How to convert a string or integer to binary in Ruby?

How do you create integers 0..9 and math operators + - * / in to binary strings. For example:

 0 = 0000,
 1 = 0001, 
 ...
 9 = 1001

Is there a way to do this with Ruby 1.8.6 without using a library?

You have Integer#to_s(base) and String#to_i(base) available to you.

Integer#to_s(base) converts a decimal number to a string representing the number in the base specified:

9.to_s(2) #=> "1001"

while the reverse is obtained with String#to_i(base):

"1001".to_i(2) #=> 9

I asked a similar question. Based on @sawa's answer, the most succinct way to represent an integer in a string in binary format is to use the string formatter:

"%b" % 245
=> "11110101"

You can also choose how long the string representation to be, which might be useful if you want to compare fixed-width binary numbers:

1.upto(10).each { |n| puts "%04b" % n }
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010

Picking up on bta's lookup table idea, you can create the lookup table with a block. Values get generated when they are first accessed and stored for later:

>> lookup_table = Hash.new { |h, i| h[i] = i.to_s(2) }
=> {}
>> lookup_table[1]
=> "1"
>> lookup_table[2]
=> "10"
>> lookup_table[20]
=> "10100"
>> lookup_table[200]
=> "11001000"
>> lookup_table
=> {1=>"1", 200=>"11001000", 2=>"10", 20=>"10100"}

You would naturally use Integer#to_s(2), String#to_i(2) or "%b" in a real program, but, if you're interested in how the translation works, this method calculates the binary representation of a given integer using basic operators:

def int_to_binary(x)
  p = 0
  two_p = 0
  output = ""

  while two_p * 2 <= x do
    two_p = 2 ** p
    output << ((two_p & x == two_p) ? "1" : "0")
    p += 1
  end

  #Reverse output to match the endianness of %b
  output.reverse
end

To check it works:

1.upto(1000) do |n|
  built_in, custom = ("%b" % n), int_to_binary(n)
  if built_in != custom
    puts "I expected #{built_in} but got #{custom}!"
    exit 1
  end
  puts custom
end