C++ Constant for PI

Where's the definition for pi? Doesn't C++ provide a constant? What about C?

The C++ standard doesn't provide a value for pi but it's simple to define yourself:

#include <iostream>
#include <cmath> // M_PI is not standard

using namespace std;

class MathConst {
  public:
    static const long double PI;
};

const long double MathConst::PI = acos((long double) -1);

int main() {
  cout.precision(100);
  cout << "PI ~ " << MathConst::PI << endl;
}

Output:

$ g++ MathConst.cpp -o pi
$ ./pi 
PI ~ 3.14159265358979323851280895940618620443274267017841339111328125

That is accurate to 18 decimal places.

Here's a definition in C:

#include <math.h>
#include <stdio.h>

double pi() {
  const double pi = acos((double) - 1);
  return pi;
}

int main() {
  printf("%.20f\n", pi());
  return 0;
}

Postgresql: Indexes on Foreign Keys

This query identifies foreign keys that are potentially missing indexes (Postgresql does not create indexes on foreign keys automatically).

/*
  Look for foreign key constraints that are missing indexes on the
  referencing table.

  Orders results by the size of the referencing table, largest first,
  on the assumption that, all else being equal, they are the most likely
  to benefit from the addition of indexes.

  This is only meant as a starting point, and isn't perfect.
  It's possible, for example, that it will report a missing index
  when in fact one is available. e.g., it won't realize that an index on
  (f1, f2) could be used with a fk on (f1). However, it will recognize
  that an index on (f1, f2) can be used with a fk on (f2, f1).

  Usage: psql -q dbname -f pg-find-missing-fk-indexes.sql
*/

CREATE FUNCTION pg_temp.sortarray(int2[]) returns int2[] as '
  SELECT ARRAY(
      SELECT $1[i]
        FROM generate_series(array_lower($1, 1), array_upper($1, 1)) i
    ORDER BY 1
  )
' language sql;

  SELECT conrelid::regclass
         ,conname
         ,reltuples::bigint
    FROM pg_constraint
         JOIN pg_class ON (conrelid = pg_class.oid)
   WHERE contype = 'f'
         AND NOT EXISTS (
           SELECT 1
             FROM pg_index
            WHERE indrelid = conrelid
                  AND pg_temp.sortarray(conkey) = pg_temp.sortarray(indkey)
         )
ORDER BY reltuples DESC
;

Linux: Download YouTube Videos

Install the youtube-dl script. With Ubuntu:

sudo apt-get install youtube-dl

Next, find the YouTube video you want to download and pass it to the script:

youtube-dl -b -t "http://www.youtube.com/watch?v=aEXFUbSbg1I"

Command-line options:

Usage: youtube-dl [options] video_url

Options:
  -h, --help            print this help text and exit
  -v, --version         print program version and exit
  -u USERNAME, --username=USERNAME
                        account username
  -p PASSWORD, --password=PASSWORD
                        account password
  -o FILE, --output=FILE
                        output video file name
  -q, --quiet           activates quiet mode
  -s, --simulate        do not download video
  -t, --title           use title in file name
  -l, --literal         use literal title in file name
  -n, --netrc           use .netrc authentication data
  -g, --get-url         print final video URL only
  -2, --title-too       used with -g, print title too
  -f FORMAT, --format=FORMAT
                        append &fmt=FORMAT to the URL
  -b, --best-quality    alias for -f 18

See also: youtube-dl Homepage