Merge pull request #347 from NPalopoli/patch-16

extra_challenges.md: new file

Author: wrightaprilm

_extras/extra_challenges.md

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+---   
+layout: page                                                                                            
+title: "Extra Challenges"                                                                           
+permalink: /extra_challenges/                                                                            
+---                                                                                                     
+
+# Extra Challenges
+
+A collection of challenges that have been either removed from or not (yet) added to the main lesson. 
+
+> ## Looping Over DataFrame
+>
+> (Please refer to lesson `06-loops-and-functions.md`)
+>
+> The file `surveys.csv` in the `data` folder contains 25 years of data from surveys,
+> starting from 1977. We can extract data corresponding to each year in this DataFrame
+> to individual CSV files, by using a `for` loop:
+> 
+> ~~~
+> import pandas as pd
+> 
+> # Load the data into a DataFrame
+> surveys_df = pd.read_csv('data/surveys.csv')
+> 
+> # Loop through a sequence of years and export selected data
+> start_year = 1977
+> end_year = 2002
+> for year in range(start_year, end_year+1):
+>
+>     # Select data for the year
+>     surveys_year = surveys_df[surveys_df.year == year]  
+>
+>     # Write the new DataFrame to a CSV file
+>     filename = 'data/surveys' + str(year) + '.csv' 
+>     surveys_year.to_csv(filename)
+> ~~~
+> {: .language-python}
+>
+> What happens if there is no data for a year in a sequence? For example,
+> imagine we used `1976` as the `start_year`
+>
+> > ## Solution
+> > We get the expected files for all years between 1977 and 2002,
+> > plus an empty `data/surveys1976.csv` file with only the headers.  
+> {: .solution}  
+{: .challenge}