function to sample variable number of substrings given string length - r

Question

I'm trying to write an R function that will sample a variable number of 5-element substrings, based on the length of the original string in each row of a data frame. I first calculated the number of times I'd like each draw to repeat, and would like to add this into the function so that the number of samples taken for each row is based on the "num_draws" column for that row. my thought was to use a generalized instance, and then use an apply statement outside of the function to act on each row, but I can't figure out how to set up the function to call col 3 as a generalized instance (without calling either the value of just the first row, or the value of all rows).
example data frame:
BP TF num_draws
1 CGGCGCATGTTCGGTAATGA TFTTTFTTTFFTTFTTTTTF 6
2 ATAAGATGCCCAGAGCCTTTTCATGTACTA TFTFTFTFFFFFFTTFTTTTFTTTTFFTTT 9
3 TCTTAGGAAGGATTC FTTTTTTTTTFFFFF 4
desired output:
[1]GGCGC FTTTF
AATGA TTTTF
TTFFT TGTTC
TAATG TTTTT
AATGA TTTTF
CGGCG TFTTT
[2]AGATG FTFTF
ATAAG TFTFT
ATGCC FTFFF
GCCCA FFFFF
ATAAG TFTFT
GTACT TFFTT
GCCCA FFFFF
TGCCC TFFFF
AGATG FTFTF
[3]TTAGG TTTTT
CTTAG TTTTT
GGAAG TTTTT
GGATT TTFFF
example code:
#make example data frame
BaseP1 <- paste(sample(size = 20, x = c("A","C","T","G"), replace = TRUE), collapse = "")
BaseP2 <- paste(sample(size = 30, x = c("A","C","T","G"), replace = TRUE), collapse = "")
BaseP3 <- paste(sample(size = 15, x = c("A","C","T","G"), replace = TRUE), collapse = "")
TrueFalse1 <- paste(sample(size = 20, x = c("T","F"), replace = TRUE), collapse = "")
TrueFalse2 <- paste(sample(size = 30, x = c("T","F"), replace = TRUE), collapse = "")
TrueFalse3 <- paste(sample(size = 15, x = c("T","F"), replace = TRUE), collapse = "")
my_df <- data.frame(c(BaseP1,BaseP2,BaseP3), c(TrueFalse1, TrueFalse2, TrueFalse3))
#calculate number of draws by length
frag_length<- 5
my_df<- cbind(my_df, (round((nchar(my_df[,1]) / frag_length) * 1.5, digits = 0)))
colnames(my_df) <- c("BP", "TF", "num_draws")
#function to sample x number of draws per row (this does not work)
Fragment = function(string) {
nStart = sample(1:(nchar(string) -5), 1)
samp<- substr(string, nStart, nStart + 4)
replicate(n= string[,3], expr = samp)
}
apply(my_df[,1:2], c(1,2), Fragment)

Answer 1

One option would be to change the function to have another argument n and create the nStart inside the replicate call
Fragment = function(string, n) {
replicate(n= n, {nStart <- sample(1:(nchar(string) -5), 1)
samp <- substr(string, nStart, nStart + 4)
})
}
apply(my_df, 1, function(x) data.frame(lapply(x[1:2], Fragment, n = x[3])))
$`1`
# BP TF
#1 GGCGC FFTTF
#2 GGTAA TFFTT
#3 GCGCA TTFTT
#4 CGCAT TFFTT
#5 GGCGC FTTTF
#6 TGTTC FTTFT
#$`2`
# BP TF
#1 GTACT TTTTF
#2 ATAAG FTTFT
#3 GTACT TFTFF
#4 TAAGA TTTTF
#5 CCTTT FFTTF
#6 TCATG TTTTF
#7 CCAGA TFTFT
#8 TTCAT TFTFT
#9 CCCAG FTFTF
#$`3`
# BP TF
#1 AAGGA TTTFF
#2 AGGAT TTTTT
#3 CTTAG TFFFF
#4 TAGGA TTTFF