How to convert a sequence of numbers from data frame to text file in special format in R?

Question

I have a data frame with one field consisting of a sequence of numbers:

test <- data.frame(N=c(1,2,3,5,7,8,9,11,13,14,15))

> test
    N
1   1
2   2
3   3
4   5
5   7
6   8
7   9
8  11
9  13
10 14
11 15

• The field N contains a sequence of integers in ascending order
• N sometimes skips some numbers, such as 2,3,5 (4 is missing).

I need to convert it into the following text format:

1-3,5,7-9,
11,13-15

This file is not data frame, but just a simple text file which contains the following conditions:

• Consecutive numbers which are located in the middle should be removed and replaced by -, i.e., 1,2,3 should be 1-3 and 1,2,3,5,6 should be 1-3,5,6
• Each number (or shortened consecutive numbers) should be separated by comma , (no space needed)
• If one line has three numbers or shortened consecutive numbers, the line should be broken to go to the next line
• End of each line should have comma, but the last line should not have

Currently I just could convert the data frame into a sequence of numbers, but the output is surrounded by c(), consecutive numbers cannot be shortened without line breaks.

> tapply(test, (seq_along(test)-1)%/%3, paste, collapse=", ")
                                       0 
"c(1, 2, 3, 5, 7, 8, 9, 11, 13, 14, 15)" 

I appreciate your idea to make it!! Thank you in advance for your support.

Answer 1

I'll say v as a vector.

v <- c(1,2,3,5,7,8,9,11,13,14,15)

then split v into consecutive sets

vv <- split(v, cumsum(c(1, diff(v) != 1)))
vv

$`1`
[1] 1 2 3

$`2`
[1] 5

$`3`
[1] 7 8 9

$`4`
[1] 11

$`5`
[1] 13 14 15

Finally, transform to form you want

lapply(vv, function(x) {
  if (length(x) == 1) {
    x
  } else(
    paste0(x[1], "-", tail(x, n=1))
  )
  
}) %>% unlist %>% as.vector

[1] "1-3"   "5"     "7-9"   "11"    "13-15"

Answer 2

Here's a possible solution using dplyr -

library(dplyr)

output <- test %>%
  #Create groups to collapse consecutive numbers
  group_by(grp = cumsum(c(TRUE, diff(N) > 1))) %>%
  #If more than 1 number in a group paste first and last value
  summarise(text = if(n() > 1) paste0(range(N), collapse = '-') else as.character(N)) %>%
  #For each 3 groups collapse the ranges as one string
  group_by(line_num = ceiling(row_number()/3)) %>%
  summarise(text = toString(text))

output

#  line_num text       
#     <dbl> <chr>      
#1        1 1-3, 5, 7-9
#2        2 11, 13-15  

#Write the output
cat(paste0(output$text, collapse = '\n'), file = 'output.txt')

The output text file looks like -