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Creating data frames
Feature engineering
Summary

## Instruction

In the previous exercise, we used numeric columns to create other numeric columns. The procedure is the same for character vectors.

For example, we can extract the quarter and store this information in a new column called quarter. We know that the quarter_prices data frame has a column named period that stores both the quarter and the year, in the following format: "Q# YYYY".

Thus, we can extract "Q#" for each quarter by using the substring() function, and store that extracted value in a new column quarter like this:

quarter_prices$quarter <- substring(quarter_prices$period, 1,2)

The first argument is the character column period. The next two arguments specify the start and end (inclusive) indexes of the substring we wish to extract. Remember that indexing begins with 1 in R. Here's the resulting data frame:

period avg_price conversion_rate avg_price_usd quarter
[1,] "Q1 2017" 170195 1.2 204234 "Q1"
[2,] "Q2 2017" 173937 1.2 208724.4 "Q2"
[3,] "Q3 2017" 177938 1.2 213525.6 "Q3"

Note that there is no error when the end index exceeds the length of the string.

## Exercise

Extract the year from the period column. Store these values in a new column named year in the quarter_prices data frame.

### Stuck? Here's a hint!

The initial index is 4, and the final index is 7.