numpy/3 Access elements + slicing.ipynb

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   "source": [
    "import numpy as np"
   ]
  },
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   "metadata": {},
   "source": [
    "## Accessing and changing \n",
    "- elements\n",
    "- rows\n",
    "- columns "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [
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     "data": {
      "text/plain": [
       "(2, 7)"
      ]
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     "execution_count": 2,
     "metadata": {},
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   ],
   "source": [
    "a = np.array([[1, 2, 3, 4, 5, 6, 7], [8, 9, 10, 11, 12, 13, 14]])\n",
    "a.shape"
   ]
  },
  {
   "attachments": {},
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Access an element"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
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     "text": [
      "13\n"
     ]
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      "text/plain": [
       "13"
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     "execution_count": 3,
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   ],
   "source": [
    "# get a specific element [row, column]\n",
    "print(a[1, 5])\n",
    "a[1, -2]"
   ]
  },
  {
   "attachments": {},
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "----\n",
    "# Slicing"
   ]
  },
  {
   "attachments": {},
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Access rows and columns"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "array([ 3, 10])"
      ]
     },
     "execution_count": 4,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "# get a specific row and column\n",
    "a[0, :]  # row\n",
    "a[:, 2]  # column"
   ]
  },
  {
   "attachments": {},
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## How to slice\n",
    "### [startindex : endindex : stepsize]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[[ 0  1  2  3  4  5  6  7  8  9]\n",
      " [10 11 12 13 14 15 16 17 18 19]] \n",
      "\n"
     ]
    },
    {
     "data": {
      "text/plain": [
       "array([1, 3, 5, 7])"
      ]
     },
     "execution_count": 5,
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   ],
   "source": [
    "a = np.arange(20).reshape(2, -1)\n",
    "print(a, \"\\n\")\n",
    "\n",
    "# 0th row, from 1th column upto last_column (not included)\n",
    "a[0, 1:-1:2]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "array([1, 3, 5, 7])"
      ]
     },
     "execution_count": 6,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "# creating a slice variable\n",
    "s = slice(1, -1, 2)\n",
    "a[0, s]"
   ]
  },
  {
   "attachments": {},
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "----"
   ]
  },
  {
   "attachments": {},
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Changing elements and rows and columns\n"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "array([[ 100,   21,   22,   23,   24,   25,   26],\n",
       "       [ 100,    9,   10,   11,   12, 1000,   14]])"
      ]
     },
     "execution_count": 7,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "a = np.array([[1, 2, 3, 4, 5, 6, 7], [8, 9, 10, 11, 12, 13, 14]])\n",
    "\n",
    "# change entire row\n",
    "a[0, :] = [20, 21, 22, 23, 24, 25, 26]\n",
    "\n",
    "# change entire column\n",
    "a[:, 0] = 100\n",
    "# note that if we give it a single value (not an array of appropriate size), all elements are set to tht particular value\n",
    "\n",
    "a[1, 5] = 1000  # change element\n",
    "a"
   ]
  },
  {
   "attachments": {},
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "----\n",
    "### 3D - example"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "array([[[1, 2],\n",
       "        [3, 4]],\n",
       "\n",
       "       [[5, 6],\n",
       "        [7, 8]]])"
      ]
     },
     "execution_count": 8,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "b = np.array([[[1, 2], [3, 4]], [[5, 6], [7, 8]]])\n",
    "b"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "6"
      ]
     },
     "execution_count": 9,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "# Get specific element - work outside in\n",
    "b[1, 0, 1]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "array([[[ 1,  2],\n",
       "        [10, 20]],\n",
       "\n",
       "       [[ 5,  6],\n",
       "        [30, 40]]])"
      ]
     },
     "execution_count": 10,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "# Replace in 3D\n",
    "b[:, 1, :] = [[10, 20], [30, 40]]\n",
    "b"
   ]
  }
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