Closes #592 -- setcolorder now handles incomplete specifications auto…

…matically

amend tests/implementation per jan

NEWS.md

@@ -5,6 +5,8 @@
 
 1. `indices()` function gain new argument `vectors` default `FALSE`, when `TRUE` provided then list of vectors is returned, single vector refers to single index. Closes #1589.
 
+2. `setcolorder` allows incomplete specification of new order (chosen columns are moved to the front), [#592](https://github.com/Rdatatable/data.table/issues/592). Thanks @MichaelChirico for the PR. 
+
 #### BUG FIXES
 
 #### NOTES

R/data.table.R

@@ -2569,22 +2569,30 @@ setnames <- function(x,old,new) {
     invisible(x)
 }
 
-setcolorder <- function(x,neworder)
+setcolorder <- function(x, neworder)
 {
+    if (any(duplicated(neworder))) stop("neworder contains duplicates")
     # if (!is.data.table(x)) stop("x is not a data.table")
-    if (length(neworder)!=length(x)) stop("neworder is length ",length(neworder)," but x has ",length(x)," columns.")
+    if (length(neworder) != length(x)) {
+      if (length(neworder) > length(x)) 
+        stop("neworder is length ", length(neworder), 
+             " but x has only ", length(x), " columns.")
+      #if shorter than length(x), pad by the missing
+      #  elements (checks below will catch other mistakes)
+      neworder = c(neworder, setdiff(if (is.character(neworder)) names(x)
+                                     else seq_along(x), neworder))
+    }
     if (is.character(neworder)) {
-        if (any(duplicated(neworder))) stop("neworder contains duplicate column names")
-        if (any(duplicated(names(x)))) stop("x has some duplicated column name(s): ",paste(names(x)[duplicated(names(x))],collapse=","),". Please remove or rename the duplicate(s) and try again.")
-        o = as.integer(chmatch(neworder,names(x)))
-        if (any(is.na(o))) stop("Names in neworder not found in x: ",paste(neworder[is.na(o)],collapse=","))
+        if (any(duplicated(names(x)))) stop("x has some duplicated column name(s): ", paste(names(x)[duplicated(names(x))], collapse=","), ". Please remove or rename the duplicate(s) and try again.")
+        o = as.integer(chmatch(neworder, names(x)))
+        if (any(is.na(o))) stop("Names in neworder not found in x: ", paste(neworder[is.na(o)], collapse=","))
     } else {
         if (!is.numeric(neworder)) stop("neworder is not a character or numeric vector")
         o = as.integer(neworder)
         m = !(o %in% seq_len(length(x)))
-        if (any(m)) stop("Column numbers in neworder out of bounds: ",paste(o[m],collapse=","))
+        if (any(m)) stop("Column numbers in neworder out of bounds: ", paste(o[m], collapse=","))
     }
-    .Call(Csetcolorder,x,o)
+    .Call(Csetcolorder, x, o)
     invisible(x)
 }
 

inst/tests/tests.Rraw

@@ -1442,7 +1442,6 @@ DT = data.table(a=1:2,b=3:4,c=5:6)
 test(495, setcolorder(DT,c(2,1,3)), data.table(b=3:4,a=1:2,c=5:6))
 test(496, setcolorder(DT,c(2,1,3)), data.table(a=1:2,b=3:4,c=5:6))
 test(497, setcolorder(DT,c("c","a","b")), data.table(c=5:6,a=1:2,b=3:4))
-test(498, setcolorder(DT,"a"), error="neworder is length")
 test(498.1, setcolorder(DT,c("d","a","b")), error="Names in neworder not found in x: d")
 
 
@@ -9789,6 +9788,11 @@ indices(DT, vectors = TRUE)
 test(1749.1, indices(DT), c("A__B","A","B"))
 test(1749.2, indices(DT, vectors = TRUE), list(c("A","B"),"A","B"))
 
+# allow incomplete specification in setcolorder, #592
+DT = data.table(a = 1:3, b = 2:4, c = 3:5)
+test(1750.1, names(setcolorder(DT, "b")), c("b", "a", "c"))
+test(1750.2, names(setcolorder(DT, c(2, 3))), c("a", "c", "b"))
+test(1750.3, setcolorder(DT, 1:4), error = "x has only")
 
 ##########################
 

man/setcolorder.Rd

@@ -13,7 +13,7 @@ setcolorder(x, neworder)
 }
 \arguments{
   \item{x}{ A \code{data.table}. }
-  \item{neworder}{ Character vector of the new column name ordering. May also be column numbers. }
+  \item{neworder}{ Character vector of the new column name ordering. May also be column numbers. If \code{length(neworder) < length(x)}, the specified columns are moved in order to the "front" of \code{x}. }
 }
 \details{
   To reorder \code{data.table} columns, the idiomatic way is to use \code{setcolorder(x, neworder)}, instead of doing \code{x <- x[, neworder, with=FALSE]}. This is because the latter makes an entire copy of the \code{data.table}, which maybe unnecessary in most situations. \code{setcolorder} also allows column numbers instead of names for \code{neworder} argument, although we recommend using names as a good programming practice. 
@@ -30,6 +30,9 @@ DT = data.table(A=sample(3, 10, TRUE),
          B=sample(letters[1:3], 10, TRUE), C=sample(10))
 
 setcolorder(DT, c("C", "A", "B"))
+
+#incomplete specification
+setcolorder(DT, "A")
 }
 \keyword{ data }