My first programming assignment for [Coursera Data Science Specialization] (https://www.coursera.org/specialization/jhudatascience/1) course [R Programming] (https://www.coursera.org/course/rprog). The requirement was to write three functions to interact with an air pollution dataset. The dataset is contained in a zip file specdata.zip.

The zip file contains 332 comma-separated-value (CSV) files containing pollution monitoring data for fine particulate matter (PM) air pollution at 332 locations in the United States. Each file contains data from a single monitor and the ID number for each monitor is contained in the file name. For example, data for monitor 200 is contained in the file "200.csv". Each file contains three variables:

• Date: the date of the observation in YYYY-MM-DD format (year-month-day)
• sulfate: the level of sulfate PM in the air on that date (measured in micrograms per cubic meter)
• nitrate: the level of nitrate PM in the air on that date (measured in micrograms per cubic meter)

This file contains the function named ‘pollutantmean’ that calculates the mean of a pollutant (sulfate or nitrate) across a specified list of monitors. The function ‘pollutantmean’ takes three arguments: ‘directory’, ‘pollutant’, and ‘id’. The function then reads the particulate matter data for the corresponding monitor(s) from the directory specified in the ‘directory’ argument and returns the mean of the pollutant across all of the monitors, ignoring any missing values coded as NA. Some things to know:

• ’directory’ is a character vector of length 1 indicating the location of the CSV files
• ’pollutant’ is a character vector of length 1 indicating the name of the pollutant for which we will calculate the mean; either "sulfate" or "nitrate".
• ‘id’ is an integer vector indicating the monitor ID number(s) to be used

This file contains the function named ‘complete’ that reads a directory full of files and reports the number of completely observed cases in each data file. The function returns a data frame where the first column is the name of the file and the second column is the number of complete cases (“nobs”). Some things to know:

• ’directory’ is a character vector of length 1 indicating the location of the CSV files
• ’id’ is an integer vector indicating the monitor ID number(s) to be used

This file contains the function named ‘corr’ that takes a directory of data files and a threshold for complete cases and calculates the correlation between sulfate and nitrate for monitor locations where the number of completely observed cases (on all variables) is greater than the specified threshold. The function returns a vector of correlations for the monitors that meet the threshold requirement. If no monitors meet the threshold requirement, then the function returns a numeric vector of length 0. Some things to know:

• ‘directory’ is a character vector of length 1 indicating the location of the CSV files
• ‘threshold’ is a numeric vector of length 1 indicating the number of completely observed observations (on all variables) required to compute the correlation between nitrate and sulfate; the default is 0

Unzip the data file which will create the directory ‘specdata’. In each file you’ll notice that there are many days where either sulfate or nitrate (or both) are missing (coded as NA). This is common with air pollution monitoring data in the United States. Once you’ve unzipped the data file, call the functions and supply the required arguments as listed above.

As this is a course assignment, it’s not open to contributions.

GNU General Public License

Copyright (C) 2014 Stephanie Brown, @stef2dotoh

This file is part of Air Pollution.

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.