corrected challenge format and added context

Updated to comply with Carpentries' format for challenges. Also added context by referring to original lesson `06-loops-and-functions.md` and adapting commands from it.

Author: NPalopoli

_extras/extra_challenges.md

@@ -8,12 +8,31 @@ permalink: /extra_challenges/
 
 A collection of challenges that have been either removed from or not (yet) added to the main lesson. 
 
-## 06-loops-and-functions
-
-### Looping Over Dataframe Challenges
-
-* What happens if there is no data for a year in the sequence (for example, imagine we had used 1976 as the start year in `range`)
-~~~
-An empty file with only the headers
-~~~
-{: .output}
+> ## 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 load the data and print all the years surveyed 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 print the year
+> start_year = 1977
+> end_year = 2002
+> for year in range(start_year, end_year+1):
+>     print(str(year))
+> ~~~
+> {: .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 in `range`
+>
+> > ## Solution
+> > An empty file with only the headers  
+> {: .solution}  
+{: .challenge}