fread nrows=0L fixed to work like nrows=0

NEWS.md

@@ -18,6 +18,8 @@
 
 2. `print(DT, trunc.cols=TRUE)` and the corresponding `datatable.print.trunc.cols` option (new feature 3 in v1.13.0) could display an extra `diff.prev` column, [#4266](https://github.com/Rdatatable/data.table/issues/4266). Thanks to @tdhock for the PR.
 
+3. `fread(..., nrows=0L)` now works as intended and the same as `nrows=0`; i.e. returning the column names and typed empty columns determined by the large sample, [#4686](https://github.com/Rdatatable/data.table/issues/4686). Thanks to @hongyuanjia for reporting, and Benjamin Schwendinger for the PR.
+
 ## NOTES
 
 1. New feature 29 in v1.12.4 (Oct 2019) introduced zero-copy coercion. Our thinking is that requiring you to get the type right in the case of `0` (type double) vs `0L` (type integer) is too inconvenient for you the user. So such coercions happen in `data.table` automatically without warning. Thanks to zero-copy coercion there is no speed penalty, even when calling `set()` many times in a loop, so there's no speed penalty to warn you about either. However, we believe that assigning a character value such as `"2"` into an integer column is more likely to be a user mistake that you would like to be warned about. The type difference (character vs integer) may be the only clue that you have selected the wrong column, or typed the wrong variable to be assigned to that column. For this reason we view character to numeric-like coercion differently and will warn about it. If it is correct, then the warning is intended to nudge you to wrap the RHS with `as.<type>()` so that it is clear to readers of your code that a coercion from character to that type is intended. For example :

R/fread.R

@@ -25,7 +25,8 @@ yaml=FALSE, autostart=NA, tmpdir=tempdir(), tz="UTC")
              isTRUEorFALSE(verbose), isTRUEorFALSE(check.names), isTRUEorFALSE(logical01), isTRUEorFALSE(keepLeadingZeros), isTRUEorFALSE(yaml) )
   stopifnot( isTRUEorFALSE(stringsAsFactors) || (is.double(stringsAsFactors) && length(stringsAsFactors)==1L && 0.0<=stringsAsFactors && stringsAsFactors<=1.0))
   stopifnot( is.numeric(nrows), length(nrows)==1L )
-  if (is.na(nrows) || nrows<0L) nrows=Inf   # accept -1 to mean Inf, as read.table does
+  nrows=as.double(nrows) #4686
+  if (is.na(nrows) || nrows<0) nrows=Inf   # accept -1 to mean Inf, as read.table does
   if (identical(header,"auto")) header=NA
   stopifnot(is.logical(header) && length(header)==1L)  # TRUE, FALSE or NA
   stopifnot(is.numeric(nThread) && length(nThread)==1L)

inst/tests/tests.Rraw

@@ -13259,14 +13259,17 @@ test(1957.3, fread("A,B\na,b\nc,d\n", stringsAsFactors=TRUE, verbose=TRUE), data
              output="stringsAsFactors=TRUE converted 2 column(s): [A, B]")
 
 # misc. coverage tests in fread
-test(1958.1, fread('\U0001f64d', encoding = 'UTF-16'), error = "Argument 'encoding' must be")
-test(1958.2, fread('a,b\n1,2', nrows = NA_real_),      data.table(a = 1L, b = 2L))
-test(1958.3, fread('a,b\n1,2', nrows = -1),            data.table(a = 1L, b = 2L))
-test(1958.4, fread('a,b\n1,2', key = 1),               error = 'must be a character vector naming columns')
-test(1958.5, fread("A,B,C\n1,2,3\n3,4,5\n0,0,0\n",   nrows=0),          data.table(A=logical(), B=logical(), C=logical()))  #2747
-test(1958.6, fread("A,B,C\n1,2,3\n3,4,5\n0,0,100\n", nrows=0, sep=','), data.table(A=logical(), B=logical(), C=logical()))
-test(1958.7, fread('A,B,C,D\n"a,b",4,5,6\n"c,d",6,7\n', fill=TRUE), data.table(A=c("a,b","c,d"), B=INT(4,6), C=INT(5,7), D=INT(6,NA)))  # 2547
-test(1958.8, fread('A,B,C,D\n"a,b",4,5\n"c,d",6,7,8\n', fill=TRUE), data.table(A=c("a,b","c,d"), B=INT(4,6), C=INT(5,7), D=INT(NA,8)))
+test(1958.01, fread('\U0001f64d', encoding = 'UTF-16'), error = "Argument 'encoding' must be")
+test(1958.02, fread('a,b\n1,2', nrows = NA_real_),      data.table(a = 1L, b = 2L))
+test(1958.03, fread('a,b\n1,2', nrows = -1),            data.table(a = 1L, b = 2L))
+test(1958.04, fread('a,b\n1,2', key = 1),               error = 'must be a character vector naming columns')
+test(1958.05, fread("A,B,C\n1,2,3\n3,4,5\n0,0,0\n",   nrows=0),          data.table(A=logical(), B=logical(), C=logical()))  #2747
+test(1958.06, fread("A,B,C\n1,2,3\n3,4,5\n0,0,100\n", nrows=0, sep=','), data.table(A=logical(), B=logical(), C=logical()))
+test(1958.07, fread('A,B,C,D\n"a,b",4,5,6\n"c,d",6,7\n', fill=TRUE), data.table(A=c("a,b","c,d"), B=INT(4,6), C=INT(5,7), D=INT(6,NA)))  # 2547
+test(1958.08, fread('A,B,C,D\n"a,b",4,5\n"c,d",6,7,8\n', fill=TRUE), data.table(A=c("a,b","c,d"), B=INT(4,6), C=INT(5,7), D=INT(NA,8)))
+# 4686
+test(1958.09, fread("A,B,C\n1,2,3\n3,4,5\n0,0,0\n",   nrows=0L),          data.table(A=logical(), B=logical(), C=logical())) 
+test(1958.10, fread("A,B,C\n1,2,3\n3,4,5\n0,0,100\n", nrows=0L, sep=','), data.table(A=logical(), B=logical(), C=logical()))
 
 # Skip should work with all types of newlines #3006
 eols = c("\n", "\r\n", "\r", "\n\r")

src/freadR.c

@@ -122,12 +122,10 @@ SEXP freadR(
   else if (LOGICAL(headerArg)[0]==TRUE) args.header = true;
 
   args.nrowLimit = INT64_MAX;
-  // checked at R level
-  if (isReal(nrowLimitArg)) {
-    if (R_FINITE(REAL(nrowLimitArg)[0]) && REAL(nrowLimitArg)[0]>=0.0) args.nrowLimit = (int64_t)(REAL(nrowLimitArg)[0]);
-  } else {
-    if (INTEGER(nrowLimitArg)[0]>=1) args.nrowLimit = (int64_t)INTEGER(nrowLimitArg)[0];
-  }
+  if (!isReal(nrowLimitArg) || length(nrowLimitArg)!=1)
+    error(_("Internal error: freadR nrows not a single real. R level catches this."));  // # nocov
+  if (R_FINITE(REAL(nrowLimitArg)[0]) && REAL(nrowLimitArg)[0]>=0.0)
+    args.nrowLimit = (int64_t)(REAL(nrowLimitArg)[0]);
 
   args.logical01 = LOGICAL(logical01Arg)[0];
   {