Apply function with an index variable as the second input.
iapply(X, FUN, ..., .method = c("sapply", "lapply", "vapply"))a list or matrix depends on .method. See lapply
a vector (atomic or list)
the function to be applied to each element of X: see `Details`.
passed to apply methods
method to use, default is sapply
FUN will be further passed to the apply methods. Unlike
lapply, FUN is expected to have at least two arguments.
The first argument is each element of X, the second argument is the
index number of the element.