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   "source": [
    "# Introduction to programming for Geoscientists through Python"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Lecture 4 solutions\n",
    "## Gerard J. Gorman (g.gorman@imperial.ac.uk) http://www.imperial.ac.uk/people/g.gorman"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "* **Exercise 4.1: Make an interactive program**</br>\n",
    "Make a program that (i) asks the user for a temperature in Fahrenheit degrees and reads the number; (ii) computes the corresponding temperature in Celsius degrees; and (iii) prints out the temperature in the Celsius scale."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [
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       "A Jupyter Widget"
      ]
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     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "from ipywidgets import widgets\n",
    "from IPython.display import display\n",
    "\n",
    "def f2c(sender):\n",
    "    F = sender.value # Read the text value; note this is a string\n",
    "    F = float(F)     # Convert this string into a float\n",
    "    print(F, \"degrees Fahrenheit is \", (F - 32)*(5./9.), \"degrees Celsius\")\n",
    "\n",
    "widget_f2c = widgets.Text()\n",
    "widget_f2c.on_submit(f2c)\n",
    "display(widget_f2c)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "* **Exercise 4.2: Prompt the user for input to a formula**</br>\n",
    "Consider the simplest program for evaluting the formula $y(t) = v_0 t − 0.5gt^2$:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "0.034199999999999786\n"
     ]
    }
   ],
   "source": [
    "v0 = 3; g = 9.81; t = 0.6\n",
    "y = v0*t - 0.5*g*t**2\n",
    "print(y)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Modify this code so that the program takes as user input *t* and *v0*, and then computes the corresponding distance traveled, $y$. Note, you can put multiple values into the same text box and then use the string method *split* to break the string value into a list."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "application/vnd.jupyter.widget-view+json": {
       "model_id": "15522825c42e4ab1bb41e735e0eb37f0",
       "version_major": 2,
       "version_minor": 0
      },
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       "A Jupyter Widget"
      ]
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     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "from ipywidgets import widgets\n",
    "from IPython.display import display\n",
    "\n",
    "def distance(sender):\n",
    "    t, v0 = sender.value.split(\",\") # Read the text value; note the use of *split*\n",
    "    t = float(t)\n",
    "    v0 = float(v0)\n",
    "    \n",
    "    g = 9.81 # Assigns g value\n",
    "    y = v0*t - 0.5*g*t**2 # Computes y\n",
    "    print(\"distance traveled: \", y, \"meters\")\n",
    "\n",
    "widget_distance = widgets.Text()\n",
    "widget_distance.on_submit(distance)\n",
    "display(widget_distance)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "* **Exercise 4.4 - Use exceptions**</br>\n",
    "Extend the program from the previous exercise with a try-except block to handle the potential error that the user enters nothing (or invalid data such as a letter) for the Fahrenheit temperature."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [
    {
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       "model_id": "3042df492df5467ca624de08a5333e1c",
       "version_major": 2,
       "version_minor": 0
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       "A Jupyter Widget"
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     "metadata": {},
     "output_type": "display_data"
    },
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "0.0 degrees Fahrenheit is  -17.77777777777778 degrees Celsius\n",
      "ERROR: invalid input, value entred must be a float (ie a real number)\n"
     ]
    }
   ],
   "source": [
    "from ipywidgets import widgets\n",
    "from IPython.display import display\n",
    "\n",
    "def f2c(sender):\n",
    "    F = sender.value # Read the text value; note this is a string\n",
    "    try:\n",
    "        F = float(F)     # Convert this string into a float\n",
    "        print(F, \"degrees Fahrenheit is \", (F - 32)*(5./9.), \"degrees Celsius\")\n",
    "    except:\n",
    "        print(\"ERROR: invalid input, value entred must be a float (ie a real number)\")\n",
    "widget_f2c = widgets.Text()\n",
    "widget_f2c.on_submit(f2c)\n",
    "display(widget_f2c)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "* **Exercise 4.5: Make the program safer**</br>\n",
    "Extend the program from the previous exercise to include exception handling such that missing (or invalid) values for *t* and *v0* are detected."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "application/vnd.jupyter.widget-view+json": {
       "model_id": "14468710b70a402fab9ff7e0160ba3c8",
       "version_major": 2,
       "version_minor": 0
      },
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       "A Jupyter Widget"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "from ipywidgets import widgets\n",
    "from IPython.display import display\n",
    "\n",
    "def distance(sender):\n",
    "    try:\n",
    "        t, v0 = sender.value.split(\",\") # Read the text value; note the use of *split*\n",
    "    except:\n",
    "        print(\"ERROR: expecting two values seperated by a ','\")\n",
    "        return\n",
    "    \n",
    "    try:\n",
    "        t = float(t)\n",
    "        v0 = float(v0)\n",
    "    except:\n",
    "        print(\"ERROR: t and v0 must be both float's (ie real numbers)\")\n",
    "        return\n",
    "    \n",
    "    g = 9.81 # Assigns g value\n",
    "    y = v0*t - 0.5*g*t**2 # Computes y\n",
    "    print(\"distance traveled: \", y, \"meters\")\n",
    "\n",
    "widget_distance = widgets.Text()\n",
    "widget_distance.on_submit(distance)\n",
    "display(widget_distance)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "* **Exercise 4.6: Test more in the program**</br>\n",
    "Test if the *t* value read in the program from the previous exercise lies between $0$ and ${2v_0}/{g}$. If not, raise a *ValueError* exception in the *if* test on legal *t* values in the program from the previous exercise. Include the legal interval for *t* in the exception message."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [
    {
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      "application/vnd.jupyter.widget-view+json": {
       "model_id": "6853dfbf6d46489e9bdc1938688794ee",
       "version_major": 2,
       "version_minor": 0
      },
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       "A Jupyter Widget"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "from ipywidgets import widgets\n",
    "from IPython.display import display\n",
    "\n",
    "def distance(sender):\n",
    "    g = 9.81 # Assigns g value\n",
    "\n",
    "    try:\n",
    "        t, v0 = sender.value.split(\",\") # Read the text value; note the use of *split*\n",
    "    except:\n",
    "        print(\"ERROR: expecting two values seperated by a ','\")\n",
    "        return\n",
    "    \n",
    "    try:\n",
    "        t = float(t)\n",
    "        v0 = float(v0)\n",
    "    except:\n",
    "        print(\"ERROR: t and v0 must be both float's (ie real numbers)\")\n",
    "        return\n",
    "    \n",
    "    try:\n",
    "        if t<0 or t> 2*v0/g:\n",
    "            raise ValueError\n",
    "    except ValueError:\n",
    "        print(\"ERROR: value of t must be between 0 and \", 2*v0/g)\n",
    "        return\n",
    "    \n",
    "    y = v0*t - 0.5*g*t**2 # Computes y\n",
    "    print(\"distance traveled: \", y, \"meters\")\n",
    "\n",
    "widget_distance = widgets.Text()\n",
    "widget_distance.on_submit(distance)\n",
    "display(widget_distance)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "* **Exercise 4.7: Compute the distance it takes to stop a car**</br>\n",
    "A car driver, driving at velocity $v_0$, suddenly puts on the brake. What braking distance $d$ is needed to stop the car? One can derive, from basic physics, that</br>\n",
    "$d = 0.5\\frac{v_0^2}{\\mu g}$</br>\n",
    "Make a program for computing $d$ using the above formula when the initial car velocity $v_0$ and the friction coefficient $\\mu$ are provided via the raw_input function. Run the\n",
    "program for two cases: $v_0$ = 120 and $v_0$ = 50 km/h, both with $\\mu$ = 0.3 ($\\mu$ is dimensionless). (Remember to convert the velocity from km/h to m/s before inserting the value in the formula!)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {},
   "outputs": [
    {
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      "application/vnd.jupyter.widget-view+json": {
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      },
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       "A Jupyter Widget"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "from ipywidgets import widgets\n",
    "from IPython.display import display\n",
    "\n",
    "def distance(sender):\n",
    "    g = 9.81 # Assigns g value\n",
    "\n",
    "    try:\n",
    "        v0, mu = sender.value.split(\",\") # Read the text value; note the use of *split*\n",
    "    except:\n",
    "        print(\"ERROR: expecting two values seperated by a ','\")\n",
    "        return\n",
    "    \n",
    "    try:\n",
    "        v0 = float(v0)\n",
    "        v0 = (v0*1000)/3600 # Conversion from km/h to m/s\n",
    "        mu = float(mu)\n",
    "    except:\n",
    "        print(\"ERROR: v0 and mu must be both float's (ie real numbers)\")\n",
    "        return\n",
    "    \n",
    "    d = (0.5*v0**2)/(mu*g) # Computes braking distance\n",
    "\n",
    "    print(\"breaking distance: \", d, \"meters\")\n",
    "\n",
    "widget_distance = widgets.Text()\n",
    "widget_distance.on_submit(distance)\n",
    "display(widget_distance)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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    "collapsed": true
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   "outputs": [],
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