Quadratic equation solver in Ruby

Question

I wrote a simple quadratic equation solver with Ruby to help me solve quadratic equations. Please tell me what I did right or wrong, and what I can do to improve it.

#!/usr/bin/ruby
if ARGV.length != 3
 STDERR << "Usage: #{0ドル} <a> <b> <c>\n"
 exit 1
end
a = ARGV[0].to_i
b = ARGV[1].to_i
c = ARGV[2].to_i
disc = b**2 - 4*a*c
d_str = (disc >= 0) ? (Math.sqrt(disc) % 1 == 0) ? Math.sqrt(disc).to_i : "√#{disc}" : "√#{disc}"
s = "#{-b} ± #{d_str}"
d = "#{2*a}"
puts "\e[4m#{s}\e[0m"
#puts "─"*(s.length)
puts d.center s.length

• Marc Rohloff: I would show the calculated value Math.sqrt(disc) even when it is a floating point number. Its also a good idea to check before using terminal control codes (A simple, though not foolproof, way is to check $stdout.isatty). The colorize gem is also, though ti does add a dependency to your code.
• anna328p: Any updates on this?

Answer 1

What you did right is solve the problem.

What you did "wrong" really depends on a number of factors, so I'll respond with what you can do to improve it, with an example that implements those improvements. Note, many improvements will make the code longer.

Improvements

• Use the shebang #!/usr/bin/env ruby. It allows the environment to have ruby in a different directory than /usr/bin/ruby.
• Use $stderr.puts "Usage: #{0ドル} <a> <b> <c>". $stderr is a global variable and can be changed. It allows standard error to be redirected, say to a file or a logger. It would be restored by $stderr = STDERR.
• Ruby indentation standard is 2 spaces and no hard tabs.
• Use disc_str to indicate it is a String version of disc.
• Use disc_sqrt_str to indicate it is a String version of disc_sqrt.
• Use a variable to reduce wasted calculations; sqrt is not a simple calculation.
• Nested ? operators can be hard to read and can lead to mistakes.
• Use numerator and denominator to convey intention, instead of s and d.
• Allow for the case when discriminant is zero.
• Use parentheses for arguments when method calls are arguments. Thus instead of f g h x do f g(h(x)) or even better f(g(h(x))).
• Use discriminant because it is a Mathematics domain term. A non-mathematics literate developer will be blocked from maintaining it because Googling "disc" won't help them.
• Eliminate common factors.
• Eliminate fraction if denominator is 1.
• Define methods for greater flexibility.
• Include some tests to test each pathway.
• Use appropriate comments and self-commenting code by using good names.
• Use Rubocop to provide similar automated improvements.
• Use the colorize gem in an optional way, since it is not actually necessary.
• Check if it is available by using ''.respond_to?(:underline), which will mean that it is more flexible because another method could be added to String which might do a similar thing or even something rather different; CSS, LaTeX, ASCII, ... .
• Use Unicode codes instead of the characters directly. Some editors may not display Unicode characters properly and could change the characters to something unexpected.

#!/usr/bin/env ruby
#
# quadratic - Output quadratic solutions as a fraction
#
# Luis Esteban 9 August 2020
# review of code of Dmitry Kudriavtsev
# Output rational solutions to the quadratic equation
# For a.x^2 + b.x + c = 0
#
# x = (-b ± √d) / 2a
# d = b^2 - 4.a.c (discriminant)
#
begin
 require 'colorize'
rescue LoadError
end
ALLOW_IMAGINARY = true
def solve_quadratic(a, b, c)
 discriminant = b**2 - 4*a*c
 denominator = 2 * a
 if discriminant > 0
 discriminant_sqrt = Math.sqrt(discriminant)
 if discriminant_sqrt % 1 == 0
 b, discriminant, denominator = simplify(b, discriminant_sqrt.to_i, denominator)
 numerator = [-b, " \u00b1 ", discriminant]
 else
 b, discriminant, denominator = simplify_with_sqrt(b, discriminant, denominator)
 numerator = [-b, " \u00b1 \u221a", discriminant]
 end
 elsif discriminant == 0
 solution = Rational(-b, denominator)
 numerator = [solution.numerator]
 denominator = solution.denominator
 else
 if ALLOW_IMAGINARY
 discriminant_sqrt = Math.sqrt(-discriminant)
 discriminant_sqrt = discriminant_sqrt.to_i if discriminant_sqrt % 1 == 0
 if discriminant_sqrt % 1 == 0
 b, discriminant, denominator = simplify(b, discriminant_sqrt.to_i, denominator)
 numerator = [-b, " \u00b1 i \u2a2f ", discriminant]
 else
 b, discriminant, denominator = simplify_with_sqrt(b, discriminant, denominator)
 numerator = [-b, " \u00b1 i \u2a2f \u221a", -discriminant]
 end
 else
 numerator = []
 end
 end
 
 [numerator, denominator]
end
def simplify(b, discriminant, denominator)
 gcd = [b, discriminant, denominator].inject(&:gcd)
 gcd = -gcd unless denominator.positive?
 
 [b, discriminant, denominator].map{|n| n / gcd }
end
def simplify_with_sqrt(b, discriminant, denominator)
 gcd = [b**2, discriminant, denominator**2].inject(&:gcd)
 gcd_sqrt = Math.sqrt(gcd).round
 if denominator.negative?
 gcd = -gcd
 gcd_sqrt = -gcd_sqrt
 end
 
 [
 b / gcd_sqrt,
 discriminant / gcd,
 denominator / gcd_sqrt
 ]
end
def display_fraction(numerator, denominator)
 size = numerator.size
 numerator = numerator.join
 denominator = denominator.to_s
 width = [numerator, denominator].map(&:length).max + 2
 if size > 0
 if denominator != "1"
 if ''.respond_to?(:underline)
 puts "#{numerator}".center(width).underline
 puts denominator.center(width)
 else
 puts numerator.center(width)
 puts "─" * width
 puts denominator.center(width)
 end
 else
 puts numerator
 end
 else
 puts "No solutions"
 end
end
if ARGV.length != 3
 $stderr.puts "Usage: #{0ドル} <a> <b> <c>"
 exit 1
else
 a, b, c = ARGV.map(&:to_i)
 puts "solution(s) to #{a}x\u00b2 + #{b}x + #{c} = 0"
 
 display_fraction(*solve_quadratic(a,b,c))
end
# Testing
# [
# [1, 1, -12], # Integer discriminant
# [-1, 1, 12], # Make denominator positive
# [1, 2, 0], # Don't show denominator if 1
# [4, 4, 1], # Don't show discriminant if 0
# [1, 2, -17], # Don't show denominator if 1 with surd
# [2, 4, 2], # Don't show denominator if 1 and discriminant if 0
# [1, 1, -13], # Show surd
# [1, 2, 17], # Imaginary solution with integer discriminant no denominator
# [1, 2, 16], # Imaginary solution with correct cancellation of surd
# [2, 0, 2], # Imaginary solution with surd, hide denominator
# [2, 8, 16], # Imaginary solution with integer discriminant
# ].each do |a,b,c|
# puts "For: a = #{a.inspect}, b = #{b.inspect}, c = #{c.inspect}"
# puts
# display_fraction(*solve_quadratic(a,b,c))
# puts
# end