grep

How To Select Multiple Columns Using Grep & R

August 18, 2019 2 Data Science Using R Recent Posts

Why you need to be using Grep when programming with R.

There’s a reason that grep is included in most if not all programming language to this day 44 years later from creation. It’s useful and simple to use. Below is an example of using grep to make selecting multiple columns in R simple and easy to read.

The dataset below has the following column names. 

names(data) # Column Names
 [1] "fips"                 "state"                "county"               "metro_area"          
 [5] "population"           "med_hh_income"        "poverty_rate"         "population_lowaccess"
 [9] "lowincome_lowaccess"  "no_vehicle_lowaccess" "s_grocery"            "s_supermarket"       
[13] "s_convenience"        "s_specialty"          "s_farmers_market"     "r_fastfood"          
[17] "r_full_service"

How can we select only the columns we need to work with?

  • metro_area
  • med_hh_income
  • poverty_rate
  • population_lowaccess
  • lowincome_lowaccess
  • no_vehicle_lowaccess
  • s_grocery
  • s_supermarket
  • s_convenience
  • s_specialty
  • s_farmers_market
  • r_fastfood
  • r_full_service

We can tell R exactly by listing each column as below

data[c("metro_area","med_hh_income", "poverty_rate", "population_lowaccess", "lowincome_lowaccess", "no_vehicle_lowaccess","s_grocery","s_supermarket","s_convenience","s_specialty","s_farmers_market", "r_fastfood", "r_full_service")]

OR

We can tell R where each column we want is.

data[c(4,6,7:17)]

First, writing out each individual column is time consuming and chances are you’re going to make a typo (I did when writing it). Second option we have to first figure out where the columns are located to then tell R. Well looking at the columns we are trying to access vs the others theirs a specific difference. All these columns have a “_” located in there name, and we can use regular expressions (grep) to select these. 

data[grep("_", names(data))])

FYI… to get the column locations you can actually use…

grep("_", names(data))
[1]  4  6  7  8  9 10 11 12 13 14 15 16 17

You will rarely have a regular expression as easy at “_” to select multiple columns, a very useful resource to learn and practice is https://regexr.com

Data was obtained from https://www.ers.usda.gov/data-products/food-access-research-atlas/download-the-data/

Egrep & Fgrep

July 30, 2019 0 Data Science Linux Recent Posts

EGREP:

            The Command egrep is the same as running grep –E. egrep is used to search for a pattern using extended regular expressions.

Terry@f:~/FinderDing>cat testsort
A line 1	
a line 2	
8 line 3	
line 4
5 line 5	
Terry@f:~/FinderDing>egrep '^[a-zA-Z]' testsort	
A line 1
a line 2
line 4

*Show lines that start with a letter from alphabet

Terry@f:~/FinderDing>cat html
<!DOCTYPE html>
<html>	
<body>
<h1>My First Heading</h1>
<p>My first paragraph.</p>
</body>
</html>
Terry@f:~/FinderDing>egrep "My|first" html
<h1>My First Heading</h1>
<p>My first paragraph.</p>

`*Find lines with pattern My first from html file

FGREP:

The command fgrep is the same as running grep –F. The Command searches for fixed character strings in a file, which means regular expressions can’t be used.

Terry@f:~/FinderDing>fgrep "My" html
<h1>My First Heading</h1>
<p>My first paragraph.</p>l