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##' Apply a Function to a Data Frame Split by Factors
##'
##' Simple wrapper of the 'by' function
##' @title Apply a Function to a Data Frame Split by Factors
##' @param x Data frame
##' @param INDICES Indices (vector or list of indices, vector of column names, or formula of column names)
##' @param FUN A function to be applied to data frame subsets of 'data'.
##' @param COLUMNS (Optional) subset of columns of x to work on
##' @param array if TRUE an array/matrix is always returned
##' @param ... Additional arguments to lower-level functions
##' @author Klaus K. Holst
##' @export
##' @examples
##' By(datasets::CO2,~Treatment+Type,colMeans,~conc)
##' By(datasets::CO2,~Treatment+Type,colMeans,~conc+uptake)
By <- function(x,INDICES,FUN,COLUMNS,array=FALSE,...) {
if (inherits(INDICES,"formula")) {
INDICES <- as.list(model.frame(INDICES,x))
} else {
if (is.character(INDICES) && length(INDICES)!=nrow(x)) {
INDICES <- as.list(x[,INDICES,drop=FALSE])
}
}
if (!missing(COLUMNS)) {
if (inherits(COLUMNS,"formula")) {
x <- model.frame(COLUMNS,x)
} else {
x <- x[,COLUMNS,drop=FALSE]
}
}
a <- by(x, INDICES, FUN=FUN, ...)
if (NCOL(x)==1 && !array) {
##DimElem <- length(a[rep(1,length(dim(a)))][[1]])
a <- a[]
attr(a,"call") <- NULL
}
return(a)
}
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