Question

I have different hashes containing units, grouped into unit types. My code aims to determine which unit type should be returned for further processing. However, a lot of duplication is going on when each list is examined. The first if is doing the exact same as the first elsif. How do I DRY the code up the best possible way?

from_unit = "gr"
to_unit = "kg"

WEIGHT = {
"gr" => 1000.0,
"kg" => 1.0,
}

MEASURE = {
"mm" => 1000.0,
"cm" => 100.0,
"m" => 1.0
}

if WEIGHT.has_key?(from_unit) or WEIGHT.has_key?(to_unit)
  if WEIGHT.has_key?(from_unit) && WEIGHT.has_key?(to_unit)
    return WEIGHT
  elsif WEIGHT.has_key?(from_unit)
    raise RuntimeError, "#{to_unit} is not a known unit"
  else
    raise RuntimeError, "#{from_unit} is not a known unit"
  end
elsif MEASURE.has_key?(from_unit) or MEASURE.has_key?(to_unit)
  if MEASURE.has_key?(from_unit) && MEASURE.has_key?(to_unit)
    return WEIGHT
  elsif MEASURE.has_key?(from_unit)
    raise RuntimeError, "#{to_unit} is not a known unit"
  else
    raise RuntimeError, "#{from_unit} is not a known unit"
  end
else
  raise RuntimeError, "You can't convert #{from_unit} into #{to_unit}"
end

Answer 1

Go for simplicity, this snippet does less checks than yours (are really that necessary?), but gets the job done:

def get_table(from_unit, to_unit)
  [WEIGHT, MEASURE].detect do |table|
    table[from_unit] && table[to_unit]
  end or fail("You can't convert #{from_unit} into #{to_unit}")
end