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Full Discussion: Incrementing with a twist - please help
Top Forums Shell Programming and Scripting Incrementing with a twist - please help Post 302400821 by alister on Thursday 4th of March 2010 07:42:10 AM
Old 03-04-2010
Hi, rdcwayx

It's not the eval which generates the data; it's brace expansion.

Code:
#Non-numeric example
$ echo f{ee,i,o,um}
fee fi fo fum

#Numeric example
$ echo {0,1}{0,1}{0,1}
000 001 010 011 100 101 110 111

Since the original poster's problem requires a fairly long sequence of digits, for brevity I wanted to avoid:
Code:
echo {0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z}{0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F,G,H,etc.....

Storing the list in the variable $digits allows me to do that. However, since brace expansion occurs before variable expansion, we need to send the command line through the shell's parser a second time, using eval. Otherwise we just get:
Code:
$ digits=0,1
$ echo {$digits}{$digits}{$digits}
{0,1}{0,1}{0,1}

Instead of:
Code:
$ eval echo {$digits}{$digits}{$digits}
000 001 010 011 100 101 110 111

Note: Brace expansion does not occur if there is no unquoted comma within the braces, so we do not need to quote {$digits}.

A downside to this approach is that if the list of digits is long enough, or if there are many braced terms to expand, the list of resulting words could exceed the command line length limit. In such a case, your approach is superior as it never needs to execute a long command line. Another way around this limitation would be to write it out explicitly in the for loop:
Code:
$ for i in {0,1}{0,1}{0,1}; do echo $i; done
000
001
010
011
100
101
110
111

IMPORTANT: Brace expansion is not a posix-standardized feature, so if that's a concern, my approach is not an option; yours would be the better choice. Also, the order of brace expansion in relation to parameter expansion as described above is BASH specific (which I chose to use for the illustrative examples since it's much more popular than ksh, and because for this particular scenario it's the more complicated case). In pdksh, ksh88, and ksh93, brace expansion occurs after parameter expansion, not before. So, if using a ksh variant, neither the eval nor the echo nor the command substition is necessary (though in this case their presence does not affect the end result); a simple 'for i in {$digits}{$digits}{$digits}' would be sufficient.

Hope that helps. If not, feel free to ask for clarification.

Cheers,
Alister
Last edited by alister; 03-04-2010 at 09:44 AM..
 

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LEARN ABOUT PHP

dateinterval.format

DATEINTERVAL.FORMAT(3)							 1						    DATEINTERVAL.FORMAT(3)

DateInterval::format - Formats the interval

SYNOPSIS

       public string DateInterval::format (string  $format)

DESCRIPTION

	Formats the interval.

PARAMETERS

	      o $format
		-

	       The following characters are recognized in the $format parameter string. Each format character must be prefixed by a percent sign (
	      %).

	      +------------------+--------------------------------------+---+
	      |$format character |					|   |
	      | 		 |					|   |
	      | 		 |	       Description		|   |
	      | 		 |					|   |
	      | 		 |	      Example values		|   |
	      | 		 |					|   |
	      +------------------+--------------------------------------+---+
	      | 		 |					|   |
	      |        %	 |					|   |
	      | 		 |					|   |
	      | 		 |		Literal %		|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      | 		 |		    %			|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      |        Y	 |					|   |
	      | 		 |					|   |
	      | 		 | Years, numeric, at  least  2  digits |   |
	      | 		 | with leading 0			|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      | 		 |		  01, 03		|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      |        y	 |					|   |
	      | 		 |					|   |
	      | 		 |	      Years, numeric		|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      | 		 |		   1, 3 		|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      |        M	 |					|   |
	      | 		 |					|   |
	      | 		 | Months,  numeric,  at least 2 digits |   |
	      | 		 | with leading 0			|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      | 		 |		01, 03, 12		|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      |        m	 |					|   |
	      | 		 |					|   |
	      | 		 |	     Months, numeric		|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      | 		 |		 1, 3, 12		|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      |        D	 |					|   |
	      | 		 |					|   |
	      | 		 | Days, numeric,  at  least  2  digits |   |
	      | 		 | with leading 0			|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      | 		 |		01, 03, 31		|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      |        d	 |					|   |
	      | 		 |					|   |
	      | 		 |	      Days, numeric		|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      | 		 |		 1, 3, 31		|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      |        a	 |					|   |
	      | 		 |					|   |
	      | 		 | Total  number of days as a result of |   |
	      | 		 | a DateTime::diff or (unknown) other- |   |
	      | 		 | wise 				|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      | 		 |	       4, 18, 8123		|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      |        H	 |					|   |
	      | 		 |					|   |
	      | 		 | Hours,  numeric,  at  least 2 digits |   |
	      | 		 | with leading 0			|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      | 		 |		01, 03, 23		|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      |        h	 |					|   |
	      | 		 |					|   |
	      | 		 |	      Hours, numeric		|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      | 		 |		 1, 3, 23		|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      |        I	 |					|   |
	      | 		 |					|   |
	      | 		 | Minutes, numeric, at least 2  digits |   |
	      | 		 | with leading 0			|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      | 		 |		01, 03, 59		|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      |        i	 |					|   |
	      | 		 |					|   |
	      | 		 |	     Minutes, numeric		|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      | 		 |		 1, 3, 59		|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      |        S	 |					|   |
	      | 		 |					|   |
	      | 		 | Seconds,  numeric, at least 2 digits |   |
	      | 		 | with leading 0			|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      | 		 |		01, 03, 57		|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      |        s	 |					|   |
	      | 		 |					|   |
	      | 		 |	     Seconds, numeric		|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      | 		 |		 1, 3, 57		|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      |        R	 |					|   |
	      | 		 |					|   |
	      | 		 | Sign " -" when negative, "  +"  when |   |
	      | 		 | positive				|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      | 		 |		   -, + 		|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      |        r	 |					|   |
	      | 		 |					|   |
	      | 		 | Sign  " -" when negative, empty when |   |
	      | 		 | positive				|   |
	      | 		 |					|   |
	      | 		 |					|   |
	      | 		 |		    -,			|   |
	      | 		 |					|   |
	      +------------------+--------------------------------------+---+
RETURN VALUES

	Returns the formatted interval.

NOTES

       Note

	       The DateInterval::format method does not recalculate carry over points in time strings nor  in  date  segments.	This  is  expected
	      because  it is not possible to overflow values like "32 days" which could be interpreted as anything from "1 month and 4 days" to "1
	      month and 1 day".

EXAMPLES

       Example #1

	      DateInterval example

	      <?php

	      $interval = new DateInterval('P2Y4DT6H8M');
	      echo $interval->format('%d days');

	      ?>

	      The above example will output:

	      4 days

       Example #2

	      DateInterval and carry over points

	      <?php

	      $interval = new DateInterval('P32D');
	      echo $interval->format('%d days');

	      ?>

	      The above example will output:

	      32 days

       Example #3

	      DateInterval and DateTime::diff with the %a and %d modifiers

	      <?php

	      $january = new DateTime('2010-01-01');
	      $february = new DateTime('2010-02-01');
	      $interval = $february->diff($january);

	      // %a will output the total number of days.
	      echo $interval->format('%a total days')."
";

	      // While %d will only output the number of days not already covered by the
	      // month.
	      echo $interval->format('%m month, %d days');

	      ?>

	      The above example will output:

	      31 total days
	      1 month, 0 days

SEE ALSO

       DateTime::diff.

PHP Documentation Group 												    DATEINTERVAL.FORMAT(3)
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