{
 "cells": [
  {
   "cell_type": "markdown",
   "id": "9291c943",
   "metadata": {
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     "slide_type": "slide"
    },
    "tags": []
   },
   "source": [
    "# Debugger"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "b235f4c3",
   "metadata": {
    "tags": [
     "remove-cell"
    ]
   },
   "source": [
    "CS1302 Introduction to Computer Programming\n",
    "___"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "c70c27e5",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    },
    "tags": []
   },
   "source": [
    "## Why debuggers?"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "b42068dc",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    },
    "tags": []
   },
   "source": [
    "As a computer programmer, your goal is to write programs which solves some tasks. But sometimes computer programs behave in an unexpected way due to *Errors* or *Bugs*. For simple programs, finding errors might be easy. However, for programs consisting many lines of code, finding errors can be very hard."
   ]
  },
  {
   "cell_type": "markdown",
   "id": "99dcd7ad",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    },
    "tags": []
   },
   "source": [
    "A *Debugger* allows us to pause/break the execution of a program at a line, called a *breakpoint*, we specify. This allows us to find errors more easily."
   ]
  },
  {
   "cell_type": "markdown",
   "id": "518321d8",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    },
    "tags": []
   },
   "source": [
    " In this lab, we will learn to use a visual debugger in JupyterLab."
   ]
  },
  {
   "cell_type": "markdown",
   "id": "8a8ba7ea",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    },
    "tags": []
   },
   "source": [
    "## Set breakpoints"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "2e530535",
   "metadata": {
    "slideshow": {
     "slide_type": "subslide"
    },
    "tags": []
   },
   "source": [
    "To use the Jupyterlab visual debugger, ensure XPython kernel is running:"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "7ea37fd0",
   "metadata": {},
   "source": [
    "![xpython](images/xpython.dio.svg)\n",
    "\n",
    "If you do not see XPython or a circle next to it on the top-right-hand corner, switch to the XPython kernel by clicking the name of the current kernel on the top-right-hand corner."
   ]
  },
  {
   "cell_type": "markdown",
   "id": "9b032c55",
   "metadata": {
    "slideshow": {
     "slide_type": "subslide"
    },
    "tags": []
   },
   "source": [
    "We will use the following code as an example to set a breakpoint. First, execute the code to see the effect."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "id": "807acc3a",
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Hello, World!"
     ]
    },
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "\n"
     ]
    }
   ],
   "source": [
    "msg = \"Hello, World!\"\n",
    "print(msg)"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "f7c158b4",
   "metadata": {
    "slideshow": {
     "slide_type": "subslide"
    },
    "tags": []
   },
   "source": [
    "To enter the debug mode, turn on the switch near on top right-hand corner: \n",
    "\n",
    "![jupyterlab debugger switch](images/jupyterlab-debug.dio.svg)"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "790f70af",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    },
    "tags": []
   },
   "source": [
    "The debugging sidebar should slide open. You can also open/close the sidebar by clicking the bug icon on the right."
   ]
  },
  {
   "cell_type": "markdown",
   "id": "6002e585",
   "metadata": {
    "slideshow": {
     "slide_type": "subslide"
    },
    "tags": []
   },
   "source": [
    "**Exercise** \n",
    "\n",
    "Code cells also looks different in debug mode. What is the difference?\n",
    "\n",
    "![code cell](images/codecell.dio.svg)"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "fa457f02",
   "metadata": {
    "deletable": false,
    "nbgrader": {
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     "grade": true,
     "grade_id": "line-number",
     "locked": false,
     "points": 1,
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     "solution": true,
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   "source": [
    "YOUR ANSWER HERE"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "7aff4682",
   "metadata": {
    "slideshow": {
     "slide_type": "subslide"
    },
    "tags": []
   },
   "source": [
    "To set a breakpoint, press the dot that appears as you hover to the left of line number 2. Run the cell to see what happens."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "id": "3d2e28ad",
   "metadata": {
    "tags": [
     "remove-output"
    ]
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Hello, World!"
     ]
    },
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "\n"
     ]
    }
   ],
   "source": [
    "msg = \"Hello, World!\"\n",
    "print(msg)"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "9ccf2c89",
   "metadata": {},
   "source": [
    "The debugger provides quite a bit of information, organized as follows:"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "1cc3e136",
   "metadata": {},
   "source": [
    "* **Orange line** is the line where the execution is paused.  \n",
    "![paused](images/paused.dio.svg)"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "c81abc1c",
   "metadata": {},
   "source": [
    "* **Variables panel** shows the current values of different variables.  \n",
    "![variables](images/variables.dio.svg)"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "7f33d6e0",
   "metadata": {},
   "source": [
    "* **Callstack panel** provides a more detailed context of where the execution is paused. It also provides the flow control buttons to resume/stop the execution.  \n",
    "![callstack](images/callstack.dio.svg)"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "cadffc2b",
   "metadata": {},
   "source": [
    "* **Breakpoints panel** shows all the breakpoints of the current debug session.  \n",
    "![breakpoints](images/breakpoints.dio.svg)"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "4a441232",
   "metadata": {},
   "source": [
    "* **Source panel** highlights the source of the breakpoint at which the execution is currently paused.  \n",
    "![source](images/source.dio.svg)"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "13ada4ec",
   "metadata": {
    "slideshow": {
     "slide_type": "subslide"
    }
   },
   "source": [
    "**Exercise** Explain whether line 1 and 2 are executed. Use the information from the debugger to support your answer."
   ]
  },
  {
   "cell_type": "markdown",
   "id": "388b3570",
   "metadata": {
    "deletable": false,
    "nbgrader": {
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     "grade": true,
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     "task": false
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   "source": [
    "YOUR ANSWER HERE"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "1a85e362",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "## Resume execution"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "72bed07f",
   "metadata": {},
   "source": [
    "To resume/terminate the execution, use the following flow control buttons at the top of the callstack panel:  \n",
    "![callstack](images/flowcontrol.dio.svg)\n",
    "- **Continue** button continues the execution and pause again if it hits a breakpoint.\n",
    "- **Terminate** button stops the execution.\n",
    "- **Next/Step-over** button continues the execution of program to the next statement without entering function."
   ]
  },
  {
   "cell_type": "markdown",
   "id": "abbc588a",
   "metadata": {
    "slideshow": {
     "slide_type": "subslide"
    }
   },
   "source": [
    "**Exercise** \n",
    "\n",
    "What does the following function `f(n)` computes? Try to set a breakpoint and use the flow control to understand how the program works."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "id": "8fc547f0",
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "6"
      ]
     },
     "execution_count": 3,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "def f(n): # definition of a function\n",
    "    return n * f(n - 1) if n > 0 else 1\n",
    "\n",
    "\n",
    "f(3)"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "6bf5c240",
   "metadata": {},
   "source": [
    "````{hint}\n",
    "\n",
    "Set the breakpoint one at a time but at different locations. If you set your breakpoint at line 5, you should use the **Step In** button to step into the execution of the function.\n",
    "\n",
    "````"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "10af3170",
   "metadata": {
    "deletable": false,
    "nbgrader": {
     "cell_type": "markdown",
     "checksum": "4977f55aff95e93021c125fe6ec165b6",
     "grade": true,
     "grade_id": "factorial",
     "locked": false,
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     "schema_version": 3,
     "solution": true,
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   "source": [
    "YOUR ANSWER HERE"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "04741656",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "## Run temporary code"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "24d25fd9",
   "metadata": {
    "slideshow": {
     "slide_type": "-"
    }
   },
   "source": [
    "Sometimes, we want to run temporary code without modifying the current notebook. To do so:"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "104c68aa",
   "metadata": {
    "slideshow": {
     "slide_type": "-"
    }
   },
   "source": [
    "- Right click anywhere on a notebook and choose `New Console for Notebook`:\n",
    "\n",
    "![new console](images/newconsole.dio.svg)"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "1dee516b",
   "metadata": {
    "slideshow": {
     "slide_type": "subslide"
    }
   },
   "source": [
    "- Type some code such as `msg` into the console input and run it with `Shift-Enter`:\n",
    "\n",
    "![console](images/console.dio.svg)"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "f03e435d",
   "metadata": {
    "slideshow": {
     "slide_type": "subslide"
    }
   },
   "source": [
    "````{tip}\n",
    "\n",
    "- You can run `f(n)` for different values of `n` in the console to understand how the function works. \n",
    "- You can set a breakpoint in line 2 of the previous code cell containing `def f(n): ...` and run `f(3)` in the console to pause at the breakpoint. The source code containing the breakpoint will also be shown in the **Source panel**. \n",
    "- You can also set a breakpoint in the console input, say `f(3)`, and then step into it.\n",
    "- While the execution is paused, you can enter a new line of code such as `print(msg)` below `f(3)` in the console and then continue the execution to run the new line of code.\n",
    "\n",
    "````"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "f7493435",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
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    "tags": []
   },
   "source": [
    "## VS Code interface (Optional)"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "cce34fd3",
   "metadata": {},
   "source": [
    "There are other debuggers in different interfaces. A powerful GUI debugger is available in the vscode interface:  \n",
    "- `File`$\\to$ `New Launcher`$\\to$ `VS Code`"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "fad986af",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    },
    "tags": []
   },
   "source": [
    "Follow the [official guide](https://code.visualstudio.com/docs/datascience/jupyter-notebooks) to install the python/jupyter extension and debug a jupyter notebook."
   ]
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