Convert integer to binary string, padded with leading zeros

Question

I want to create new method Integer#to_bin that convert decimal to a binary string. The argument of #to_bin is the number of digits. The result should be padded with leading zeros to make it have that many digits.

Example:

1.to_bin(4)
#=> "0001"
1.to_bin(3)
#=> "001"
1.to_bin(2)
#=> "01"
7.to_bin(1)
#=> nil
7.to_bin
#=> "111"
etс.

What I've tried:

class Integer

    def to_bin(number=nil)
        if number == nil
          return self.to_s(2)
        else 
          s = self.to_s(2).size
          e = number-s
          one = '0'
          two = '00'
          three = '000'

        if e==one.size
          one+self.to_s(2)
        elsif e==two.size
          two+self.to_s(2)
        elsif e==three.size
          three+self.to_s(2)
        end
        end
    end
end

How do I convert an integer to a binary string padded with leading zeros?

No, you're not trying to convert decima; to bin, you try to do some black magic. I'm asking what is that exactly. (to convert dec to bin to_s(2) is enough) — Mike Szyndel
– Mike Szyndel, Commented Jul 3, 2013 at 8:02

the Tin Man · Accepted Answer · 2013-07-03 08:30:14Z

35

The appropriate way to do this is to use Kernel's sprintf formatting:

'%03b' % 1 # => "001"
'%03b' % 2 # => "010"
'%03b' % 7 # => "111"

'%08b' % 1 # => "00000001"
'%08b' % 2 # => "00000010"
'%08b' % 7 # => "00000111"

But wait, there's more!:

'%0*b' % [3, 1] # => "001"
'%0*b' % [3, 2] # => "010"
'%0*b' % [3, 7] # => "111"

'%0*b' % [8, 1] # => "00000001"
'%0*b' % [8, 2] # => "00000010"
'%0*b' % [8, 7] # => "00000111"

So defining a method to extend Fixnum or Integer is easy and cleanly done:

class Integer
  def to_bin(width)
    '%0*b' % [width, self]
  end
end

1.to_bin(8) # => "00000001"
0x55.to_bin(8) # => "01010101"
0xaaa.to_bin(16) # => "0000101010101010"

edited Jul 3, 2013 at 8:30

answered Jul 3, 2013 at 8:16

the Tin Man

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Comments

Chris Heald · Accepted Answer · 2013-07-03 08:05:22Z

16

Ruby already has a built-in mechanism to convert a number to binary: #to_s accepts a base to convert to.

30.to_s(2) # => "11110"

If you want to left-pad it with zeroes:

30.to_s(2).rjust(10, "0") => "0000011110"

You could extend this into a little method that combines the two:

class Fixnum
  def to_bin(width = 1)
    to_s(2).rjust(width, "0")
  end
end

> 1234.to_bin
=> "10011010010"
> 1234.to_bin(20)
=> "00000000010011010010"

edited Jul 3, 2013 at 8:05

answered Jul 3, 2013 at 8:00

Chris Heald

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5 Comments

Dogbert Over a year ago

String#rjust would be a little cleaner - 30.to_s(2).rjust(10, "0")

Chris Heald Over a year ago

Ah, good point. I was looking for #format since I knew there was some way to right-justify with the string as the receiver!

sawa Over a year ago

This does not give nil to 7.to_bin(1).

Chris Heald Over a year ago

I presumed that was a bug, since it wasn't actually specified in the requirements as far as I can tell.

sawa Over a year ago

It looks like the expected behavior is to return nil when the argument is given and is less than the string length in binary before padding. In addition, the OPs code returns nil when the argument is other than 1, 2, or 3. I am not sure whether that was intended. But the question is badly phrased. It is mostly the OP's fault.

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