Air Pollution Project

This was a project, or the first Programming Assignment of the Johns Hopkins

R course, of the Data Science Specialization path, in Coursera.

This assignment consists in 3 exercises, or writing 3 functions.

I will describe those functions below.

1. Write a function that takes some (given) .csv files from a directory,
   and calculate the mean from a specified column, of a given number or
   of files and return the mean of the column/s of one or more files.

2. Write a function that takes the directory of the .csv files, and the id
   of the files to work on and return a data frame with two columns, id
   (id of files) and nobs (number of complete cases or complete rows).

3. Write a function that returns the correlation, using cor() function,
   from a collection of files with a given threshold of observed complete
   cases, or complete lines.