ある整数の各桁ごとに四則演算した結果を全パターン求める

class Integer
    def operations
        os = operators_series
        ns = self.to_s.each_char.map{|i| i.to_f}
        os.inject([]) {|r, e| r << ns.zip(e).flatten.compact.join}
    end

    private
    def operators_series
        length = self.to_s.length
        return self if length < 2
        operators = %w(+ - * /)
        length = self.to_s.length
        (length - 2).times
            .inject(operators) {|r, i| r.product(operators)}
            .flatten(length - 2)
            .each_slice(length - 1)
            .to_a
    end
end

# 小町算
20140112.operations.
    reject{|e| eval(e).infinite? || eval(e).nan?}.
    select{|e| eval(e) == 10.0}.
    each{|e| p e + " = " + eval(e).to_s}

# =>
"2.0+0.0+1.0+4.0+0.0+1.0+1.0*2.0 = 10.0"
"2.0+0.0+1.0+4.0+0.0+1.0*1.0+2.0 = 10.0"
"2.0+0.0+1.0+4.0+0.0+1.0/1.0+2.0 = 10.0"
"2.0+0.0+1.0+4.0+0.0*1.0+1.0+2.0 = 10.0"
"2.0+0.0+1.0+4.0+0.0/1.0+1.0+2.0 = 10.0"
"2.0+0.0+1.0+4.0-0.0+1.0+1.0*2.0 = 10.0"
"2.0+0.0+1.0+4.0-0.0+1.0*1.0+2.0 = 10.0"
"2.0+0.0+1.0+4.0-0.0+1.0/1.0+2.0 = 10.0"
"2.0+0.0+1.0+4.0-0.0*1.0+1.0+2.0 = 10.0"
"2.0+0.0+1.0+4.0-0.0/1.0+1.0+2.0 = 10.0"
"2.0+0.0+1.0*4.0+0.0+1.0+1.0+2.0 = 10.0"
"2.0+0.0+1.0*4.0-0.0+1.0+1.0+2.0 = 10.0"
"2.0+0.0*1.0+4.0+0.0+1.0+1.0+2.0 = 10.0"
"2.0+0.0*1.0+4.0-0.0+1.0+1.0+2.0 = 10.0"
"2.0+0.0/1.0+4.0+0.0+1.0+1.0+2.0 = 10.0"
"2.0+0.0/1.0+4.0-0.0+1.0+1.0+2.0 = 10.0"
"2.0-0.0+1.0+4.0+0.0+1.0+1.0*2.0 = 10.0"
"2.0-0.0+1.0+4.0+0.0+1.0*1.0+2.0 = 10.0"
"2.0-0.0+1.0+4.0+0.0+1.0/1.0+2.0 = 10.0"
"2.0-0.0+1.0+4.0+0.0*1.0+1.0+2.0 = 10.0"
"2.0-0.0+1.0+4.0+0.0/1.0+1.0+2.0 = 10.0"
"2.0-0.0+1.0+4.0-0.0+1.0+1.0*2.0 = 10.0"
"2.0-0.0+1.0+4.0-0.0+1.0*1.0+2.0 = 10.0"
"2.0-0.0+1.0+4.0-0.0+1.0/1.0+2.0 = 10.0"
"2.0-0.0+1.0+4.0-0.0*1.0+1.0+2.0 = 10.0"
"2.0-0.0+1.0+4.0-0.0/1.0+1.0+2.0 = 10.0"
"2.0-0.0+1.0*4.0+0.0+1.0+1.0+2.0 = 10.0"
"2.0-0.0+1.0*4.0-0.0+1.0+1.0+2.0 = 10.0"
"2.0-0.0*1.0+4.0+0.0+1.0+1.0+2.0 = 10.0"
"2.0-0.0*1.0+4.0-0.0+1.0+1.0+2.0 = 10.0"
"2.0-0.0/1.0+4.0+0.0+1.0+1.0+2.0 = 10.0"
"2.0-0.0/1.0+4.0-0.0+1.0+1.0+2.0 = 10.0"