Use getPrice to extract other columns

The extractor functions you learned in the previous two exercises do not cover all use cases. Sometimes you might have one object that contains the same price column for multiple instruments. Other times, you might have an object with price data (e.g. bid and/or ask prices) that do not have an explicit extractor function.

The quantmod package also provides a getPrice() function that is flexible enough to handle these cases. You can extract a column with any name (e.g. bid, ask, trade) with it by specifying the name via the prefer argument. It is also useful if you have an object that contains multiple instruments with the same price type, because you can use the symbol argument to extract columns for a specific instrument.

You can use regular expressions for both the prefer and symbol arguments, because they are passed to the base::grep() function internally.

Instructions

Finish the Quandl() call to download CME futures data for CL and BZ. First, specify the database in the code argument using "CME/CLH2016" and "CME/BZH2016". Then set the type to return an xts object. Store in oil_data.
Use colnames() to view the column names of the oil_data object.
Finish the getPrice() call to extract the Open price for CL. Use prefer to avoid matching both "Open" and "Open Interest". Store in cl_open.
Use a ISO-8601 string to subset cl_open for the month of January, 2016 (don't use a range, and you don't need to specify any days).

Take Hint (-30xp)