Running_Code.ipynb

{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Running Code"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "First and foremost, the Jupyter Notebook is an interactive environment for writing and running code. The notebook is capable of running code in a wide range of languages. However, each notebook is associated with a single kernel.  This notebook is associated with the IPython kernel, therefore runs Python code."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Code cells allow you to enter and run code"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Run a code cell using `Shift-Enter` or pressing the <button class='btn btn-default btn-xs'><i class=\"icon-step-forward fa fa-step-forward\"></i></button> button in the toolbar above:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "a = 10"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "print(a)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "There are two other keyboard shortcuts for running code:\n",
    "\n",
    "* `Alt-Enter` runs the current cell and inserts a new one below.\n",
    "* `Ctrl-Enter` run the current cell and enters command mode."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Note that the `In []:` symbol next to each cell tells you about the state of the cell\n",
    "* `In [ ]:` means that the cell has not been run yet.\n",
    "* `In [*]:` means that the cell is currently running.\n",
    "* `In [1]:` means that the cell has finished running, and was the 1st cell run.\n",
    "\n",
    "The number tells you the order that the cells have been run in. This is important as some cells require the previous cells to have been run. This is the case above: attempting to `print` the variable `a` before it's defined will fail. You can try this by restarting the kernel and running the second cell before the first if you're interested (see below for instructions on restarting the kernel)."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Managing the Kernel"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Code is run in a separate process called the Kernel.  The Kernel can be interrupted or restarted. You may wish to do this if you think there's an issue with your code and you want to rerun it. Try running the following cell and then hit the <button class='btn btn-default btn-xs'><i class='icon-stop fa fa-stop'></i></button> button in the toolbar above."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "import time\n",
    "time.sleep(10)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "If the Kernel dies you will be prompted to restart it. Here we call the low-level system libc.time routine with the wrong argument via\n",
    "ctypes to segfault the Python interpreter. You'll need to uncomment the last three lines of code to see the segfault. You can do this by highlighting multiple lines and pressing `Ctrl+/` to uncomment them."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "import sys\n",
    "from ctypes import CDLL\n",
    "# This will crash a Linux or Mac system\n",
    "# equivalent calls can be made on Windows\n",
    "\n",
    "# Uncomment these lines if you would like to see the segfault\n",
    "\n",
    "# dll = 'dylib' if sys.platform == 'darwin' else 'so.6'\n",
    "# libc = CDLL(\"libc.%s\" % dll) \n",
    "# libc.time(-1)  # BOOM!!"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Cell menu"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "The \"Cell\" menu has a number of menu items for running code in different ways. These include:\n",
    "* Run and Select Below\n",
    "* Run and Insert Below\n",
    "* Run All\n",
    "* Run All Above\n",
    "* Run All Below"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Restarting the kernels"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "The kernel maintains the state of a notebook's computations. You can reset this state by restarting the kernel. This is done by clicking on the <button class='btn btn-default btn-xs'><i class='fa fa-repeat icon-repeat'></i></button> in the toolbar above. You will be asked to confirm that you wish to restart the kernal."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## sys.stdout and sys.stderr"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "The stdout and stderr streams are displayed as text in the output area. Errors appear on a red background."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "print(\"hi, stdout\")"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "print('hi, stderr', file=sys.stderr)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Output is asynchronous"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "All output is displayed as it is generated in the kernel. If you execute the next cell, you will see the output one piece at a time, not all at the end."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "import time, sys\n",
    "for i in range(8):\n",
    "    print(i)\n",
    "    time.sleep(0.5)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Large outputs"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "To better handle large outputs, the output area can be collapsed. Run the following cell and then single- or double- click on the active area to the left of the output:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "scrolled": true
   },
   "outputs": [],
   "source": [
    "for i in range(50):\n",
    "    print(i)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Beyond a certain point, output will scroll automatically:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "for i in range(500):\n",
    "    print(2**i - 1)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": []
  }
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