Assign vector component using string in R

Question

I have a string, say "ti[123]", and a corresponding value, say 1.2 What is the command I should use in R to assign 1.2 to the 123rd position of vector ti ? I tried assign("ti[123]",1.2) but it creates a new variable with name ti[123], which is not what I expected.

The reason I need to do this is that I use rstan's optimizing function to optimize a model, and it returns a named vector, with names like this (if you happen to use vectors of parameters in your model).

Answer 1

We can extract the number from the string using str_extract and assign the value

library(stringr)
assign('ti', '[[<-'(ti, as.numeric(str_extract("ti[123]",
                  '(?<=\\[)\\d+')), 1.2))

Or

ti[as.numeric(str_extract("ti[123]", '(?<=\\[)\\d+'))] <- 1.2

Another option would be rm_square from library(qdapRegex) to extract the numbers

library(qdapRegex) 
ti[as.numeric(rm_square('ti[123]', extract=TRUE))] <- 1.2

Update

If we don't know the object name,

str1 <- 'ti[123]'
v1 <- strsplit(str1, '[][]')[[1]]
assign(v1[1], `[[<-`(get(v1[1]), as.numeric(v1[2]), 1.2))
get(v1[1])[123]
#[1] 1.2

Or

ti[123]
#[1] 1.2