How to calculate the number of occurrence of a given character in each row of a column of strings?

Solution 1:

The stringr package provides the str_count function which seems to do what you're interested in

# Load your example data
q.data<-data.frame(number=1:3, string=c("greatgreat", "magic", "not"), stringsAsFactors = F)
library(stringr)

# Count the number of 'a's in each element of string
q.data$number.of.a <- str_count(q.data$string, "a")
q.data
#  number     string number.of.a
#1      1 greatgreat           2
#2      2      magic           1
#3      3        not           0

Solution 2:

If you don't want to leave base R, here's a fairly succinct and expressive possibility:

x <- q.data$string
lengths(regmatches(x, gregexpr("a", x)))
# [1] 2 1 0