Allow passing data.frame directly to dcast(), melt() (#7634)

* feat(7614): added s3 method at dcast and melt for data.frame

* feat(7614): added to news

* feat(7614): code styling

* feat(7614): bug fix on dcast

* feat(7614): added s3method to namespace

* feat(7614): fix test 2365.1 as its calling undefined columns

* feat(7614): removed s3method and used redirection incase of data.frame for dcast/melt

* feat(7614): code linting

* feat(7614): use call to capture args and redirect to dcast.data.table and melt.data.table

* feat(7614): updated tests and dcast, melt

* Tidy up NEWS

* sync deprecated test comments

---------

Co-authored-by: Michael Chirico <chiricom@google.com>

Author: manmita

NEWS.md

@@ -26,6 +26,8 @@
       - Type conversion support in GForce expressions (e.g., `sum(as.numeric(x))` will use GForce, saving the need to coerce `x` in a setup step) [#2934](https://github.com/Rdatatable/data.table/issues/2934)
       - Arithmetic operation support in GForce (e.g., `max(x) - min(x)` will use GForce on both `max(x)` and `min(x)`, saving the need to do the subtraction in a follow-up step) [#3815](https://github.com/Rdatatable/data.table/issues/3815)
 
+4. `dcast()` and `melt()` "just work" when passed a data.frame, not just data.tables, with no need for coercion, [#7614](https://github.com/Rdatatable/data.table/issues/7614). Thanks @MichaelChirico for the suggestion and @manmita for the PR. Note that to avoid potential conflicts with {reshape2}'s data.frame methods, we do the dispatch to the data.table method manually.
+
 ### BUG FIXES
 
 1. `fread()` with `skip=0` and `(header=TRUE|FALSE)` no longer skips the first row when it has fewer fields than subsequent rows, [#7463](https://github.com/Rdatatable/data.table/issues/7463). Thanks @emayerhofer for the report and @ben-schwen for the fix.

R/fcast.R

@@ -12,6 +12,11 @@ dcast = function(
   data, formula, fun.aggregate = NULL, ..., margins = NULL,
   subset = NULL, fill = NULL, value.var = guess(data)
 ) {
+  if (!is.data.table(data) && is.data.frame(data)){
+    mc <- match.call()
+    mc[[1L]] <- as.name("dcast.data.table")
+    return(eval(mc, parent.frame()))
+  }
   UseMethod("dcast", data)
 }
 
@@ -119,7 +124,6 @@ aggregate_funs = function(funs, vals, sep="_", ...) {
 }
 
 dcast.data.table = function(data, formula, fun.aggregate = NULL, sep = "_", ..., margins = NULL, subset = NULL, fill = NULL, drop = TRUE, value.var = guess(data), verbose = getOption("datatable.verbose"), value.var.in.dots = FALSE, value.var.in.LHSdots = value.var.in.dots, value.var.in.RHSdots = value.var.in.dots) {
-  if (!is.data.table(data)) stopf("'%s' must be a data.table", "data")
   drop = as.logical(rep_len(drop, 2L))
   if (anyNA(drop)) stopf("'drop' must be logical vector with no missing entries")
   if (!isTRUEorFALSE(value.var.in.dots))

R/fmelt.R

@@ -4,6 +4,11 @@
 #   redirection as well
 
 melt = function(data, ..., na.rm = FALSE, value.name = "value") {
+  if (!is.data.table(data) && is.data.frame(data)){
+    mc <- match.call()
+    mc[[1L]] <- as.name("melt.data.table")
+    return(eval(mc, parent.frame()))
+  }
   UseMethod("melt", data)
 }
 
@@ -176,7 +181,6 @@ measurev = function(fun.list, sep="_", pattern, cols, multiple.keyword="value.na
 melt.data.table = function(data, id.vars, measure.vars, variable.name = "variable",
        value.name = "value", ..., na.rm = FALSE, variable.factor = TRUE, value.factor = FALSE,
        verbose = getOption("datatable.verbose")) {
-  if (!is.data.table(data)) stopf("'%s' must be a data.table", "data")
   for(type.vars in c("id.vars","measure.vars")){
     sub.lang <- substitute({
       if (missing(VAR)) VAR=NULL

inst/tests/tests.Rraw

@@ -13042,9 +13042,7 @@ test(1953.2, melt(DT, id.vars = 'id', measure.vars = patterns(a = 'a', b = 'b',
              error = 'Patterns not found')
 test(1953.3, melt(DT, id.vars = 'id', measure.vars = patterns(1L)),
              error = 'Input patterns must be of type character')
-setDF(DT)
-test(1953.4, melt.data.table(DT, id.vars = 'id', measure.vars = 'a'),
-             error = "must be a data.table")
+# 1953.4 was a test of melt() erroring when passed a data.frame, no longer the case, #7614
 
 # appearance order of two low-cardinality columns that were squashed in pr#3124
 DT = data.table(A=INT(1,3,2,3,2), B=1:5)  # respect groups in 1st column (3's and 2's)
@@ -13389,9 +13387,7 @@ test(1962.083, guess(DT), '(all)')
 setnames(DT, 'V')
 test(1962.084, guess(DT), 'V',
      message = 'Using.*value column.*override')
-setDF(DT)
-test(1962.085, dcast.data.table(DT), error = 'must be a data.table')
-setDT(DT)
+# 1962.085 was a test of dcast() erroring when passed a data.frame, no longer the case, #7614
 test(1962.086, dcast(DT, a ~ a, drop = NA),
      error = "'drop' must be logical vector with no missing entries")
 DT = data.table(a = c(1, 1, 2, 2), b = list(1, 2, 3, 4), c = c(4, 4, 2, 2))
@@ -21509,3 +21505,13 @@ setdroplevels(x)
 setdroplevels(y)
 test(2364.2, levels(x$a), levels(y$a))
 rm(x, y)
+
+# test for data.frame reshape for melt
+df_melt = data.frame(a = 1:2, b = 3:4)
+dt_melt = data.table(a = 1:2, b = 3:4)
+test(2365.1, melt(df_melt, id.vars=1:2), melt(dt_melt, id.vars=1:2))
+
+# test for data.frame reshape for dcast
+df_dcast = data.frame(a = c("x", "y"), b = 1:2, v = 3:4)
+dt_dcast = data.table(a = c("x", "y"), b = 1:2, v = 3:4)
+test(2365.2, dcast(df_dcast, a ~ b, value.var = "v"), dcast(dt_dcast, a ~ b, value.var = "v"))