Insert string in vector before every string containing pattern

I have the following vector:

test <- c("here is some text", "here is some other text", "here is my formula", "2+2", "here is my second formula", "4+4", "here is even more text", "here is my final formula", "6+6")

What I'm hoping to do is take every instance of a string containing "formula" and inserting a random string like "CCC" in front of it so that I'd have something like the following:

test <- c("here is some text", "here is some other text", "CCC", "here is my formula", "2+2", "CCC", "here is my second formula", "4+4", "here is even more text", "CCC", "here is my final formula", "6+6")

These use only base R:

1) Use append as shown:

test2 <- test
ix <- rev(grep("formula", test)) - 1
for(i in ix) test2 <- append(test2, "CCC", i)
test2
##  [1] "here is some text"         "here is some other text"  
##  [3] "CCC"                       "here is my formula"       
##  [5] "2+2"                       "CCC"                      
##  [7] "here is my second formula" "4+4"                      
##  [9] "here is even more text"    "CCC"                      
## [11] "here is my final formula"  "6+6"         

2) Here are three different one-liners.

The first creates a matrix whose first row contains "CCC" and NA elements and whose second row is test. It then unravels that and removes the NA's.

The second iterates over test outputting the element if formula is not contained in it or a vector "CCC" followed by the element. This produces a list which is unlisted.

The third prefaces any element containing formula with "CCC\n" and then splits it up.

# 2a
c(na.omit(c(rbind(ifelse(grepl("formula", test), "CCC", NA), test))))

# 2b
unlist(lapply(test, function(x) if (grepl("formula", x)) c("CCC", x) else x))

# 2c
scan(text = sub("(.*formula)", "CCC\n\", test), what="", quiet=TRUE, sep="\n")

Update: Improved code:

library(tidyverse)

test %>%
  tibble(value = .) %>%
  mutate(index = row_number(), new_val = ifelse(str_detect(value, "formula"), "CCC", NA_character_)) %>%
  pivot_longer(c(new_val, value), values_drop_na = TRUE) %>%
  pull(value)

Here is another approach:
I already mentioned this: For me it is much easier to think in terms of a data frame or a tibble in this situation:

library(tidyverse)

test %>%
  as_tibble() %>% 
  mutate(index = row_number()) %>% 
  mutate(new_val = ifelse(str_detect(test, "formula"), "CCC", NA_character_)) %>% 
  pivot_longer(c(new_val, value)) %>% 
  drop_na() %>% 
  pull(value)

 [1] "here is some text"         "here is some other text"  
 [3] "CCC"                       "here is my formula"       
 [5] "2+2"                       "CCC"                      
 [7] "here is my second formula" "4+4"                      
 [9] "here is even more text"    "CCC"                      
[11] "here is my final formula"  "6+6"  

Here is one option in tidyverse

library(dplyr)
library(stringr)
library(tidyr)
tibble(test) %>% 
 uncount(str_detect(test, 'formula') + 1) %>%
  mutate(test = replace(test, duplicated(test, fromLast = TRUE), "CCC"))

-output

# A tibble: 12 × 1
   test                     
   <chr>                    
 1 here is some text        
 2 here is some other text  
 3 CCC                      
 4 here is my formula       
 5 2+2                      
 6 CCC                      
 7 here is my second formula
 8 4+4                      
 9 here is even more text   
10 CCC                      
11 here is my final formula 
12 6+6            

Here is a base R way with grepl/cumsum to create a grouping vector with each group starting at a "formula" string. Then by will insert "CCC" in its place.

test <- c("here is some text", "here is some other text",
          "here is my formula", "2+2", "here is my second formula",
          "4+4", "here is even more text", 
          "here is my final formula", "6+6")

j <- cumsum(grepl("formula", test))
test <- unname(unlist(by(test, j, FUN = \(x) c("CCC", x))))
if(j[1L] == 0) test <- test[-1L]
test
#>  [1] "here is some text"         "here is some other text"  
#>  [3] "CCC"                       "here is my formula"       
#>  [5] "2+2"                       "CCC"                      
#>  [7] "here is my second formula" "4+4"                      
#>  [9] "here is even more text"    "CCC"                      
#> [11] "here is my final formula"  "6+6"

^{Created on 2023-03-30 with reprex v2.0.2}