02_lists_loops.ipynb

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   "source": [
    "# Lists and Loops\n",
    "\n",
    "This notebook is based on materials kindly provided by the [IN1900]( https://www.uio.no/studier/emner/matnat/ifi/IN1900/h19/) team.\n",
    "\n",
    "## Lists\n",
    "\n",
    "Python lists can contain `int`, `float`, `String` and other items.\n",
    "We make a list by placing the items in square brackets, `[]`, separated by commas."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "my_list = [1, 2]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "There are many functions for working with lists. One example is `len()`, which returns the length of the list, in other words the number of items it contains."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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   "source": [
    "odd = [1, 3, 5, 7, 9]\n",
    "\n",
    "print(\"odd: \", odd)\n",
    "print(\"length of list 'odd' :\", len(odd))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Adding (Appending) Elements to a List\n",
    "\n",
    "We can add/append elements to an existing list. Below we first make an empty list, and then append one element at a time using `append()`. Note that `append()` adds items at the end of the list."
   ]
  },
  {
   "cell_type": "code",
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   "source": [
    "empty_list = []\n",
    "\n",
    "empty_list.append(\"not\")\n",
    "empty_list.append(\"so\")\n",
    "empty_list.append(\"empty\")\n",
    "empty_list.append(\"anymore\")\n",
    "\n",
    "print(empty_list)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### <span style=\"color:green\"> Exercise: Adding Elements to a List </span>  \n",
    "\n",
    "The list `days` below contains the weekdays, but is missing the weekend. Your task will be to add *Saturday* and *Sunday* to the existing list `days`. \n",
    "\n",
    "Start by printing out the length of the list `days` using the function `len()`. Then, add the two days of the weekend to `days`. Finish by printing out the new length of the list using the `len()`-function. \n",
    "\n",
    "An example of how your output could be:\n",
    "```\n",
    "There are 5 days!\n",
    "There are 7 days!\n",
    "``` \n",
    "The text is not strictly necessary, the numbers are the important part!"
   ]
  },
  {
   "cell_type": "code",
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   "source": [
    "days = [\"Monday\", \"Tuesday\", \"Wednesday\", \"Thursday\", \"Friday\"]  # DO NOT CHANGE THIS LINE\n",
    "\n",
    "# PRINT THE LENGTH OF THE LIST \n",
    "\n",
    "# ADD THE DAYS OF THE WEEKEND HERE\n",
    "\n",
    "# PRINT THE LENGTH OF THE LIST AGAIN"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "<span style=\"color:green\">Hint  </span>  : remember that *Saturday* and *Sunday* need to be of type `str`, else you will get a `NameError`."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Accessing List Elements - List Indexing\n",
    "\n",
    "To get an item from a list, we use an index number. \n",
    "\n",
    "**Python counts from 0, not 1!**\n",
    "\n",
    "We can get the items in `odd` one at a time:"
   ]
  },
  {
   "cell_type": "code",
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   "source": [
    "print('odd[0] = ', odd[0])\n",
    "print('odd[1] = ', odd[1])\n",
    "print('odd[2] = ', odd[2])\n",
    "print('odd[3] = ', odd[3])\n",
    "print('odd[4] = ', odd[4])"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### <span style=\"color:green\"> Exercise: Indexing Lists</span>  \n",
    "\n",
    "Change the variable `index` in the code below such that the word *something* is printed when the code snippet is run."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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   "outputs": [],
   "source": [
    "Margaret_Lanterman = ['my', 'log', 'has', 'something', 'to', 'tell', 'you.']\n",
    "\n",
    "index = -1\n",
    "print(Margaret_Lanterman[index])"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Nested Lists \n",
    "\n",
    "Lists are frequently used in Python. Later we will see an example of lists of cases from US case law.\n",
    "Lists can contain other lists. In the list of cases, each case has a list of attorneys.\n",
    "\n",
    "We can make a new list `even` to complement our list `odd`."
   ]
  },
  {
   "cell_type": "code",
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   "source": [
    "even = [2, 4, 6, 8, 10]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Making a List of Lists \n",
    "\n",
    "Now, we can make a new list containing the lists `odd` and `even`."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "odd_and_even = [odd, even]\n",
    "print(\"odd_and_even : \", odd_and_even)\n",
    "print(\"Second object in odd_and_even is the list even : \", odd_and_even[1])\n",
    "print(\"First number in even: \", odd_and_even[1][0])\n",
    "print(\"Third number in even: \", odd_and_even[1][2])"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### <span style=\"color:green\">Exercise: Dates and Indexing Nested Lists</span> \n",
    "\n",
    "Change the indexes in the code below, so that the printed information is correct."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "summer = [\"June\", \"July\", \"August\"]\n",
    "autum = [\"September\", \"October\", \"November\"]\n",
    "winter = [\"December\", \"January\", \"February\"]\n",
    "spring = [\"March\", \"April\", \"May\"]\n",
    "\n",
    "months = [summer, autum, winter, spring]\n",
    "\n",
    "\n",
    "# change the indices below\n",
    "print(\"My birthday is the Xth of {}.\".format(months[0][2]))\n",
    "print(\"Christmas is the Xth of {}.\".format(months[1][1]))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Loops\n",
    "\n",
    "We saw above how we could manually get each element in a list. However, this kind of repetition is tedious and error-prone. For example, if we change ```odd``` to be a shorter list, we will get an error when re-running the code we wrote above:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "odd = [1, 3, 5, 7]\n",
    "\n",
    "print('odd[0] = ', odd[0])\n",
    "print('odd[1] = ', odd[1])\n",
    "print('odd[2] = ', odd[2])\n",
    "print('odd[3] = ', odd[3])\n",
    "print('odd[4] = ', odd[4])"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "This code violates an important principle in programming: *Don't Repeat Yourself* (DRY).\n",
    "To avoid such errors, we should leave the work of keeping track of the list elements to the computer."
   ]
  },
  {
   "cell_type": "code",
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   "outputs": [],
   "source": [
    "for number in odd:\n",
    "    print(number)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "We can read the code above as:\n",
    "for each `number` in `odd`, do the following:\n",
    "print the number."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### <span style=\"color:green\">Exercise: Loop</span> \n",
    "\n",
    "We can also loop or iterate over lists of strings.\n",
    "Complete the following code, to loop (iterate) over the summer months.\n",
    "The expected output is:\n",
    "\n",
    "    June\n",
    "    July\n",
    "    August"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [],
   "source": [
    "months = [\"June\", \"July\", \"August\"]\n",
    "\n",
    "for month in months:\n",
    "    \"your code here\""
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Visualizing loops\n",
    "\n",
    "It might be helpful to illustrate loops with an animation. Here, we iterate over a list of strings:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "sentence = ['a', 'short', 'list']\n",
    "\n",
    "print(\"Iterating over the sentence:\")\n",
    "for word in sentence:\n",
    "    print(word)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "For each iteration of the loop, the variable `word` changes to the next element in the list:"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "<iframe src=\"https://www.openprocessing.org/sketch/906164/embed/\" width=\"510\" height=\"600\"></iframe>"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "collapsed": true
   },
   "source": [
    "## Nested Loops \n",
    "\n",
    "When we put a loop inside another loop, we call them nested loops. We can use nested loops to iterate over nested lists.\n",
    "\n",
    "Remember that ``` odd_and_even = [odd, even] ```."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "for items in odd_and_even:\n",
    "    for number in items:\n",
    "        print(number)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### <span style=\"color:green\">Exercise: Nested Loops</span> \n",
    "\n",
    "Next, we iterate over a nested list of strings:\n",
    "Complete the following code, to loop (iterate) over all the months.\n",
    "\n",
    "You will need two nested `for`-loops."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "summer = [\"June\", \"July\", \"August\"]\n",
    "autum = [\"September\", \"October\", \"November\"]\n",
    "winter = [\"December\", \"January\", \"February\"]\n",
    "spring = [\"March\", \"April\", \"May\"]\n",
    "\n",
    "seasons = [summer, autum, winter, spring]\n",
    "\n",
    "for # your code here"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Bonus Material: Iterating over Numbers with `range()`\n",
    "\n",
    "`for`-loops iterate over all the elements in a sequence, frequently a list.\n",
    "We can also loop over other kinds of sequences. The function `range()` returns a sequence of numbers:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "print(\"Using range:\")\n",
    "for i in range(4):\n",
    "    print(i)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### <span style=\"color:green\">Exercise: Indexing in a Loop</span> \n",
    "\n",
    "The list `names` contains a few names and the list `birth_years` contain their corresponding birth years. There is also a `for`-loop that should print out the names with their corresponding birth years. The print-statement is incomplete, such that only the name is printed. \n",
    "\n",
    "Your task is to add the correct birth year in the print-statement in the loop by indexing `birth_years`."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "names = [\"John Cleese\", \"Terry Gilliam\", \"Eric Idle\", \"Michael Palin\", \"Graham Chapman\", \"Terry Jones\"]\n",
    "birth_years = [1939, 1940, 1943, 1943, 1941, 1942]\n",
    "\n",
    "for i in range(len(names)):\n",
    "    print(names[i], \"was born in\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### `range(start, stop, step_length)`\n",
    "\n",
    "`range()` can have up to three arguments. If you give all three arguments, range will yield integers starting from and *including* `start`, up to but *excluding* `stop`, in steps of size `step_length`.\n",
    "\n",
    "If you only give two arguments, the step length will be 1.\n",
    "\n",
    "If you only give a single argument, the starting point will be zero."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### <span style=\"color:green\"> Exercise: Using `range` </span>\n",
    " \n",
    " Predict what will be printed when running the code snippet below:\n",
    " \n",
    "```\n",
    "for i in range(0, 3, 2):\n",
    "    print(i)\n",
    "```"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### <span style=\"color:green\"> Exercise: Using `range` when Indexing </span>\n",
    " \n",
    "Predict what will be printed when running the code snippet below:\n",
    "\n",
    "```\n",
    "message = [\"You\", \"don't\", \"understand!\"]\n",
    "\n",
    "for i in range(0, 3, 2):\n",
    "    print(message[i])\n",
    "```"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "###  <span style=\"color:green\"> Exercise: Nested Loops and `range()`</span>\n",
    "We can also use nested loops with `range`. Explain the loop below."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "for i in range(4):\n",
    "    print(\"Main loop i =\", i)\n",
    "    for j in range(i+1):\n",
    "        print(\"\\tInner loop j =\", j, \"  (i =\", i, \")\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "##  <span style=\"color:blue\">Key Points</span>\n",
    "\n",
    "- We can make lists of items\n",
    "- We use indexes to access items in lists: `things[3]` or `many_things[0][4]`\n",
    "- Modify lists using `append()` and other functions\n",
    "- Use `for`-loops to iterate over lists"
   ]
  }
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