R: Counting Sequences of "Grouped Coin Flips"-CodePudding

I am working with the R programming language.

I have this dataset over here - different students flip a coin a different number of times:

set.seed(123)
ids = 1:100
student_id = sample(ids, 1000, replace = TRUE)
coin_result = sample(c("H", "T"), 1000, replace = TRUE)
my_data = data.frame(student_id, coin_result)

my_data =  my_data[order(my_data$student_id),]

Based on my data, I want to count the number of "3 sequence" coin flips sequences for each student.

I know how to do this for the entire dataset at once:

# https://stackoverflow.com/questions/74758896/r-counting-the-frequencies-of-coin-flips

results = my_data$coin_result

n_sequences <- function(n, results) {
  helper <- function(i, n) if (n < 1) "" else sprintf(
    "%s%s", 
    helper(i, n - 1), 
    results[i   n - 1]
  )
  result <- data.frame(
    table(
      sapply(
        1:(length(results) - n   1),
        function(i) helper(i, n)
      )
    )
  )
  colnames(result) <- c("Sequence", "Frequency")
  result
}


n_sequences(3, results)

  Sequence Frequency
1      HHH       140
2      HHT       129
3      HTH       132
4      HTT       119
5      THH       129
6      THT       121
7      TTH       119
8      TTT       109

Now, I am trying to perform similar calculations - but for individual students - and then grouped over all students. That is, I want the "counter" to restart every time a new student starts flipping the coin. Thus, this would allow me to find out the total number of times "HHH" appears for all students individually.

I thought of a very slow and inefficient way to do this:

 library(dplyr)

 my_list = list()

for (i in 1:length(unique(ids))) {
    tryCatch({
        frame_i = my_data[my_data$student_id == i,]
        results_i = frame_i$coin_result
        results = results_i
        results_i = n_sequences(3, results)
        final_i = cbind(student_id = i, results_i)
        my_list[[i]] = final_i
        #print(final_i)
    }, error = function(e) {})
}


goal = do.call(rbind.data.frame, my_list)

# EXPECTED OUTPUT
summary = goal %>% group_by(Sequence) %>% summarise(sums = sum(Frequency))

> summary
# A tibble: 8 x 2
  Sequence  sums
  <fct>    <int>
1 HTT         93
2 TTH         93
3 HHH        112
4 HHT        106
5 HTH        108
6 THH         97
7 TTT         94
8 THT         97

Even if my approach is correct - I have a feeling that running this loop for big datasets (e.g. when there over 1 million student_id) will take a long time to run.

Can someone please suggest a more efficient way to solve this problem?

Thanks!

Note: I am not sure the n_sequence() function can work if any student in the data frame has fewer than "n" sequences - e.g n_sequences(n =5, results) . This is why I added a tryCatch() statement to override such occurrences.

CodePudding user response：

Here‘s some dplyr code:

library(tidyverse)
my_data %>%
  group_by(student_id) %>%
  summarize(Sequence = str_c(coin_result, lead(coin_result), lead(coin_result, 2)), .groups = 'drop') %>%
  filter(!is.na(Sequence)) %>%
  count(Sequence)

# A tibble: 8 x 2
  Sequence     n
  <chr>    <int>
1 HHH        112
2 HHT        106
3 HTH        108
4 HTT         93
5 THH         97
6 THT         97
7 TTH         93
8 TTT         94