Logical Operations on Strings

ruby-talk
1

Is there a bitwise and for strings that I am missing? I tried the documentation and Google with no luck. I coded a simple method to handle this, it obviously needs more error checking, but don't want to reinvent the wheel if not necessary.

def AND(s, m)
   t = s.dup
   for i in 0...(s.size <= m.size ? s.size : m.size)
     t[i] = (s[i] & m[i]).chr
   end
   return t
end

On a related note, is there any way to create a string from a hex number other than separating each two digits and using .chr to convert them to a number and adding them to the string? If I have a string such as 0x7FFFFFFFFFFF I have to use something like str1 = 0x7F.chr followed by 5 str1 << 0xFF.chr which seems to be unnecessarily clumsy.

2

Is there a bitwise and for strings that I am missing? I tried the
documentation and Google with no luck. I coded a simple method to
handle this, it obviously needs more error checking, but don't want to
reinvent the wheel if not necessary.

def AND(s, m)
   t = s.dup
   for i in 0...(s.size <= m.size ? s.size : m.size)
     t[i] = (s[i] & m[i]).chr
   end
   return t
end

I believe this implementation is flawed because it returns a string
that is too long if self.size > m.size.

This is what I'd probably do

class String
  def &(s)
    t = ""
    [size, s.size].min.times do |i|
      t << (self[i] & s[i])
    end
    t
  end

  def |(s)
    t = ""
    [size, s.size].max.times do |i|
      t << ((self[i] || 0) | (s[i] || 0))
    end
    t
  end
end

Now you can do

irb(main):020:0> 0.chr | "a"
=> "a"
irb(main):021:0> 0.chr | "abc"
=> "abc"
irb(main):022:0> 0.chr & "abc"
=> "\000"
irb(main):023:0> a="s"
=> "s"
irb(main):024:0> a|="123"
=> "s23"
irb(main):025:0> a&="\012"
=> "\002"

On a related note, is there any way to create a string from a hex number
other than separating each two digits and using .chr to convert them to
a number and adding them to the string? If I have a string such as
0x7FFFFFFFFFFF I have to use something like str1 = 0x7F.chr followed by
5 str1 << 0xFF.chr which seems to be unnecessarily clumsy.

I am not sure which direction you mean (there is no such thing as a
"hex number" in Ruby, a number is a number, hex is just one
representation - internally they are most likely stored binary). Does
this help?

irb(main):001:0> 123.to_s 16
=> "7b"
irb(main):002:0> "%04x" % 123
=> "007b"
irb(main):003:0> "7b".to_i 16
=> 123

Kind regards

robert

···

2008/1/28, Michael W. Ryder <_mwryder@worldnet.att.net>:

--
use.inject do |as, often| as.you_can - without end

3

Robert Klemme wrote:

Is there a bitwise and for strings that I am missing? I tried the
documentation and Google with no luck. I coded a simple method to
handle this, it obviously needs more error checking, but don't want to
reinvent the wheel if not necessary.

def AND(s, m)
   t = s.dup
   for i in 0...(s.size <= m.size ? s.size : m.size)
     t[i] = (s[i] & m[i]).chr
   end
   return t
end

I believe this implementation is flawed because it returns a string
that is too long if self.size > m.size.

Actually I designed it that way so that I could change only part of the original string without having to create a mask of 0xFF for the remaining characters. Your implementations look a little cleaner than my first try and I will probably use them in the finished version. I still wonder why these weren't implemented in Ruby as I have been using them for over 25 years in other languages.

This is what I'd probably do

class String
  def &(s)
    t = ""
    [size, s.size].min.times do |i|
      t << (self[i] & s[i])
    end
    t
  end

  def |(s)
    t = ""
    [size, s.size].max.times do |i|
      t << ((self[i] || 0) | (s[i] || 0))
    end
    t
  end
end

Now you can do

irb(main):020:0> 0.chr | "a"
=> "a"
irb(main):021:0> 0.chr | "abc"
=> "abc"
irb(main):022:0> 0.chr & "abc"
=> "\000"
irb(main):023:0> a="s"
=> "s"
irb(main):024:0> a|="123"
=> "s23"
irb(main):025:0> a&="\012"
=> "\002"

On a related note, is there any way to create a string from a hex number
other than separating each two digits and using .chr to convert them to
a number and adding them to the string? If I have a string such as
0x7FFFFFFFFFFF I have to use something like str1 = 0x7F.chr followed by
5 str1 << 0xFF.chr which seems to be unnecessarily clumsy.

I am not sure which direction you mean (there is no such thing as a
"hex number" in Ruby, a number is a number, hex is just one
representation - internally they are most likely stored binary). Does
this help?

The reason I asked is that sometimes it is easier for me to think in hex when creating masks for logical operations. Plus, I learned the PCL codes for printing using hex and it is easier for me to use them that way. In Business Basic to enter a group of hex characters into a string I just use A$=$1B2C062C$ to create a 4 character string. No need to enter each character individually.

···

2008/1/28, Michael W. Ryder <_mwryder@worldnet.att.net>:

irb(main):001:0> 123.to_s 16
=> "7b"
irb(main):002:0> "%04x" % 123
=> "007b"
irb(main):003:0> "7b".to_i 16
=> 123

Kind regards

robert

4

Are you looking for something like this?

irb(main):010:0> ["414243"].pack('H*')
=> "ABC"
irb(main):011:0> ["7FFFFFFFFFFF"].pack('H*')
=> "\177\377\377\377\377\377"
irb(main):012:0> A=['1B2C062C'].pack('H*')
=> "\e,\006,"

-Adam

···

On 1/28/08, Michael W. Ryder <_mwryder@worldnet.att.net> wrote:

Robert Klemme wrote:
> 2008/1/28, Michael W. Ryder <_mwryder@worldnet.att.net>:
>> On a related note, is there any way to create a string from a hex number
>> other than separating each two digits and using .chr to convert them to
>> a number and adding them to the string? If I have a string such as
>> 0x7FFFFFFFFFFF I have to use something like str1 = 0x7F.chr followed by
>> 5 str1 << 0xFF.chr which seems to be unnecessarily clumsy.
>
The reason I asked is that sometimes it is easier for me to think in hex
when creating masks for logical operations. Plus, I learned the PCL
codes for printing using hex and it is easier for me to use them that
way. In Business Basic to enter a group of hex characters into a string
I just use A$=$1B2C062C$ to create a 4 character string. No need to
enter each character individually.

5

Robert Klemme wrote:
> I believe this implementation is flawed because it returns a string
> that is too long if self.size > m.size.

Actually I designed it that way so that I could change only part of the
original string without having to create a mask of 0xFF for the
remaining characters.

Ah, ok, then this was on purpose. Personally I'd consider an element
that is not there as 0x00 for OR and AND - that's why I did it this
way.

Your implementations look a little cleaner than
my first try and I will probably use them in the finished version. I
still wonder why these weren't implemented in Ruby as I have been using
them for over 25 years in other languages.

Since Ruby is so flexible chances are that you will find an equally
easy (or easier) way. :slight_smile:

The reason I asked is that sometimes it is easier for me to think in hex
when creating masks for logical operations. Plus, I learned the PCL
codes for printing using hex and it is easier for me to use them that
way. In Business Basic to enter a group of hex characters into a string
I just use A$=$1B2C062C$ to create a 4 character string. No need to
enter each character individually.

Then #pack and #unpack are your friends (see Adam's reply). You could
easily put that in a method if you want a more concise representation,
e.g.

def h(s)
  [s.to_str].pack 'H*'
end

irb(main):005:0> h '212223'
=> "!\"#"
irb(main):006:0> h '414243'
=> "ABC"
irb(main):010:0> h %Q{414243}
=> "ABC"
irb(main):011:0> h %q{414243}
=> "ABC"

Note also, that you can use hex constants in Ruby:

irb(main):001:0> 0x7b
=> 123

but it seems you are more after construction of strings.

Kind regards

robert

···

2008/1/28, Michael W. Ryder <_mwryder@worldnet.att.net>:

--
use.inject do |as, often| as.you_can - without end

6

Adam Shelly wrote:

···

On 1/28/08, Michael W. Ryder <_mwryder@worldnet.att.net> wrote:

Robert Klemme wrote:

2008/1/28, Michael W. Ryder <_mwryder@worldnet.att.net>:

On a related note, is there any way to create a string from a hex number
other than separating each two digits and using .chr to convert them to
a number and adding them to the string? If I have a string such as
0x7FFFFFFFFFFF I have to use something like str1 = 0x7F.chr followed by
5 str1 << 0xFF.chr which seems to be unnecessarily clumsy.

The reason I asked is that sometimes it is easier for me to think in hex
when creating masks for logical operations. Plus, I learned the PCL
codes for printing using hex and it is easier for me to use them that
way. In Business Basic to enter a group of hex characters into a string
I just use A$=$1B2C062C$ to create a 4 character string. No need to
enter each character individually.

Are you looking for something like this?

irb(main):010:0> ["414243"].pack('H*')
=> "ABC"
irb(main):011:0> ["7FFFFFFFFFFF"].pack('H*')
=> "\177\377\377\377\377\377"
irb(main):012:0> A=['1B2C062C'].pack('H*')
=> "\e,\006,"

-Adam

That is much better, I will have to read up on pack and template strings now but at least I have a starting point. It's not as concise as what I am used to but it is usable. Thank you for pointing this out.