updating notes 02

linearalgebrayhz · linearalgebrayhz · commit 304411e2b508 · 2024-07-12T00:29:48.000+08:00
diff --git a/02-array-seq/02-notes.ipynb b/02-array-seq/02-notes.ipynb
@@ -462,6 +462,232 @@
     "2. 用`_make()`通过接受一个可迭代对象来生成这个类的一个实例，它的作用跟`City(*delhi_data)`是一样的。\n",
     "3. `_asdict()`把具名元组以`collections.OrderedDict`的形式返回，我们可以利用它来把元组里的信息清晰的呈现出来。"
    ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "### 2.3.5 作为不可变列表的元组\n",
+    "\n",
+    "除了和增减元素相关的方法之外，元组支持列表的其他所有方法。有一个例外是元组没有`__reversed__`方法 (书上描述:这个方法只是个优化) ，但是可以使用`reversed()`函数。\n",
+    "\n",
+    "注: `__reversed__` method returns an iterator of reversed items"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 3,
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "<reversed object at 0x00000291885FE9B0>\n",
+      "(3, 2, 1)\n"
+     ]
+    }
+   ],
+   "source": [
+    "a = (1,2,3)\n",
+    "a = reversed(a)\n",
+    "print(a)\n",
+    "print(tuple(a))"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 5,
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "<list_reverseiterator object at 0x00000291885FF9A0>\n",
+      "[3, 2, 1]\n"
+     ]
+    }
+   ],
+   "source": [
+    "a = [1,2,3]\n",
+    "a = reversed(a)\n",
+    "print(a)\n",
+    "print(list(a))"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## 2.4 切片\n",
+    "`a:b:c` 这种用法只能作为索引或者下标用在[]中来返回一个切片对象：`slice(a, b, c)` 。对`seq[start:stop:step]` 进行求值的时候，Python会调用`seq`. \n",
+    "`__getitem__(slice(start, stop, step))`。\n",
+    "\n",
+    "(may refer to a video about slicing <a href = \"https://www.bilibili.com/video/BV1ki421v7Ao/?share_source=copy_web&vd_source=0f2805b5dbbfea4fe3a5cd27c708c741\">【Python】Slice：被低估的小技巧，减少重复工作量 </a>)\n",
+    "\n",
+    "### 2.4.3 多维切片和省略\n",
+    "`[]` 运算符里还可以使用以逗号分开的多个索引或者是切片, numpy库就利用了这个特性。\n",
+    "\n",
+    "要正确处理这种`[]`运算符的话，对象的特殊方法`__getitem__`和`__setitem__`需要以元组的形式来接收`a[i, j]`中的索引。也就是说，如果要得到`a[i, j]`的值，Python会调用`a.__getitem__((i, j))`\n",
+    "\n",
+    "省略`...`是Ellipsis对象的别名，它可以表示任意多的冒号\n",
+    "\n",
+    "书本注：fun fact, `Ellipsis` object is a singleton object of `ellipsis` class (Ellipsis是一个内置实例). Yes, it is a class with lower letters. Similar to `bool` class with `True` and `False` instances.\n"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 12,
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "[[[ 1  2  3  4]\n",
+      "  [ 5  6  7  8]]\n",
+      "\n",
+      " [[12 34 56 78]\n",
+      "  [56 78 90 12]]]\n"
+     ]
+    }
+   ],
+   "source": [
+    "import numpy as np\n",
+    "\n",
+    "a = np.array([[[[1,2,3,4],[5,6,7,8]],[[12,34,56,78],[56,78,90,12]]],[[[1,2,3,4],[5,6,7,8]],[[12,34,56,78],[56,78,90,12]]]])\n",
+    "b = a[0, ...] # very interesting example\n",
+    "print(b)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "### 2.4.4 给切片赋值（有意思）\n",
+    "如果把切片放在赋值语句的左边，或把它作为`del`操作的对象，我们就可以对序列进行**嫁接、切除或就地修改**操作。"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 19,
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]\n"
+     ]
+    }
+   ],
+   "source": [
+    "l = list(range(10))\n",
+    "print(l)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 20,
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "[0, 1, 20, 30, 5, 6, 7, 8, 9]"
+      ]
+     },
+     "execution_count": 20,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "l[2:5] = [20,30]\n",
+    "l"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 21,
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "[0, 1, 20, 30, 5, 8, 9]"
+      ]
+     },
+     "execution_count": 21,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "del l[5:7]\n",
+    "l"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 22,
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "[0, 1, 20, 11, 5, 22, 9]"
+      ]
+     },
+     "execution_count": 22,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "l[3::2] = [11,22]\n",
+    "l"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 23,
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "[0, 1, 100, 22, 9]"
+      ]
+     },
+     "execution_count": 23,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "# l[2:5] = 100 error because 100 is not iterable\n",
+    "l[2:5] = [100] \n",
+    "l"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "如果赋值的对象是一个切片，那么赋值语句的右侧**必须是个可迭代对象**。即便只有单独一个值，也要把它转换成可迭代的序列。\n",
+    "\n",
+    "## 对序列使用`+`和`*`\n",
+    "`+`和`*`都遵循这样的规律：不修改原有的操作对象，而是构建一个全新的序列。\n",
+    "\n",
+    "`*`操作符的一个潜在的缺点是，它会把一个单一的元素复制多次以构建新的列表。这意味着，如果这个元素是**可变的**，就可能导致意想不到的副作用。(萌新时期噩梦!)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": []
   }
  ],
  "metadata": {
