vapply

vapply is a function in the R programming language, part of the `utils` package, designed to apply a function over a list or vector and return a list of the same length. It is a more constrained and often safer alternative to the generic `lapply` function. The primary distinguishing feature of `vapply` is its requirement to specify the expected output type and dimensions for each element of the returned list. This explicit specification helps catch errors early by ensuring that the function applied to each element produces a result consistent with the declared format.

The syntax for `vapply` is `vapply(X, FUN, FUN.VALUE, ..., USE.NAMES = TRUE)`. `X` is the input list or vector. `FUN` is the function to be applied. `FUN.VALUE` is a template for the expected output of `FUN` for a single element of `X`, specifying its type (e.g., numeric, character, logical) and dimensions (e.g., a single value, a vector of length 3). The ellipses `...` represent additional arguments to be passed to `FUN`. `USE.NAMES` controls whether the names of `X` are preserved in the output.

By enforcing the `FUN.VALUE`, `vapply` can prevent unexpected behavior that might arise from `lapply` when the applied function returns results of varying types or lengths. This makes `vapply` particularly useful in situations where data consistency and predictable output structures are important, such as in data analysis and package development.