{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# 4 Decisions: if.. elif.. else"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "collapsed": true
   },
   "source": [
    "Decision making plays a key role in programming and Python provides several ways to do that."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 4.1 The If statement"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "The `if` statement is used to check if a condition is fulfilled and then a task (or set of tasks) is executed. The following example shows how it is used to check if a number is even:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "number = 2\n",
    "if number % 2 == 0:\n",
    "    print(\"number is even.\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "The operator `%` is called modulus and gives the remainder of an integer division (see section 1). The *conditional expression* follows directly after the word `if` and ends with a colon `:` (just as in the syntax for loops, see section 3). The task(s) to be executed in case the condition is `True`, need to be indented (again as in the syntax for loops). Note that if the condition is not `True`, the task is simply not executed."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "number = 3\n",
    "if number % 2 == 0:\n",
    "    print(\"number is even.\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 4.2 The if.. else statement"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "An `else` statement can be combined with an `if` statement. An else statement contains the block of code that is executed if the conditional expression in the `if` statement resolves as `False`."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "number = 3\n",
    "if number % 2 == 0:\n",
    "    print(\"number is even.\")\n",
    "else:\n",
    "    print(\"number is odd.\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Just as loops can be nested within loops, `if` statements can be nested within `if` statements."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Now that you know the concept of loops and decision making, by combining them you can do a range of different tasks!"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Exercise 4.2.1\n",
    "\n",
    "While `n < 10`, print whether each number is odd or even."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "# Exercise 4.2.1\n",
    "for n in range(1,11):\n",
    "    if n % 2 == 0:\n",
    "        print('%d is even' % n)\n",
    "    else:\n",
    "        print('%d is odd' % n)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Exercise 4.2.2\n",
    "\n",
    "While `n < 10`, find the number of values that are even."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "# Exercise 4.2.2\n",
    "counter = 0\n",
    "for n in range(1,11):\n",
    "    if n % 2 == 0:\n",
    "        counter += 1\n",
    "print(f\"Number of even values: {counter}\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "If there is more than one condition you want to check, you can add `elif` statements to check each condition. You can have as many `elif`s between an `if` and `else` as you want."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "for n in range(-2, 2):\n",
    "    if n < 0:\n",
    "        print(f'{n} is negative')\n",
    "    elif n > 0:\n",
    "        print(f'{n} is positive')\n",
    "    else:\n",
    "        print(f'{n} is zero')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "It is worth noting that in an `if`.. `elif`.. `else` block, the first statement that returns as `True` is executed."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Review:\n",
    "\n",
    "In this section, we have covered:\n",
    "- How to do decision making\n",
    "- The `if` statement.\n",
    "- The `if .. else` statment.\n",
    "- The `if .. elif .. else` statement."
   ]
  }
 ],
 "metadata": {
  "kernelspec": {
   "display_name": "Python 3",
   "language": "python",
   "name": "python3"
  },
  "language_info": {
   "codemirror_mode": {
    "name": "ipython",
    "version": 3
   },
   "file_extension": ".py",
   "mimetype": "text/x-python",
   "name": "python",
   "nbconvert_exporter": "python",
   "pygments_lexer": "ipython3",
   "version": "3.7.3"
  }
 },
 "nbformat": 4,
 "nbformat_minor": 1
}