Reshaping multiple sets of measurement columns (wide format) into single columns (long format)

I have a dataframe in a wide format, with repeated measurements taken within different date ranges. In my example there are three different periods, all with their corresponding values. E.g. the first measurement (Value1) was measured in the period from DateRange1Start to DateRange1End:

ID DateRange1Start DateRange1End Value1 DateRange2Start DateRange2End Value2 DateRange3Start DateRange3End Value3
1 1/1/90 3/1/90 4.4 4/5/91 6/7/91 6.2 5/5/95 6/6/96 3.3

I'm looking to reshape the data to a long format such that the DateRangeXStart and DateRangeXEnd columns are grouped,. Thus, what was 1 row in the original table becomes 3 rows in the new table:

ID DateRangeStart DateRangeEnd Value
1 1/1/90 3/1/90 4.4
1 4/5/91 6/7/91 6.2
1 5/5/95 6/6/96 3.3

I know there must be a way to do this with reshape2/melt/recast/tidyr, but I can't seem to figure it out how to map the multiple sets of measure variables into single sets of value columns in this particular way.

Solution

Reshaping from wide to long format with multiple value/measure columns is possible with the function pivot_longer() of the tidyr package since version 1.0.0.

This is superior to the previous tidyr strategy of gather() than spread() (see answer by @AndrewMacDonald), because the attributes are no longer dropped (dates remain dates and numerics remain numerics in the example below).

library("tidyr")
library("magrittr")

a <- structure(list(ID = 1L, 
                    DateRange1Start = structure(7305, class = "Date"), 
                    DateRange1End = structure(7307, class = "Date"), 
                    Value1 = 4.4, 
                    DateRange2Start = structure(7793, class = "Date"),
                    DateRange2End = structure(7856, class = "Date"), 
                    Value2 = 6.2, 
                    DateRange3Start = structure(9255, class = "Date"), 
                    DateRange3End = structure(9653, class = "Date"), 
                    Value3 = 3.3),
               row.names = c(NA, -1L), class = c("tbl_df", "tbl", "data.frame"))

pivot_longer() (counterpart: pivot_wider()) works similar to gather(). However, it offers additional functionality such as multiple value columns. With only one value column, all colnames of the wide data set would go into one long column with the name given in names_to. For multiple value columns, names_to may receive multiple new names.

This is easiest if all column names follow a specific pattern like Start_1, End_1, Start_2, etc. Therefore, I renamed the columns in the first step.

(names(a) <- sub("(\\d)(\\w*)", "\\2_\\1", names(a)))
#>  [1] "ID"               "DateRangeStart_1" "DateRangeEnd_1"  
#>  [4] "Value_1"          "DateRangeStart_2" "DateRangeEnd_2"  
#>  [7] "Value_2"          "DateRangeStart_3" "DateRangeEnd_3"  
#> [10] "Value_3"

pivot_longer(a, 
             cols = -ID, 
             names_to = c(".value", "group"),
             # names_prefix = "DateRange",
             names_sep = "_")
#> # A tibble: 3 x 5
#>      ID group DateRangeEnd DateRangeStart Value
#>   <int> <chr> <date>       <date>         <dbl>
#> 1     1 1     1990-01-03   1990-01-01       4.4
#> 2     1 2     1991-07-06   1991-05-04       6.2
#> 3     1 3     1996-06-06   1995-05-05       3.3

Alternatively, the reshape may be done using a pivot spec that offers finer control (see link below):

spec <- a %>%
    build_longer_spec(cols = -ID) %>%
    dplyr::transmute(.name = .name,
                     group = readr::parse_number(name),
                     .value = stringr::str_extract(name, "Start|End|Value"))

pivot_longer(a, spec = spec)

^{Created on 2019-03-26 by the reprex package (v0.2.1)}