# gpa: compute gpa based on semester grades - assumes each class same 
#      number of credits

# string(ascii) -> array(float)
# precondition: text is not empty
def convert_to_numbers(text)
  letters = text.split(' ')
  numeric_values = letters.map do |c|
    if c == 'A' || c == 'a'
      4.0
    elsif c == 'B' || c == 'b'
      3.0
    elsif c == 'C' || c == 'c'
      2.0
    elsif c == 'D' || c == 'd'
      1.0
    else
      0.0 # treat all others as F
    end
  end
end

puts "Enter grades (as A, B, C, etc.), entering a blank line or end-of-file to stop:"
total = 0.0
count = 0
while ( (line = gets) && !line.chomp.empty? )
  nums = convert_to_numbers line
  # the following statement is equivalent to
  #    i = 0; while i < nums.length; total += nums[i]; i += 1; end
  nums.each { |x| total += x }
  count += nums.length
end

if count == 0
  puts "You must take classes to have a semester GPA!"
end
puts "Semester GPA: #{total / count}"