{
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  {
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   "source": [
    "# Euler Problem 2\n",
    "\n",
    "Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:\n",
    "\n",
    "1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...\n",
    "\n",
    "By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "def fibonacci_generator():\n",
    "    a = 0\n",
    "    b = 1\n",
    "    while True:\n",
    "        yield a + b\n",
    "        a, b = b, (a + b)\n",
    "        "
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Test fibonacci sequence generator by comparing the result to the example in the task statement."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "i = fibonacci_generator()\n",
    "fib_10 = [next(i) for _ in range(10)]\n",
    "assert(fib_10 == [1, 2, 3, 5, 8, 13, 21, 34, 55, 89,])"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Greate a function which returns all even-valued fibonacci values smaller or equal to four million."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "def get_even_fibonaccis_smaller_or_equal_four_million():\n",
    "    r = []\n",
    "    i = fibonacci_generator()\n",
    "    current_value = next(i)\n",
    "    while current_value <= 4000000:\n",
    "        if current_value % 2 == 0:\n",
    "            r.append(current_value)\n",
    "        current_value = next(i)\n",
    "    return r"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Calculate the solution."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "4613732\n"
     ]
    }
   ],
   "source": [
    "f = get_even_fibonaccis_smaller_or_equal_four_million()\n",
    "print(sum(f))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "I looked at the solutions in the forum and I kind of forgot about simple straight forward approaches. There is no need to create a list and the sum it up. Instead I can simply increment a counter which will be much faster, but less readable maybe."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "4613732\n"
     ]
    }
   ],
   "source": [
    "r, a, b = 0, 0, 1\n",
    "\n",
    "while b <= 4000000:\n",
    "    if b % 2 == 0:\n",
    "        r += b\n",
    "    a, b = b, a + b\n",
    "    \n",
    "print(r)"
   ]
  }
 ],
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  "completion_date": "Tue, 19 Aug 2014, 20:36",
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