如何找到R中向量中每个元素的文本字符串的匹配位置？

Question

I have a vector of text characters, say month.name:

> month.name
 [1] "January"   "February"  "March"     "April"     "May"       "June"      "July"     
 [8] "August"    "September" "October"   "November"  "December" 

What R function should I use to find the position of "ber" such that it returns a numeric vector in the form of c(-1,-1,-1,-1,-1,-1,-1,-1,7,5,6,6), ie, -1 for no match and 5 for the fifth character?

Answer 1

You could use stringr::str_locate . It returns a matrix:

library(stringr)
str_locate(month.name, "ber")

      start end
 [1,]    NA  NA
 [2,]    NA  NA
 [3,]    NA  NA
 [4,]    NA  NA
 [5,]    NA  NA
 [6,]    NA  NA
 [7,]    NA  NA
 [8,]    NA  NA
 [9,]     7   9
[10,]     5   7
[11,]     6   8
[12,]     6   8

So str_locate(month.name, "ber")[, 'start'] returns a vector:

 [1] NA NA NA NA NA NA NA NA  7  5  6  6

Personally I think NA is a better choice for "no match" than -1. You could always substitute -1 later if you really want to do so. For example:

pos <- str_locate(month.name, "ber")[, 'start']
ifelse(is.na(pos), -1, pos)

 [1] -1 -1 -1 -1 -1 -1 -1 -1  7  5  6  6

Answer 2

This is the exact output of ?regexpr (along with some other helpful attributes):

regexpr("ber", month.name)
# [1] -1 -1 -1 -1 -1 -1 -1 -1  7  5  6  6
#attr(,"match.length")
# [1] -1 -1 -1 -1 -1 -1 -1 -1  3  3  3  3
#attr(,"index.type")
#[1] "chars"
#attr(,"useBytes")
#[1] TRUE