# Best way to create a string with gap (0x00's) in the middle?

**URL:** <https://www.ruby-forum.com/t/best-way-to-create-a-string-with-gap-0x00s-in-the-middle/92824>\
**Category:** Ruby\
**Created:** [April 24, 2007, 5:47pm UTC](https://www.ruby-forum.com/t/best-way-to-create-a-string-with-gap-0x00s-in-the-middle/92824 "2007-04-24T17:47:39Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![eclubb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/ecc23a/32.png) [@eclubb](https://www.ruby-forum.com/u/eclubb)\
**Post date:** [April 24, 2007, 5:47pm UTC](https://www.ruby-forum.com/t/best-way-to-create-a-string-with-gap-0x00s-in-the-middle/92824/1 "2007-04-24T17:47:39Z")

</div>

I’m trying to build a packet to send to UDPSocket::send. Send requires  
a string, but I’m not sure of the best way to build this string. The  
packet’s structure requires two strings: one beginning at byte 0 and the  
other at byte 64. The packet is 128 bytes long and must contain 0’s in  
the bytes that the strings don’t use. Here is an example packet:

54 68 69 73 20 69 73 20 73 74 72 69 6E 67 20 41 This is string A  
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 …  
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 …  
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 …  
4E 6F 77 20 77 65 20 68 61 76 65 20 73 74 72 69 Now we have stri  
6E 67 20 42 00 00 00 00 00 00 00 00 00 00 00 00 ng B…  
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 …  
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 …

I have a solution, but it’s not very elegant and I’m looking for a  
better way of doing this. Here’s what I have so far:

I tried this first, but ended up with a bunch of 0x30’s where the 0x00’s  
should be:

dgram = Array.new(128, 0)  
dgram[0] = “This is string A”.to\_a  
dgram[64] = “Now we have string B”.to\_a  
UDPSocket.open.send(dgram.to\_s, 0, host, port)

dgram.to\_s converts all of the 0x00’s in the array to 0x30’s in the  
string, which is not acceptable.

This is what I have right now:

dgram = “”  
94.times {dgram \<\< 0}  
dgram[0] = “This is string A”  
dgram[64] = “Now we have string B”  
UDPSocket.open.send(dgram, 0, host, port)

Like I said: it works, but I don’t really like it. Any ideas for a  
better implementation?

Thanks,  
Earle

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<div class="post-metadata">

**Author:** ![eclubb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/ecc23a/32.png) [@eclubb](https://www.ruby-forum.com/u/eclubb)\
**Post date:** [April 24, 2007, 6:02pm UTC](https://www.ruby-forum.com/t/best-way-to-create-a-string-with-gap-0x00s-in-the-middle/92824/2 "2007-04-24T18:02:25Z")

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Earle C. wrote:

> 4E 6F 77 20 77 65 20 68 61 76 65 20 73 74 72 69 Now we have stri  
> dgram = Array.new(128, 0)  
> dgram = “”

BLANK\_DGRAM = “\0”\*128  
dgram = BLANK\_DGRAM.dup

s1 = “This is string A”  
s2 = “Now we have string B”

dgram[0…s1.length] = s1  
dgram[64…64+s2.length] = s2

If you think you might need to fill in other kinds of fields besides  
fixed-length strings, you could use a BitStruct:

require ‘bit-struct’ # [BitStruct](http://redshift.sourceforge.net/bit-struct)

class Dgram \< BitStruct  
char :s1, 64_8  
char :s2, 64_8  
end

dgram = Dgram.new  
dgram.s1 = “This is string A”  
dgram.s2 = “Now we have string B”

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<div class="post-metadata">

**Author:** ![eclubb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/ecc23a/32.png) [@eclubb](https://www.ruby-forum.com/u/eclubb)\
**Post date:** [April 24, 2007, 6:16pm UTC](https://www.ruby-forum.com/t/best-way-to-create-a-string-with-gap-0x00s-in-the-middle/92824/3 "2007-04-24T18:16:31Z")

</div>

On 24.04.2007 17:47, Earle C. wrote:

> 4E 6F 77 20 77 65 20 68 61 76 65 20 73 74 72 69 Now we have stri  
> dgram = Array.new(128, 0)  
> dgram = “”  
> 94.times {dgram \<\< 0}  
> dgram[0] = “This is string A”  
> dgram[64] = “Now we have string B”  
> UDPSocket.open.send(dgram, 0, host, port)
> 
> Like I said: it works, but I don’t really like it. Any ideas for a  
> better implementation?

> > %w{foo bar}  
> > =\> [“foo”, “bar”]
> 
> > %w{foo bar}.pack(“a64a64”)  
> > =\>  
> > “foo\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000  
> > 000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000bar\000\000\000\000\000\000\0  
> > 00\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\0  
> > 00\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000”

Or, printed more nicely:

> > %w{foo bar}.pack(“a64a64”).to\_enum(:each\_byte).each\_slice(16) {|s|  
> > puts s.map {|x| “%02x” % x}.join(" ")}  
> > 66 6f 6f 00 00 00 00 00 00 00 00 00 00 00 00 00  
> > 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  
> > 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  
> > 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  
> > 62 61 72 00 00 00 00 00 00 00 00 00 00 00 00 00  
> > 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  
> > 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  
> > 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  
> > =\> nil

Kind regards

robert