Removing strings from rows for specific words

Question

My Data looks like:

Weather                           
   <chr>                             
 1 Snow Low clouds                   
 2 Snow Cloudy                       
 3 Drizzle Fog                       
 4 Thundershowers Partly cloudy      
 5 Thunderstorms More clouds than sun
 6 Sprinkles Partly cloudy           
 7 Heavy rain Broken clouds          
 8 Light rain Partly cloudy     

I am trying to use mutate to remove some text. For example I would like the above to look like:

Weather                           
   <chr>                             
 1 Snow                   
 2 Snow                       
 3 Drizzle                      
 4 Thundershowers      
 5 Thunderstorms More clouds than sun
 6 Sprinkles Partly cloudy           
 7 Heavy rain           
 8 Light rain 

So I would like to remove the text after some specific words. If I have a vector of the following:

c("Snow", "Drizzle", "Heavy rain", "Light rain") 

Remove the text after these. However I do not want to grep words such as Cloudy , Fog since they occure as their own row in the data but something like Snow Light fog can be cut down to Snow .

Data:

d <- structure(list(Weather = c("Snow Low clouds", "Snow Cloudy", 
"Drizzle Fog", "Thundershowers Partly cloudy", "Thunderstorms More clouds than sun", 
"Sprinkles Partly cloudy", "Heavy rain Broken clouds", "Light rain Partly cloudy", 
"Rain showers Passing clouds", "Thundershowers Scattered clouds", 
"Thundershowers Passing clouds", "Light snow Overcast", "Snow Light fog", 
"Drizzle Broken clouds", "Light rain Fog", "Cloudy", "Thunderstorms Partly cloudy", 
"Heavy rain More clouds than sun", "Partly cloudy", NA)), class = c("tbl_df", 
"tbl", "data.frame"), row.names = c(NA, -20L))

Answer 1

A general approach you can take here is to build a regex alternation of all target terms. Then, match those terms followed by anything up until the end of the input, and replace with just the term.

terms <- c("Snow", "Drizzle", "Heavy rain", "Light rain")
regex <- paste0("\\b(", paste(terms, collapse="|"), ")\\b")
sub(paste0(regex, "\\s.*"), "\\1", d$Weather)

 [1] "Snow"                               "Snow"                              
 [3] "Drizzle"                            "Thundershowers Partly cloudy"      
 [5] "Thunderstorms More clouds than sun" "Sprinkles Partly cloudy"           
 [7] "Heavy rain"                         "Light rain"                        
 [9] "Rain showers Passing clouds"        "Thundershowers Scattered clouds"   
[11] "Thundershowers Passing clouds"      "Light snow Overcast"               
[13] "Snow"                               "Drizzle"                           
[15] "Light rain"                         "Cloudy"                            
[17] "Thunderstorms Partly cloudy"        "Heavy rain"                        
[19] "Partly cloudy"                      NA

Note that my output does not line up exactly with your expected output, but then again you did not include all target words in the suggested vector.

The regex I used was:

\b(Snow|Drizzle|Heavy rain|Light rain)\b

The trick here is that the above alternation is also a capture group, letting us easily replace the match with just the term you want. You may add more terms to this to get the desired output.

Answer 2

• Maybe you can use the code below

v <- c("Snow", "Drizzle", "Heavy rain", "Light rain") 
pat <- paste0(v,collapse = "|")
unlist(regmatches(d$Weather,gregexpr(pat,d$Weather)))

such that

> unlist(regmatches(d$Weather,gregexpr(pat,d$Weather)))
[1] "Snow"       "Snow"       "Drizzle"    "Heavy rain" "Light rain" "Snow"      
[7] "Drizzle"    "Light rain" "Heavy rain"

• If you want to add the extracted value and append them to d in a new column, then you can use the following code:

d <- within(d,X <- ifelse(grepl(pat,Weather),unlist(regmatches(Weather,gregexpr(pat,Weather))),NA))

such that

> d
# A tibble: 20 x 2
   Weather                            X         
   <chr>                              <chr>     
 1 Snow Low clouds                    Snow      
 2 Snow Cloudy                        Snow      
 3 Drizzle Fog                        Drizzle   
 4 Thundershowers Partly cloudy       NA        
 5 Thunderstorms More clouds than sun NA        
 6 Sprinkles Partly cloudy            NA        
 7 Heavy rain Broken clouds           Drizzle   
 8 Light rain Partly cloudy           Light rain
 9 Rain showers Passing clouds        NA        
10 Thundershowers Scattered clouds    NA        
11 Thundershowers Passing clouds      NA        
12 Light snow Overcast                NA        
13 Snow Light fog                     Heavy rain
14 Drizzle Broken clouds              Light rain
15 Light rain Fog                     Snow      
16 Cloudy                             NA        
17 Thunderstorms Partly cloudy        NA        
18 Heavy rain More clouds than sun    Heavy rain
19 Partly cloudy                      NA        
20 NA                                 NA