Replace values in dplyr

Question

I've got a dataset with some lower detection limit values (values are not consistent) and I want to replace them with half of the detection value.I tried with following r codes but it doesn't work. Can anyone help with this?

dat %>% mutate(value=sub("<0.02500","0.0125",value))

Site    value
NR  <0.02500
NR  <0.02500
NR  <0.02500
NR  <0.02500
NR  <0.02500
NR  <0.02500
NR  0.01
NR  0.01
NR  0.01
NR  0.02
NR  0.01
NR  0.01
NR  0.01
NR  0.01
NR  0.01
NR  <0.05100
NR  <0.05100
NR  <0.05100
NR  <0.05000
NR  <0.05000
NR  <0.05000
NR  <0.05000
NR  <0.05000
NR  <0.05000
NR  0.02
NR  0.017
NR  0.031
NR  0.025
NR  0.023
NR  0.024
NR  0.023

Answer 1

Assuming that the value column is character (as in the Note at the end or if not convert them first) remove the < character, convert to numeric and multiply each value by 0.5 or 1.

library(dplyr)

dat %>% 
  mutate(value = as.numeric(sub("<", "", value)) * if_else(grepl("<", value), .5, 1))

or using only base R:

transform(dat, value = as.numeric(sub("<", "", value)) * ifelse(grepl("<", value), .5, 1))

Note

Lines <- "Site    value
NR  <0.02500
NR  <0.02500
NR  <0.02500
NR  <0.02500
NR  <0.02500
NR  <0.02500
NR  0.01
NR  0.01
NR  0.01
NR  0.02
NR  0.01
NR  0.01
NR  0.01
NR  0.01
NR  0.01
NR  <0.05100
NR  <0.05100
NR  <0.05100
NR  <0.05000
NR  <0.05000
NR  <0.05000
NR  <0.05000
NR  <0.05000
NR  <0.05000
NR  0.02
NR  0.017
NR  0.031
NR  0.025
NR  0.023
NR  0.024
NR  0.023"
dat <- read.table(text = Lines, header = TRUE, as.is = TRUE)

Answer 2

We can use data.table :

library (data.table)

setDT(df1)[, value := ifelse(as.character(value)=="<0.02500", 
                             "0.0125", as.character(value)), ][]

#     Site    value
# 1:    NR   0.0125
# 2:    NR   0.0125
# 3:    NR   0.0125
# 4:    NR   0.0125
# 5:    NR   0.0125
# 6:    NR   0.0125
# 7:    NR     0.01
# ...
# 24:   NR <0.05000
# 25:   NR     0.02
# 26:   NR    0.017
# 27:   NR    0.031
# 28:   NR    0.025
# 29:   NR    0.023
# 30:   NR    0.024
# 31:   NR    0.023
#     Site    value