Sunday, July 16, 2017

(R) Data Frame Extraction

In this article, we will be discussing how to extract data from existing data frames within R.

If you aren’t already familiar with the function of braces( ‘[‘ and ‘]’ ) within R, we will briefly review their usage.

When you encounter braces in R, the variables specified within the braces themselves, are instructing R to query and return data.

[ X , Y ]

Above is an example of how such a query would appear within the R code base.

X - Specifies Row

Y - Specifies Column

So if a programmer were to write the code:

E <- DataFrameA[ 1 , 2, drop = FALSE]

R would interpret this to mean: return the data from Row: 1, Column: 2, and store this data in factor variable ‘E’.

Leaving either the left or the right position empty in a braces related query, instructs R to return ALL data.


E <- DataFrameA[ 1 ,  , drop = FALSE]

Would instruct R to return ALL Column data from Row:1. (And store this data in ‘E’)


E <- DataFrameA[ , 1 ] 
Would instruct R to return ALL Row data from Column:1. (And store this data in ‘E’)

The following are examples of code samples which extract data from R Data Frames.

E <- DataFrameA[3, 2, drop = FALSE] Extracts the third element in the second column of DataFrameA, and stores that element in factor variable ‘E'.

E <- DataFrameA[c(1 , 3), 2, drop = FALSE] Extracts the data within row 1 and row 3, within column 2, of DataFrameA. The data is then stored in factor variable ‘E’.

E <- DataFrameA[5, ] Extracts row 5, and all column data contained within row 5. The data will be stored in DataFrame ‘E'.

E <- DataFrameA[ , 8] Extract all rows data from column 8. The data is then stored in factor variable ‘E’.

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