{
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  {
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   "source": [
    "# NumPy: Structured arrays"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "3b2e2335",
   "metadata": {},
   "source": [
    "## Introduction\n",
    "Structured arrays are ndarrays whose datatype is a composition of simpler datatypes organized as a sequence of named [`fields`](https://numpy.org/doc/stable/glossary.html#term-field)."
   ]
  },
  {
   "cell_type": "markdown",
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   "source": [
    "```python\n",
    ">>> x = np.array([('Rex', 9, 81.0), ('Fido', 3, 27.0)],\n",
    "...             dtype=[('name', 'U10'), ('age', 'i4'), ('weight', 'f4')])\n",
    ">>> x\n",
    "array([('Rex', 9, 81.), ('Fido', 3, 27.)],\n",
    "      dtype=[('name', '<U10'), ('age', '<i4'), ('weight', '<f4')])\n",
    "```"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "3524ee07",
   "metadata": {},
   "source": [
    "See how `dtype` is used [here](https://numpy.org/doc/stable/reference/arrays.dtypes.html#specifying-and-constructing-data-types). The prototype of this `dtype` construction is `[(field_name, field_type, field_shape), ...]`, `field_shape` is optional."
   ]
  },
  {
   "cell_type": "markdown",
   "id": "8abcb2ce",
   "metadata": {},
   "source": [
    "Here `x` is a 1-dimensional array with 2 rows, 3 columns, and the corresponding 3 fields for each column: 1. A string of length 10 or less named `name`, 2. a 32-bit integer named `age`, 3. a 32-bit floating-point number named `weight`"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "8eebf167",
   "metadata": {},
   "source": [
    "```python\n",
    ">>> x[1]\n",
    "np.void(('Fido', 3, 27.0), dtype=[('name', '<U10'), ('age', '<i4'), ('weight', '<f4')])\n",
    "```"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "b45573fc",
   "metadata": {},
   "source": [
    "You can access and modify individual fields/items by indexing the field name, like python `dict`"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "9feb21e8",
   "metadata": {},
   "source": [
    "```python\n",
    ">>> x['age']\n",
    "array([9, 3], dtype=int32)\n",
    ">>> x['age'] = 5\n",
    ">>> x\n",
    "array([('Rex', 5, 81.), ('Fido', 5, 27.)],\n",
    "      dtype=[('name', '<U10'), ('age', '<i4'), ('weight', '<f4')])\n",
    ">>> x['age'][x['name'] == 'Rex'] = 9\n",
    ">>> x\n",
    "array([('Rex', 9, 81.), ('Fido', 5, 27.)],\n",
    "      dtype=[('name', '<U10'), ('age', '<i4'), ('weight', '<f4')])\n",
    "```"
   ]
  }
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