Identify contiguous segments of data in a vector

segments() and changes() are extremely useful functions for finding contiguous "segments" indicated in a vector. It can be particularly useful to use segments() to create grouping factors.

Usage

segments(..., first = TRUE, any = TRUE, reverse = FALSE)

changes(..., first = TRUE, value = FALSE, any = TRUE, reverse = FALSE)

Arguments

...

A list of atomic vectors.

If the vectors differ in length, they are all recycled to match the length of the longest vector.

first

Is the first index (or last index if reverse == TRUE) marked as a "change."

Defaults to TRUE.

Must be a singleton logical value: an on/off switch.

any

Whether to mark changes any or all input vectors.

Defaults to TRUE.

Must be a singleton logical value: an on/off switch.

If TRUE, a change in any input vector is marked a change. If FALSE, changes must occur in all input vectors to be marked as a change.

reverse

Whether the excecution order is reversed.

Defaults to FALSE.

Must be a singleton logical value: an on/off switch.

If TRUE the function is excecuted backwards through the input vector(s).

value

Whether to return the changed value matrix.

Defaults to FALSE.

Must be a singleton logical value: an on/off switch.

If TRUE, the input values where changes occur are returned in a matrix, with each row matching a change and each column containing the value from the associated input vector.

Changes

changes takes and input vector and finds all indices i where the value of x[i] != x[i-1]---i.e., where the value at one index has "changed" since the last index. By default, changes returns a logical vector the same length as the input, with TRUE only at indices where a change occured. The first argument indicates whether the first index (i == 1) is marked TRUE.

changes can accept more than one input vector. If the any argument is set to TRUE (the default), a change in any input is marked as a change (TRUE) in the output. If any == FALSE, changes must happen in all vectors to be marked in the output.

Finally, the reverse argument reverses the behavior of changes, checkig instead if x[i] != x[i + 1].

Values

By default, the values of the input vector(s) where a change occurs are placed in a matrix and put in the values attribute of the logical output. However, if the value argument is set to TRUE, the values themselves are returned.

Segments

The segments builds off of the changes function. The segments function takes a logical input and cummulatively tallies each TRUE value in the vector, from left to right (or right to left, if reverse == TRUE). Thus, the input c(TRUE, FALSE, TRUE, FALSE, FALSE, TRUE, TRUE, FALSE) would return c(1, 1, 2, 2, 2, 3, 4, 4). This creates contiguous blocks of values which can be used for a groupby argument in a call to within.humdrumR(), or similar functions like base::tapply().

Any input vector(s) to segments which are not logical, are first fed to changes to create a logical input.

Examples

segments(letters %~% '[aeiou]')
#>  [1] 1 1 1 1 2 2 2 2 3 3 3 3 3 3 4 4 4 4 4 4 5 5 5 5 5 5

changes(c(1, 1, 1, 2, 2, 2, 3, 3, 3, 4, 4, 4), 
        c(1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3),
        any = TRUE)
#>  [1]  TRUE FALSE FALSE  TRUE  TRUE FALSE  TRUE FALSE  TRUE  TRUE FALSE FALSE
#> attr(,"values")
#>    [,1] [,2]
#> 1     1    1
#> 4     2    1
#> 5     2    2
#> 7     3    2
#> 9     3    3
#> 10    4    3
# result is T,F,F,T,T,F,T,F,T,T,F,F