{
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  {
   "cell_type": "code",
   "execution_count": 1,
   "id": "a9165db4",
   "metadata": {},
   "outputs": [],
   "source": [
    "import numpy as np"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "9f44a5bd",
   "metadata": {},
   "source": [
    "### Make some arrays that hold star temp and mass"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "id": "1079a811",
   "metadata": {},
   "outputs": [],
   "source": [
    "# set up arrays holding temperature and sizes of some stars\n",
    "temp = np.array([  3000,   8000,  6000,  4000, 9000,  3500,  7000,  10000])\n",
    "mass = np.array([ 0.514,  2.015,  1.05,  0.75,  2.5,  2.03,  1.51,   0.50])"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "9b002dcd",
   "metadata": {},
   "source": [
    "### Define a boolean flag that defines hot stars to be hotter than the Sun"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "id": "80f5f92c",
   "metadata": {
    "scrolled": true
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[ 3000  8000  6000  4000  9000  3500  7000 10000]\n",
      "[False  True  True False  True False  True  True]\n"
     ]
    }
   ],
   "source": [
    "Tsun = 5770\n",
    "hot = temp > Tsun\n",
    "print(temp)\n",
    "print(hot)"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "44d17740",
   "metadata": {},
   "source": [
    "### Work out how many stars there are, and how many of them are hot"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "id": "c62e9a43",
   "metadata": {
    "scrolled": true
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "There are 8 stars\n",
      "There are 5 hot stars\n"
     ]
    }
   ],
   "source": [
    "# len() tells you how many elements are in an array\n",
    "print('There are {} stars'.format(len(temp)))\n",
    "\n",
    "# but you can also sum a boolean array to figure out how many satisfy the condition\n",
    "print('There are {} hot stars'.format(np.sum(hot)))"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "4310ffd2",
   "metadata": {},
   "source": [
    "### Work out average mass of all stars, and average mass of just the hot stars"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "id": "70db1d7f",
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "The average mass of ALL stars is 1.36 Msun\n",
      "The average mass of HOT stars is 1.52 Msun\n",
      "The average mass of HOT stars is 1.5150000000000001 Msun  <--- yuck!\n"
     ]
    }
   ],
   "source": [
    "# this averages all the star masses in the array\n",
    "avmass_all = np.average(mass)\n",
    "# this averages the star mass of just the stars with hot temps: temp[hot]\n",
    "avmass_hot = np.average(mass[hot])\n",
    "\n",
    "# print out the average mass, notice how I make sure not to print out tons of digits!\n",
    "print('The average mass of ALL stars is {:.2f} Msun'.format(avmass_all))\n",
    "print('The average mass of HOT stars is {:.2f} Msun'.format(avmass_hot))\n",
    "\n",
    "# if I had omitted the ':.2f' it would have looked terrible:\n",
    "print('The average mass of HOT stars is {} Msun  <--- yuck!'.format(avmass_hot))\n"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "66791946",
   "metadata": {},
   "source": [
    "### Figure out which stars are more massive than the Sun"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "id": "19584f6a",
   "metadata": {
    "scrolled": true
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[False  True  True False  True  True  True False]\n",
      "There are 5 massive stars\n"
     ]
    }
   ],
   "source": [
    "bigmass = mass > 1\n",
    "print(bigmass)\n",
    "print('There are {} massive stars'.format(np.sum(bigmass)))"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "9041a55d",
   "metadata": {},
   "source": [
    "### Stacking boolean flags"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "id": "1402ec66",
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "There are 4 hot main sequence stars\n"
     ]
    }
   ],
   "source": [
    "# a hot main sequence star should be hotter than the Sun and more massive than the Sun\n",
    "hot_ms = np.logical_and(hot, bigmass)\n",
    "print('There are {} hot main sequence stars'.format(np.sum(hot_ms)))"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "c5f77e84",
   "metadata": {},
   "source": [
    "### Using the '~' modifier"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "id": "3be5ec74",
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "There are 1 white dwarf stars\n"
     ]
    }
   ],
   "source": [
    "# with a boolean flag, a '~' in front means 'not'\n",
    "# so white dwarfs are stars that are hot but that are not big mass\n",
    "wd = np.logical_and(hot, ~bigmass)\n",
    "\n",
    "print('There are {} white dwarf stars'.format(np.sum(wd)))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "id": "5eb637f1",
   "metadata": {},
   "outputs": [],
   "source": []
  }
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