euler/ipython/EulerProblem035.ipynb

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  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Euler Problem 35\n",
    "\n",
    "The number, 197, is called a circular prime because all rotations of the digits: 197, 971, and 719, are themselves prime.\n",
    "\n",
    "There are thirteen such primes below 100: 2, 3, 5, 7, 11, 13, 17, 31, 37, 71, 73, 79, and 97.\n",
    "\n",
    "How many circular primes are there below one million?"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "First we get all primes into a look-up table. Then we iterate them and check whether they are circular."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "def get_primes_smaller(number):\n",
    "    primes = []\n",
    "    prospects = [n for n in range(2, number)]\n",
    "    while prospects:\n",
    "        p = prospects[0]\n",
    "        prospects = [x for x in prospects if x % p != 0]\n",
    "        primes.append(p)\n",
    "        if p * p > number:\n",
    "            return primes + prospects\n",
    "    return primes\n",
    "\n",
    "ps = get_primes_smaller(1000000)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "def get_combinations(xs):\n",
    "    if not xs:\n",
    "        return []\n",
    "    rs = []\n",
    "    for i in range(len(xs)):\n",
    "        yss = get_combinations(xs[:i] + xs[i + 1:])\n",
    "        if not yss:\n",
    "            rs.append(xs[i])\n",
    "        for ys in yss:\n",
    "            rs.append(xs[i] + ys)\n",
    "    return rs\n",
    "\n",
    "assert(get_combinations(\"ab\") == [\"ab\", \"ba\"])"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "from itertools import permutations\n",
    "prime_set = set(ps)\n",
    "\n",
    "def is_circular(p):\n",
    "    cs = permutations(str(p))\n",
    "    for c in cs:\n",
    "        if not int(\"\".join(c)) in prime_set:\n",
    "            return False\n",
    "    return True\n",
    "\n",
    "assert(is_circular(\"2\") == True)\n",
    "assert(is_circular(\"11\") == True)\n",
    "assert(is_circular(\"47\") == False)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "False solution 22\n"
     ]
    }
   ],
   "source": [
    "s = len([p for p in ps if is_circular(p)])\n",
    "print(\"False solution {}\".format(s))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "We did not read the problem properly. Cycles are obviously not the same as permutations. If we change that we should  get the solution in no time."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "def cyles(xs):\n",
    "    if len(xs) <= 1:\n",
    "        return xs\n",
    "    return [xs[i:] + xs[:i]  for i in range(len(xs))]\n",
    "\n",
    "prime_set = set(ps)\n",
    "\n",
    "def is_circular(p):\n",
    "    cs = cyles(str(p))\n",
    "    for c in cs:\n",
    "        if not int(\"\".join(c)) in prime_set:\n",
    "            return False\n",
    "    return True"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "55"
      ]
     },
     "execution_count": 6,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "s = len([p for p in ps if is_circular(p)])\n",
    "s"
   ]
  }
 ],
 "metadata": {
  "completion_date": "Tue, 13 Feb 2018, 09:14",
  "kernelspec": {
   "display_name": "Python 3",
   "language": "python",
   "name": "python3"
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  "language_info": {
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  "tags": [
   "circular",
   "combinations"
  ]
 },
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