Inserting random letters at random locations within a string - r

Question

I am trying to make a little script to demonstrate how DNA sequences can evolve using a sentence as an example. I would like to repeatedly replace or insert letters or words into a string in R. I would like this to happen repeatedly so one can watch the string change over time. Finally I would like there to be a greater probability of letters changing than words changing.
So far I have defined a string and created lists of both letters and words and sample randomly from both these lists.
However I do not know how to then modify the text with a set probability. For example how do I make it so there is a 50% chance of a letter in the text being replaced with a letter from my letter list and if this happens, it should occur at a random location in the text?
I also want this process to occur X times so I can show the text changing over time. Any help or suggestions are greatly appreciated. My current incomplete code is below
#First I define the string
text <- c("This sentence is changing")
#Then make a vector of words from the string
word_list <- strsplit(text, " ")
word_list <- unlist(word_list)
#Also make a vector of letters from the string
letters_and_gaps <- substring(text, seq(1, nchar(text), 1), seq(1, nchar(text), 1))
letters_and_gaps <- unlist(letters_and_gaps)
#Now for probability 1 in 2 or it occuring, select a random character from letters_and_gaps:
sample(letters_and_gaps, 1)
#Then choose a random character in text and replace it with this randomly sampled character:
#Now with probability 1 in 10 or it occuring, select a random word from word_list
sample(letters_and_gaps, 1)
#Then choose a random word in text and replace it with this randomly sampled word:
#Then print the updated text:
text
#Iteratively repeat this process X times
My goal is to ultimately put this in a Shiny app where one can select the probability of different events occuring (letter vs word replacement) and then watch how this influence how the text evolves.

Answer 1

Here is the beginning of an implementation. We just wrap your logic up in a function and use a for loop to apply it again and again. Here I put the output in a table and then display only unique rows (possibly excluding times where it mutated back to the same string as a previous iteration but probably not significant) so you can see that changes happening. Note that because we are sampling from the words and characters of the previous sentence, and we are including spaces, new words can form when spaces are inserted and the distribution will tend to become more uniform (if a character is common it will tend to be substituted more often)
library(tidyverse)
evolve_sentence <- function(sentence, arg2) {
chars <- str_split(sentence, "") %>% pluck(1)
if (runif(1) > 0.5) {
chars[sample(1:length(chars), 1)] <- sample(chars, 1)
}
sentence <- str_c(chars, collapse = "")
words <- str_split(sentence, " ") %>% pluck(1)
if (runif(1) > 0.9) {
words[sample(1:length(words), 1)] <- sample(words, 1)
}
sentence <- str_c(words, collapse = " ")
sentence
}
tbl_evolve <- tibble(iteration = 1:500, text = "This sentence is changing")
for (i in 2:500) {
tbl_evolve$text[i] <- evolve_sentence(tbl_evolve$text[i - 1])
}
tbl_evolve %>%
distinct(text, .keep_all = TRUE)
#> # A tibble: 204 x 2
#> iteration text
#> <int> <chr>
#> 1 1 This sentence is changing
#> 2 3 hhis sentence is changing
#> 3 4 hhis sentence is chasging
#> 4 6 hhis sestence is chasging
#> 5 10 hhi sestence is chasging
#> 6 12 hhi sesnence is chasging
#> 7 14 hhi sesnesce is chasging
#> 8 15 hhi se nesce is chasging
#> 9 18 hhi se nesceiis chasging
#> 10 20 hhi se nesceiis chasgihg
#> # … with 194 more rows
Created on 2019-04-17 by the reprex package (v0.2.1)