finish reduce

Author: rmccorm4

built_in_functions/.ipynb_checkpoints/reduce-checkpoint.ipynb

@@ -5,161 +5,147 @@
    "metadata": {},
    "source": [
     "# reduce()\n",
-    "As the name suggests, filter creates a list of elements for which a function returns true. Here is a simple example:"
+    "\n",
+    "`reduce(function_to_apply, list_to_apply_to)` is a really useful function for performing some computation on a list and returning the result. It applies a rolling computation to sequential pairs of values in a list. For example, if you wanted to compute the product of a list of integers.\n",
+    "\n",
+    "So the normal way you might go about doing this task in python is using a basic for loop:"
    ]
   },
   {
    "cell_type": "code",
-   "execution_count": 13,
+   "execution_count": 4,
    "metadata": {},
    "outputs": [
     {
      "name": "stdout",
      "output_type": "stream",
      "text": [
-      "[-5, -4, -3, -2, -1, 0, 1, 2, 3, 4]\n",
-      "[-5, -4, -3, -2, -1]\n"
+      "24\n"
      ]
     }
    ],
    "source": [
-    "def negative(x):\n",
-    "    return x < 0\n",
-    "\n",
-    "number_list = list(range(-5, 5))\n",
-    "less_than_zero = list(filter(negative, number_list))\n",
+    "product = 1\n",
+    "list = [1, 2, 3, 4]\n",
+    "for num in list:\n",
+    "    product = product * num\n",
     "\n",
-    "print(number_list)\n",
-    "print(less_than_zero)"
+    "print(product)"
    ]
   },
   {
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "Just like `map()` you can use a lambda function for more concise code instead of defining one separately."
+    "Now let’s try it with `reduce()`:"
    ]
   },
   {
    "cell_type": "code",
-   "execution_count": 14,
+   "execution_count": 5,
    "metadata": {},
    "outputs": [
     {
      "name": "stdout",
      "output_type": "stream",
      "text": [
-      "[-5, -4, -3, -2, -1, 0, 1, 2, 3, 4]\n",
-      "[-5, -4, -3, -2, -1]\n"
+      "24\n"
      ]
     }
    ],
    "source": [
-    "number_list = list(range(-5, 5))\n",
-    "less_than_zero = list(filter(lambda x: x < 0, number_list))\n",
+    "def multiply(x, y):\n",
+    "    return x*y\n",
     "\n",
-    "print(number_list)\n",
-    "print(less_than_zero)"
+    "from functools import reduce\n",
+    "product = reduce(multiply, [1, 2, 3, 4])\n",
+    "print(product)"
    ]
   },
   {
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "The filter resembles a for loop but it is a builtin function and faster.\n",
-    "Note: If map & filter do not appear beautiful to you then you can also use \n",
-    " list/dict/tuple comprehensions."
+    "Just like `filter()` and `map()`, `reduce()` takes a function to apply, so we can use a `lambda` function\n",
+    "for more concise code:"
    ]
   },
   {
    "cell_type": "code",
-   "execution_count": 15,
+   "execution_count": 7,
    "metadata": {},
    "outputs": [
     {
      "name": "stdout",
      "output_type": "stream",
      "text": [
-      "[-5, -4, -3, -2, -1, 0, 1, 2, 3, 4]\n",
-      "[-5, -4, -3, -2, -1]\n"
+      "24\n"
      ]
     }
    ],
    "source": [
-    "number_list = list(range(-5, 5))\n",
-    "less_than_zero = [x for x in number_list if x < 0]\n",
-    "\n",
-    "print(number_list)\n",
-    "print(less_than_zero)"
+    "from functools import reduce\n",
+    "product = reduce((lambda x, y: x * y), [1, 2, 3, 4])\n",
+    "print(product)"
    ]
   },
   {
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "You can use `filter()` for many things that require some condition, but just to give another example, you\n",
-    "can use it to get just the even or odd numbers of a list"
+    "You can use `reduce()` for a lot of things, so I'll just give a concise example for creating a\n",
+    "Factorial function:"
    ]
   },
   {
    "cell_type": "code",
-   "execution_count": 1,
+   "execution_count": 26,
    "metadata": {},
    "outputs": [
     {
      "name": "stdout",
      "output_type": "stream",
      "text": [
-      "[-5, -4, -3, -2, -1, 0, 1, 2, 3, 4]\n",
-      "[-4, -2, 0, 2, 4]\n",
-      "[-5, -3, -1, 1, 3]\n"
+      "120\n"
      ]
     }
    ],
    "source": [
-    "number_list = list(range(-5, 5))\n",
-    "even_numbers = list(filter(lambda x : x % 2 == 0, number_list))\n",
-    "odd_numbers = list(filter(lambda x : x % 2 == 1, number_list))\n",
+    "def factorial(n):\n",
+    "    return reduce((lambda x,y : x*y), range(1, n+1))\n",
     "\n",
-    "print(number_list)\n",
-    "print(even_numbers)\n",
-    "print(odd_numbers)"
+    "print(factorial(5))"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "This is the same as using a for loop, but in one line instead of four:"
    ]
   },
   {
    "cell_type": "code",
-   "execution_count": 3,
+   "execution_count": 25,
    "metadata": {},
    "outputs": [
     {
-     "ename": "TypeError",
-     "evalue": "'int' object is not iterable",
-     "output_type": "error",
-     "traceback": [
-      "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
-      "\u001b[0;31mTypeError\u001b[0m                                 Traceback (most recent call last)",
-      "\u001b[0;32m<ipython-input-3-1da792b5c7a0>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[1;32m      5\u001b[0m         \u001b[0;32mreturn\u001b[0m \u001b[0mi\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m      6\u001b[0m \u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 7\u001b[0;31m \u001b[0;32mfor\u001b[0m \u001b[0mnum\u001b[0m \u001b[0;32min\u001b[0m \u001b[0mgen\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m\u001b[1;32m      8\u001b[0m     \u001b[0mprint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mnum\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n",
-      "\u001b[0;31mTypeError\u001b[0m: 'int' object is not iterable"
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "120\n"
      ]
     }
    ],
    "source": [
-    "number_list = list(range(-5, 5))\n",
-    "\n",
-    "def gen():\n",
-    "    for i in range(10):\n",
-    "        yield i\n",
+    "def factorial(n):\n",
+    "    product = 1\n",
+    "    for i in range(1, n+1):\n",
+    "        product *= i\n",
+    "    return product\n",
     "\n",
-    "for num in gen():\n",
-    "    print(num)\n"
+    "print(factorial(5))"
    ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": null,
-   "metadata": {},
-   "outputs": [],
-   "source": []
   }
  ],
  "metadata": {