A short time ago in a galaxy not so far, far away I came across a nice blog post: 15 Exercises for Learning a new Programming Language.
One could argue if these are *really* the most appropriate 15(+) exercises to learn a new programming language - however, the task of answering this rather complex question is left as an exercise for the reader. Instead of this I will show you their implementation in Ruby - rubyrailways.com style.
Why did I bother to solve these problems (including not really trivial ones, like a scientific calculator with a GUI) ? Well, actually to learn a new programming language! I still consider myself a beginner Ruby apprentice just playing it by ear in my somewhat scarce free time, so I thought that systematically implementing a task list like this will mean great step forward for me compared to just coding random things at random times. Fortunately I was perfectly right!
Before we move onto the code, one last disclaimer: the fact that I am still a Ruby n00b implies that the code can be somewhat hairy/not optimal/[insert any other language than Ruby here]-ish so don’t use these snippets as a textbook solution of the problems or anything like that. I would be glad if someone could suggest a bit of refactoring of the bad parts but I also hope that that there are some nice parts which you can learn from (actually I am quite sure about this since I used some magick formulas from a few Ruby (grand)masters in some cases).
OK, enough talk for now. Let’s see the stuff!
1. Problem: “Display series of numbers (1,2,3,4, 5….etc) in an infinite loop. The program should quit if someone hits a specific key (Say ESCAPE key).”
Solution: Hmm, well, errr…uh-oh… I could not solve this problem fully (what a terrific start 
). If Henry Ford would sit beside me now, he would say : You can hit any key to exit - so long as it’s ‘C’ - and one more advice: don’t forget to hold CTRL during this action . More on this after the code snippet:
i = 0
loop { print "#{i+=1}, " }
i = 0
loop { print "#{i+=1}, " }
Comments :
If anyone knows how to add code which will cause this program to stop with a specific keyhit (say ‘ESC’) please, please, please drop me a note. I have been researching this for at least 10% of the time of solving all the tasks, nearly spitting blood when I gave up . It seems (to me) that there is no simple (i.e. no threads and similar) and clean platform-independent solution for this problem. I guess (hope) the author’s idea here was different than to introduce threading or writing platform specific-code…
2. Problem: “Fibonacci series, swapping two variables, finding maximum/minimum among a list of numbers.”
Solution:
#Fibonacci series
Fib = Hash.new{ |h, n| n < 2 ? h[n] = n : h[n] = h[n - 1] + h[n - 2] }
puts Fib[50]
#Swapping two variables
x,y = y,x
#Finding maximum/minimum among a list of numbers
puts [1,2,3,4,5,6].max
puts [7,8,9,10,11].min
#Fibonacci series
Fib = Hash.new{ |h, n| n < 2 ? h[n] = n : h[n] = h[n - 1] + h[n - 2] }
puts Fib[50]
#Swapping two variables
x,y = y,x
#Finding maximum/minimum among a list of numbers
puts [1,2,3,4,5,6].max
puts [7,8,9,10,11].min
Comments: The Fibonacci code was written by Andrew Johnson (found via Ruby Quiz). I like it so much that I think it would be a shame to present a trivial version here. I guess the rest of the code is self-explanatory.
3. Problem: “Accepting series of numbers, strings from keyboard and sorting them ascending, descending order.”
Solution:
a = []
loop { break if (c = gets.chomp) == 'q'; a << c }
p a.sort
p a.sort { |a,b| b<=>a }
a = []
loop { break if (c = gets.chomp) == ‘q’; a << c }
p a.sort
p a.sort { |a,b| b<=>a }
Comments: This version is accepting strings - I think anybody who got to this point can adapt it to work with numbers.
4. Problem: “Reynolds number is calculated using formula (D*v*rho)/mu Where D = Diameter, V= velocity, rho = density mu = viscosity Write a program that will accept all values in appropriate units (Don’t worry about unit conversion) If number is < 2100, display Laminar flow, If it’s between 2100 and 4000 display 'Transient flow' and if more than '4000', display 'Turbulent Flow' (If, else, then...)"
Solution:
vars = %w{D V Rho Mu}
vars.each do |var|
print "#{var} = "
val = gets
eval("#{var}=#{val.chomp}")
end
reynolds = (D*V*Rho)/Mu.to_f
if (reynolds < 2100)
puts "Laminar Flow"
elsif (reynolds > 4000)
puts "Turbulent Flow"
else
puts "Transient Flow"
end
vars = %w{D V Rho Mu}
vars.each do |var|
print "#{var} = "
val = gets
eval("#{var}=#{val.chomp}")
end
reynolds = (D*V*Rho)/Mu.to_f
if (reynolds < 2100)
puts "Laminar Flow"
elsif (reynolds > 4000)
puts "Turbulent Flow"
else
puts "Transient Flow"
end
Comments: Can you spot the trick in the part which is filling up the variables? They don’t go out of scope after the loop ends because they are constants. Other possibility would be to use $global variables but I guess it is usually not a very good programming practice to do that.
5. Problem: “Modify the above program such that it will ask for ‘Do you want to calculate again (y/n), if you say ‘y’, it’ll again ask the parameters. If ‘n’, it’ll exit. (Do while loop)
While running the program give value mu = 0. See what happens. Does it give ‘DIVIDE BY ZERO’ error? Does it give ‘Segmentation fault..core dump?’. How to handle this situation. Is there something built in the language itself? (Exception Handling)”
Solution:
vars = { "d" => nil, "v" => nil, "rho" => nil, "mu" => nil }
begin
vars.keys.each do |var|
print "#{var} = "
val = gets
vars[var] = val.chomp.to_i
end
reynolds = (vars["d"]*vars["v"]*vars["rho"]) / vars["mu"].to_f
puts reynolds
if (reynolds < 2100)
puts "Laminar Flow"
elsif (reynolds > 4000)
puts "Turbulent Flow"
else
puts "Transient Flow"
end
print "Do you want to calculate again (y/n)? "
end while gets.chomp != "n"
vars = { "d" => nil, "v" => nil, "rho" => nil, "mu" => nil }
begin
vars.keys.each do |var|
print "#{var} = "
val = gets
vars[var] = val.chomp.to_i
end
reynolds = (vars["d"]*vars["v"]*vars["rho"]) / vars["mu"].to_f
puts reynolds
if (reynolds < 2100)
puts "Laminar Flow"
elsif (reynolds > 4000)
puts "Turbulent Flow"
else
puts "Transient Flow"
end
print "Do you want to calculate again (y/n)? "
end while gets.chomp != "n"
Comments: As you can see, I could not use the same trick here when asking for the variables, because when somebody wants to calculate again, Ruby will complain (although by printing a warning only) that the constants have been already set up. Therefore I went for the hash solution. I think the do-you-want-to-calculate-again part is straightforward so I won’t analyze that here.
“While running the program give value mu = 0.”
Ruby gives a rather interesting result in this case: infinity .
“Is there something built in the language itself?”
Sure: exception handling. Division by zero could be caught with a ZeroDivisionError rescue clause.
6. Problem: “Scientific calculator supporting addition, subtraction, multiplication, division, square-root, square, cube, sin, cos, tan, Factorial, inverse, modulus”
Solution:
Since this code snippet is longer It would look ugly here - you can download it from here instead.
Screenshot:
If you would like to try it, you will need the Tk bindings for Ruby (maybe you have them already, here on Ubuntu I did not). Also note that only the regular 0-9 keys (and of course the mouse) work, the numpad ones do not. One more little detail: % stands for modulo, not percent.
Comments: Phew, this was a real challenge, mostly because I never did any GUI in Ruby before. I was amazed that I could code up a relatively feature rich calculator in 100+ lines of code, without any golfing or trying to optimize for shortness. What I wanted to say with this is that the shortness does not praise my programming skills (since I did not eve try to golf) but the superb terseness of Ruby. OK, of course there are some problems (e.g. cube, cos, tan, inverse are not implemented) but the usability/amount of code ratio is unbelievably high.
The GUI is also not the nicest since I have used Tk - wxRuby or qt-ruby would produce much nicer results, but since I did not code any GUI in Ruby previously, I have decided to try the good-old-skool Tk for the first time.
7. Problem: “Printing output in different formats (say rounding up to 5 decimal places, truncating after 4 decimal places, padding zeros to the right and left, right and left justification)(Input output operations)”
Solution:
#rounding up to 5 decimal pleaces
puts sprintf("%.5f", 124.567896)
#truncating after 4 decimal places
def truncate(number, places)
(number * (10 ** places)).floor / (10 ** places).to_f
end
puts truncate(124.56789, 4)
#padding zeroes to the left
puts 'hello'.rjust(10,'0')
#padding zeroes to the right
puts 'hello'.ljust(10,'0')
#right justification
puts ">>#{'hello'.rjust(20)}<<"
#left justification
puts ">>#{’hello’.ljust(20)}<<"
#rounding up to 5 decimal pleaces
puts sprintf("%.5f", 124.567896)
#truncating after 4 decimal places
def truncate(number, places)
(number * (10 ** places)).floor / (10 ** places).to_f
end
puts truncate(124.56789, 4)
#padding zeroes to the left
puts ‘hello’.rjust(10,’0‘)
#padding zeroes to the right
puts ‘hello’.ljust(10,’0‘)
#right justification
puts ">>#{’hello’.rjust(20)}<<"
#left justification
puts ">>#{’hello’.ljust(20)}<<"
Comments: Amazingly lot of things can be done with sprintf() - I could solve nearly all the problems with it - but that would not really be rubyish, so I have decided for built-in (and one homegrown) functions. However, mastering (s)printf() is a very handy thing, since nearly all big players (C (of course ), C++, Java, PHP, … ) have it so you get a powerful function in more languages for the price of learning one). As you can see, r/ljust is a nice one, too.
8. Problem: “Open a text file and convert it into HTML file. (File operations/Strings)”
Solution: Well, this problem was not specified in a great detail, to say the least - or to put it otherwise, the solvers are given a great freedom to provide a solution spiced up with their fantasy. This is what I came up with:
doc = <strong tag.
DOC
final_doc = <
embed_doc_here
