Turn repeat-sales data into sales pairs that are suitable for making repeat-sales matrices.
Arguments
- period
A vector that gives the time period for each sale. Usually a date vector, or a factor with the levels in chronological order, but other values are possible if they can be sorted in chronological order (i.e., with
order()).- product
A vector that gives the product identifier for each sale. Usually a factor or vector of integer codes for each product.
Value
A numeric vector of indices giving the position of the previous sale
for each product, with the convention that the previous sale for the
first sale is itself. Ties are resolved according to the order they
appear in period.
Note
order() is the workhorse of rs_pairs(), so performance can be
sensitive to the types of period and product, and can be slow for large
character vectors.
See also
rs_matrix() for using sales pairs to make a repeat-sales index.
rtCreateTrans() in the hpiR package for a feature-rich but
slower and less flexible function to make sales pairs.
Examples
# Make sales pairs
x <- data.frame(
id = c(1, 1, 1, 3, 2, 2, 3, 3),
date = c(1, 2, 3, 2, 1, 3, 4, 1),
price = c(1, 3, 2, 3, 1, 1, 1, 2)
)
pairs <- rs_pairs(x$date, x$id)
x[c("date_prev", "price_prev")] <- x[c("date", "price")][pairs, ]
x
#> id date price date_prev price_prev
#> 1 1 1 1 1 1
#> 2 1 2 3 1 1
#> 3 1 3 2 2 3
#> 4 3 2 3 1 2
#> 5 2 1 1 1 1
#> 6 2 3 1 1 1
#> 7 3 4 1 2 3
#> 8 3 1 2 1 2