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‎notebooks/L3/conditional-statements.ipynb‎

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{
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"cells": [
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"# Conditional statements\n",
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"\n",
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"## Sources\n",
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"\n",
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"This lesson is based on the [Software Carpentry group’s](http://software-carpentry.org/) lessons on [Programming with Python](http://swcarpentry.github.io/python-novice-inflammation/).\n",
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"\n",
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"## Basics of conditional statements\n",
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"\n",
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"Conditional statements can change the code behaviour based on meeting certain conditions.\n",
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"\n",
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"### A simple conditional statement\n",
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"\n",
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"Let’s take a simple example."
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]
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},
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"outputs": [
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{
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"name": "stdout",
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"text": [
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"it is not hot\n"
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]
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}
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],
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"source": []
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"What did we do here?\n",
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"First, we used the `if` and `else` statements to determine what parts of the code to execute.\n",
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"Note that both lines containing `if` or `else` end with a `:` and the text beneath is indented.\n",
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"What do these tests do?\n",
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"The if test checks to see whether the variable value for `temperature` is greater than 25.\n",
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"If so, `'it is hot'` would be written to the screen.\n",
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"Since 17 is smaller than 25, the code beneath the else is executed.\n",
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"The `else` statement code will run whenever the `if` test is false."
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"#### A familiar conditional scenario\n",
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"\n",
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"As it turns out, we all use logic similar to `if` and `else` conditional statements daily.\n",
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"Imagine you’re getting ready to leave your home for the day and want to decide what to wear.\n",
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"You might look outside to check the weather conditions.\n",
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"If it is raining, you will wear a rain jacket.\n",
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"Otherwise, you will not.\n",
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"In Python we could say:\n",
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"\n",
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"```python\n",
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"weather = 'Rain'\n",
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"\n",
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"if weather == 'Rain':\n",
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" print('Wear a raincoat')\n",
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"else:\n",
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" print('No raincoat needed')\n",
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"\n",
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"Wear a raincoat\n",
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"```\n",
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"Note here that we use the `==` to test if a value is exactly equal to another."
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"### else is not required\n",
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"\n",
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"The combination of if and else is very common, but both are not strictly required."
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]
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},
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{
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"Note that here we use only the `if` statement, and because 13 is not greater than 25, nothing is printed to the screen."
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"### Introducing second test\n",
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"\n",
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"We can also have a second test for an if statment by using the elif (else-if) statement."
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]
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},
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{
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"name": "stdout",
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"text": [
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"-3 is below freezing\n"
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]
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}
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],
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{
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"cell_type": "markdown",
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"Makes sense, right? Note here that we again use the == to test if a value is exactly equal to another. The complete list of these comparison operators is given in the table below.\n",
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"\n",
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"| Operator | Meaning |\n",
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"| -------- | ------------------------ |\n",
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"| < | Less than |\n",
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"| <= | Less than or equal to |\n",
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"| == | Equal to |\n",
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"| >= | Greater than or equal to |\n",
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"| > | Greater than |\n",
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"| != | Not equal to |"
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]
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},
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{
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"cell_type": "markdown",
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"source": [
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"<hr>\n",
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"\n",
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"#### Poll question pause\n",
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"\n",
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"Time to check your understanding.\n",
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"Let's assume that yesterday it was 14°C, it is 10°C outside today, and tomorrow it will be 13°C.\n",
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"The following code compares these temperatures and prints something to the screen based on the comparison.\n",
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"\n",
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"```python\n",
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"yesterday = 14\n",
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"today = 10\n",
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"tomorrow = 13\n",
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"\n",
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"if yesterday <= today:\n",
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" print('A')\n",
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"elif today != tomorrow:\n",
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" print('B')\n",
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"elif yesterday > tomorrow:\n",
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" print('C')\n",
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"elif today == today:\n",
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" print('D')\n",
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"```\n",
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"\n",
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"Which of the letters `A`, `B`, `C`, and `D` would be printed to the screen?\n",
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"Select your answer from the poll options at https://geo-python.github.io/poll/.\n",
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"\n",
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"<hr>"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"### Combining conditions\n",
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"\n",
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"We can also use `and` and `or` to have multiple conditions."
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]
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},
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{
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"outputs": [
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{
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"name": "stdout",
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"text": [
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"One part is not true\n"
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]
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}
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],
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"source": []
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},
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{
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"cell_type": "code",
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"outputs": [
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{
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"name": "stdout",
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"text": [
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"At least one test is true\n"
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]
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}
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],
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"source": []
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},
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{
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"cell_type": "markdown",
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"These are just simple examples, but concepts that can be quite handy."
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"#### Another familiar conditional scenario\n",
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"\n",
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"Again, making decisions based on multiple conditions is something we regularly do.\n",
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"Imagine that we consider not only the rain, but also whether or not it is windy.\n",
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"If it is windy and raining, we’ll just stay home.\n",
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"Otherwise, we need appropriate clothing to go out.\n",
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"We can again handle this kind of decision with Python.\n",
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"\n",
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"```python\n",
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"weather = 'Rain'\n",
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"wind = 'Windy'\n",
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"\n",
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"if (weather == 'Rain') and (wind == 'Windy'):\n",
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" print('Just stay home')\n",
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"elif weather == 'Rain':\n",
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" print('Wear a raincoat')\n",
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"else:\n",
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" print('No raincoat needed')\n",
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"\n",
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"Just stay home\n",
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"```\n",
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"\n",
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"As you can see, we better just stay home if it is windy and raining."
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]
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}
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