Python/Lambda Functions/Python Lambda Functions.ipynb

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   "cell_type": "markdown",
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   "source": [
    "# Python Lambda Functions\n",
    "Anonymous, single-use, or throw-away functions.  \n",
    "**lambda arguments : expression**  \n",
    "Here are some single-argument lambdas:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 59,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "12\n"
     ]
    }
   ],
   "source": [
    "add5 = lambda x: x + 5\n",
    "print(add5(7))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 58,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "64\n"
     ]
    }
   ],
   "source": [
    "square = lambda x: x * x\n",
    "print(square(8))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 67,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "4\n",
      "3\n"
     ]
    }
   ],
   "source": [
    "get_tens = lambda p: int(p/10)%10\n",
    "print(get_tens(749))\n",
    "print(get_tens(836.21))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**Lambdas as an argument in other functions**   \n",
    "One of the most popular uses for lambda functions is as an argument inside sort, or filter functions.  \n",
    "### Sorting a List of Tuples using Lambda"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 80,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[('carrots', 1.1), ('peaches', 2.45), ('eggs', 5.25), ('honey', 9.7)]\n"
     ]
    }
   ],
   "source": [
    "list1 = [('eggs', 5.25), ('honey', 9.70), ('carrots', 1.10), ('peaches', 2.45)]\n",
    "list1.sort(key = lambda x: x[1])\n",
    "print(list1)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Sorting a List of Dictionaries using Lambda"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 63,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[{'make': 'Tesla', 'model': 'X', 'year': 1999},\n",
      " {'make': 'Mercedes', 'model': 'C350E', 'year': 2008},\n",
      " {'make': 'Ford', 'model': 'Focus', 'year': 2013}]\n"
     ]
    }
   ],
   "source": [
    "import pprint as pp\n",
    "list1 = [{'make':'Ford', 'model':'Focus', 'year':2013}, {'make':'Tesla', 'model':'X', 'year':1999}, {'make':'Mercedes', 'model':'C350E', 'year':2008}]\n",
    "list2 = sorted(list1, key = lambda x: x['year'])\n",
    "pp.pprint(list2)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Filtering a List of Integers using Lambda"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 64,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[2, 4, 6]\n"
     ]
    }
   ],
   "source": [
    "list1 = [1, 2, 3, 4, 5, 6]\n",
    "list2 = list(filter(lambda x: x%2 == 0, list1))\n",
    "print(list2)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 49,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[1, 3, 5]\n"
     ]
    }
   ],
   "source": [
    "odds = lambda x: x%2 == 1\n",
    "list1 = [1, 2, 3, 4, 5, 6]\n",
    "list2 = list(filter(odds, list1))\n",
    "print(list2)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Lambda Function on a List using Map\n",
    "Python's map function applies the lambda to every element in the list."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 74,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[1, 4, 9, 16, 25, 36]\n"
     ]
    }
   ],
   "source": [
    "list1 = [1, 2, 3, 4, 5, 6]\n",
    "list2 = list(map(lambda x: x ** 2, list1))\n",
    "print(list2)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Lambda Conditionals\n",
    "**lambda args: a if boolean_expression else b**  "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 70,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "True\n"
     ]
    }
   ],
   "source": [
    "starts_with_J = lambda x: True if x.startswith('J') else False\n",
    "print(starts_with_J('Joey'))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 81,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "and\n"
     ]
    }
   ],
   "source": [
    "wordb4 = lambda s, w: s.split()[s.split().index(w)-1] if w in s else None\n",
    "sentence = 'Four score and seven years ago'\n",
    "print(wordb4(sentence, 'seven'))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Lambdas on DataTime Objects\n",
    "You sometimes want to get just the year, month, date or time for comparision.  \n",
    "This would typically be most useful as a parameter in sort or filter functions."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 82,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "2019-03-07 19:36:58.442863\n",
      "2019\n"
     ]
    }
   ],
   "source": [
    "import datetime\n",
    "\n",
    "now = datetime.datetime.now()\n",
    "print(now)\n",
    "year = lambda x: x.year\n",
    "print(year(now))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 79,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "4096\n",
      "125\n"
     ]
    }
   ],
   "source": [
    "def do_something(f, val):\n",
    "    return f(val)\n",
    "\n",
    "func = lambda x: x**3\n",
    "print(func(16))\n",
    "print(do_something(func, 5))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Extreme Lambdas\n",
    "This is probably a stretch -- you shouldn't be trying to do this much with Lambdas.   \n",
    "Some things are better done in a regular function. But this shows what's possible with Lambdas."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 66,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "True\n",
      "True\n",
      "False\n",
      "False\n",
      "True\n",
      "-1\n",
      "-21.67 <class 'float'>\n"
     ]
    }
   ],
   "source": [
    "isnum = lambda q: q.replace('.','',1).isdigit()\n",
    "print(isnum('25983'))\n",
    "print(isnum('3.1415'))\n",
    "print(isnum('T57'))\n",
    "print(isnum('-16'))\n",
    "\n",
    "is_num = lambda r: isnum(r[1:]) if r[0]=='-' else isnum(r)\n",
    "print(is_num('-16.4'))\n",
    "\n",
    "tonum = lambda s: float(s) if is_num(s) else -1\n",
    "print(tonum('30y'))\n",
    "print(tonum('-21.67'), type(tonum('-21.67')))"
   ]
  }
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