Expand c(1:3) and c(1, 2, 3) to c(1, 2, 2, 3, 3, 3) and c(1, 1, 2, 1, 2, 3) ready for purrr::map() functions in R

get_expansion.R

# This isn't optimal and there's probably
# a function that does all this already

# Packages
library(dplyr)
library(purrr)
library(tibble)

# Write function
get_expansion <- function(x, y) {
  
  # x: number of categories
  # y: number of pages
  
  expand_df <-
    # repeat the page number by the number of pages
    data.frame(x = rep(x, y)) %>%
    # consecutive ascending row numbers within each page number
    group_by(x) %>% mutate(y = row_number()) %>% ungroup()

  return(expand_df)
  
}

# Use function to create dataframe of page-category expansions
df <- get_expansion(c(1:5), sample(1:5, 5))  # made-up data

# Map the function over the two columns of data in the df dataframe.
# The columns have the same names as the arguments to get_expansion(),
# so you can just give pmap() the dataframe instead of supplying 
# .x and .y separately. pmap_dbl() outputs to a numeric vector.
output <- pmap_dbl(df, ~ .x + .y)

# Add the numeric vector result to the original dataframe as a new column
bind_cols(df, as.data.frame(output))