CMSC330 - SUMMER 2006 - PSEUDO-MIDTERM 1
INSTRUCTOR: GUILHERME FONSECA
(1) (20 points) What is the output of each Ruby program below? Ignore any possible
warning message. All programs are syntatically valid.
(a) if 0
puts "ab"
elsif ""
puts "cd"
else
puts "ef"
end
(b) a = b = [3, 1, 2]
a.sort!
puts b
(c) a = [[1,2], [1,2,1], [-1], [1,3]]
puts a.sort.inspect
(d) h = Hash.new([])
h["a"] = 3
h["b"] = h["b"] + [5]
h["c"] = h["c"] + [7]
puts "#{h["a"]} #{h["b"][0]} #{h["c"][-1]}"
(2) (10 points) Write a ruby program that reads several whole numbers from the standard
input, and prints them sorted by the absolute value. In Ruby, if xis a Fixnum, x.abs
returns the absolute value of x. The input numbers have no leading zeroes or spaces,
and are presented as one number per line. Output them as one number per line too.
(3) (21 points) Write a formal regular expression for each of the languages below. The
alphabet is Σ = {a, b}. The only operators allowed are ∗and |(do not write a Ruby
regular expression).
(a) {w|the number of a’s before the first bin wis at least 3 }
wis trivially in the language if there is no b.
(b) {w|the number of a’s before the first bin wis at most 5 }
wis trivially in the language if there is no b.
(c) The intersection of the two languages above.
(4) (8 points) Write a formal regular expression for each of the languages below. The
alphabet is Σ = {a, b, c}. The only operators allowed are ∗and |(do not write a
Ruby regular expression).
(a) {w|there is no aadjacent to b}
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