01-python-fundamentals.ipynb

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   "source": [
    "# Python Fundamentals"
   ]
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   "source": [
    "## Variables\n",
    "Any Python interpreter can be used as a calculator:"
   ]
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    "3 + 5 * 4"
   ]
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    "This is great but not very interesting. To do anything useful with data, we need to assign its value to a _variable_. In Python, we can assign a value to a variable, using the equals sign `=`. For example, to assign value `60` to a variable `weight_kg`, we would execute:"
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    "weight_kg = 60"
   ]
  },
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   "source": [
    "From now on, whenever we use weight_kg, Python will substitute the value we assigned to it. In layman’s terms, a **variable is a name for a value**.\n",
    "\n",
    "In Python, variable names:\n",
    "\n",
    "- can include letters, digits, and underscores\n",
    "- cannot start with a digit\n",
    "- are case sensitive.\n",
    "\n",
    "This means that, for example:\n",
    "\n",
    "- `weight0` is a valid variable name, whereas `0weight` is not\n",
    "- `weight` and `Weight` are different variables"
   ]
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   "source": [
    "## Types of data"
   ]
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    "Python knows various types of data. Three common ones are:\n",
    "\n",
    "- integer numbers\n",
    "- floating point numbers, and\n",
    "- strings.\n",
    "\n",
    "In the example above, variable weight_kg has an integer value of 60. To create a variable with a floating point value, we can execute:"
   ]
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    "weight_kg = 60.0"
   ]
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    "weight_kg_text = 'weight in kilograms:'"
   ]
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    "## Using Variables in Python\n",
    "To display the value of a variable to the screen in Python, we can use the `print` function:"
   ]
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  {
   "cell_type": "code",
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   "source": [
    "print(weight_kg)"
   ]
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   "source": [
    "Moreover, we can do arithmetic with variables right inside the print function:\n"
   ]
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   "source": [
    "print('weight in pounds:', 2.2 * weight_kg)"
   ]
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   "source": [
    "The above command, however, did not change the value of `weight_kg`:"
   ]
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  {
   "cell_type": "code",
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   "source": [
    "print(weight_kg)"
   ]
  },
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   "source": [
    "To change the value of the `weight_kg` variable, we have to **assign** `weight_kg` a new value using the equals `=` sign:\n"
   ]
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   "source": [
    "weight_kg = 65.0\n",
    "print('weight in kilograms is now:', weight_kg)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "A variable is analogous to a sticky note with a name written on it: assigning a value to a variable is like putting that sticky note on a particular value.\n",
    "\n",
    "\"\"Value of 65.0 with weight_kg label stuck on it\"\"\n",
    "\n",
    "This means that assigning a value to one variable does **not** change values of other variables. For example, let’s store the subject’s weight in pounds in its own variable:"
   ]
  },
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   "cell_type": "code",
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   "outputs": [],
   "source": [
    "# There are 2.2 pounds per kilogram\n",
    "weight_lb = 2.2 * weight_kg\n",
    "print(weight_kg_text, weight_kg, 'and in pounds:', weight_lb)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "weight_kg = 100.0\n",
    "print('weight in kilograms is now:', weight_kg, 'and weight in pounds is still:', weight_lb)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Since `weight_lb` doesn’t “remember” where its value comes from, it is not updated when we change `weight_kg`."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "What values do the variables `mass` and `age` have after each statement in the following program? Test your answers by executing the commands.\n"
   ]
  },
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   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "mass = 47.5\n",
    "age = 122\n",
    "mass = mass * 2.0\n",
    "age = age - 20\n",
    "print(mass, age)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "What does the following program print out?"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "first, second = 'Grace', 'Hopper'\n",
    "third, fourth = second, first\n",
    "print(third, fourth)"
   ]
  }
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