Extra Challenges

A collection of challenges that have been either removed from or not (yet) added to the main lesson.

06-loops-and-functions

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03-index-slice-subset

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Additional slicing challenge

You can also select every Nth row by providing a third number inside the [], e.g. surveys_df[1:10:2] returns every other row in the DataFrame, from the second to the tenth:

OUTPUT

   record_id  month  day  year  plot_id species_id sex  hindfoot_length  weight
1          2      7   16  1977        3         NL   M             33.0     NaN
3          4      7   16  1977        7         DM   M             36.0     NaN
5          6      7   16  1977        1         PF   M             14.0     NaN
7          8      7   16  1977        1         DM   M             37.0     NaN
9         10      7   16  1977        6         PF   F             20.0     NaN

Given this, what do you think will happen when you run surveys_df[::-1]? After you have predicted the result, run the code to see if you were correct.

Show me the solution

surveys_df[::-1] provides every row of the DataFrame, in reverse order.

Looping Over DataFrame

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:

PYTHON

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)

What happens if there is no data for a year in a sequence? For example, imagine we used 1976 as the start_year

Show me the solution

We get the expected files for all years between 1977 and 2002, plus an empty data/surveys1976.csv file with only the headers.