R count number of commas and string

Question

I have a string:

    str1 <- "This is a string, that I've written 
        to ask about a question, or at least tried to."

How would I :

1) count the number of commas

2) count the occurences of '-ion'

Any suggestions?

Answer 1

The stringr package has a function str_count that does this for you nicely.

library(stringr)

str_count(str1, ',')
[1] 2
str_count(str1, 'ion')
[1] 1

EDIT:

Cause I was curious:

vec <- paste(sample(letters, 1e6, replace=T), collapse=' ')

system.time(str_count(vec, 'a'))
   user  system elapsed 
  0.052   0.000   0.054 

system.time(length(gregexpr('a', vec, fixed=T)[[1]]))
   user  system elapsed 
  2.124   0.016   2.146 

system.time(length(gregexpr('a', vec, fixed=F)[[1]]))
   user  system elapsed 
  0.052   0.000   0.052 

Answer 2

The general problem of mathcing text requires regular expressions. In this case you just want to match specific characters, but the functions to call are the same. You want gregexpr.

matched_commas <- gregexpr(",", str1, fixed = TRUE)
n_commas <- length(matched_commas[[1]])

matched_ion <- gregexpr("ion", str1, fixed = TRUE)
n_ion <- length(matched_ion[[1]])

If you want to only match "ion" at the end of words, then you do need regular expressions. \b represents a word boundary, and you need to escape the backslash.

gregexpr(
  "ion\\b", 
  "ionisation should only be matched at the end of the word", 
  perl = TRUE
)