在R中使用ifelse提取字符串中模式的位置

Question

I have a set of strings x for example:

[1] "0000000000000000000000000000000000000Y"   "9000000000D00000000000000000000Y"        
[3] "0000000000000D00000000000000000000X"      "000000000000000000D00000000000000000000Y"
[5] "000000000000000000D00000000000000000000Y" "000000000000000000D00000000000000000000Y"
[6]"000000000000000000000000D0000000011011D1X"

I want to extract the last position of a particular character like 1. I am running this code:

ifelse(grepl("1",x),rev(gregexpr("1",x)[[1]])[1],50)

But this is returning -1 for all elements. How do I correct this?

Answer 1

In base R, the following returns the position of the last matched "1" .

# Make some toy data
toydata <- c("001", "007", "00101111Y", "000AAAYY")

# Find last postion
last_pos <- sapply(gregexpr("1", toydata), function(m) m[length(m)])
print(last_pos)
#[1]  3 -1  8 -1

It returns -1 whenever the pattern is not matched.

Answer 2

We can use stri_locate_last from stringi . If there are no matches, it will return NA .

library(stringi)
r1 <- stri_locate_last(v1, fixed=1)[,1]
r1
#[1] NA NA NA NA NA NA 40
nchar(v1)
#[1] 38 32 35 40 40 40 41

If we need to replace the NA values with number of characters

ifelse(is.na(r1), nchar(v1), r1)

data

v1 <-  c("0000000000000000000000000000000000000Y", 
      "9000000000D00000000000000000000Y", 
      "0000000000000D00000000000000000000X",
      "000000000000000000D00000000000000000000Y",
      "000000000000000000D00000000000000000000Y", 
      "000000000000000000D00000000000000000000Y", 
      "000000000000000000000000D0000000011011D1X")