards
winning_hand = hand
end
end
puts “Best hand from #{seven_cards.join(“, “)} ”
puts “\n\t #{winning_hand} : #{best_cards.join(“, “)}”
end
end
The golden ratio exercise from July 10 2009 in Scheme, recursively:
(define (golden_r n) (if (= 0 n) 1 (+ 1 (/ 1 (golden (- n 1))))))and iteratively (I’ve started working my way through SICP as you can probably tell…):
(define (golden_i n) (define (golden-iter n result count) (if (= count n) result (golden-iter n (+ 1 (/ 1 result)) (+ count 1)))) (golden-iter n 1 0))