匹配序列。获取遵循模式的元素的索引

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pattern <- letters[1:4]
keep <- logical(length(vec))

s <- 0
for (i in seq_along(keep)){
    if (pattern[s +1] == vec[i]){
        keep[i] <- T
        s <- (s + 1) %% length(pattern)
    } else {
        keep[i] <- F
    }
}


vec_filtered <- vec[keep]

which(keep)

#Initialise index to loop through "pattern" and "vec"
i = 1
j = 1
#Create a new variable to store index values
index = numeric()
#Until "vec" is not over
while (i < length(vec)) {
   #Check if a match is found
   if(vec[i] == pattern[j]) {
     #If found store the index
     index = c(index, i)
     j = j + 1
     #Recycle the "pattern" object and continue searching till "vec" is not over
     if (j > length(pattern))
       j = 1
    }
   i = i + 1
}

index
#[1]  1  3  5 15 17 20