sep2 can be an empty string. Closes #5414

Author: philippechataignon

R/fwrite.R

@@ -46,7 +46,7 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
     is.list(x),
     identical(quote,"auto") || isTRUEorFALSE(quote),
     is.character(sep) && length(sep)==1L && (nchar(sep) == 1L || identical(sep, "")),
-    is.character(sep2) && length(sep2)==3L && nchar(sep2[2L])==1L,
+    is.character(sep2) && length(sep2)==3L && nchar(sep2[2L])<=1L,
     is.character(dec) && length(dec)==1L && nchar(dec) == 1L,
     dec != sep,  # sep2!=dec and sep2!=sep checked at C level when we know if list columns are present
     is.character(eol) && length(eol)==1L,

inst/tests/tests.Rraw

@@ -10964,6 +10964,8 @@ test(1736.08, capture.output(fwrite(DT)), c("A","foo","ba|r","baz"))
 DT = data.table(A=c("foo","ba|r","baz"), B=list(1:3,1:4,c("fo|o","ba,r","baz"))) # now list column and need to quote
 test(1736.09, capture.output(fwrite(DT)), c("A,B", "foo,1|2|3", "\"ba|r\",1|2|3|4", "baz,\"fo|o\"|\"ba,r\"|baz"))
 test(1736.10, capture.output(fwrite(DT,quote=TRUE)), c("\"A\",\"B\"", "\"foo\",1|2|3", "\"ba|r\",1|2|3|4", "\"baz\",\"fo|o\"|\"ba,r\"|\"baz\""))
+test(1736.11, capture.output(fwrite(DT[c(1,3,4)], col.names = FALSE, sep = " ", sep2 = c("","",""))), 
+    c("1 12345678910 stuvw", "3 7 foobar", "4 910 TRUETRUEFALSE"))
 
 # any list of same length vector input
 test(1737.1, fwrite(list()), NULL, warning="fwrite was passed an empty list of no columns")

src/fwriteR.c

@@ -85,9 +85,10 @@ void writeList(const void *col, int64_t row, char **pch) {
   writer_fun_t *fun = funs[wf];
   for (int j=0; j<LENGTH(v); j++) {
     (*fun)(data, j, &ch);
-    *ch++ = sep2;
+    if (sep2)
+        *ch++ = sep2;
   }
-  if (LENGTH(v)) ch--; // backup over the last sep2 after the last item
+  if (LENGTH(v) && sep2) ch--; // backup over the last sep2 after the last item
   write_chars(sep2end, &ch);
   *pch = ch;
 }
@@ -280,7 +281,7 @@ SEXP fwriteR(
   args.sep2 = sep2 = *CHAR(STRING_ELT(sep2_Arg, 1));
   args.dec = *CHAR(STRING_ELT(dec_Arg,0));
 
-  if (!firstListColumn) {
+  if (!firstListColumn && sep2) {
     if (args.verbose) Rprintf(_("No list columns are present. Setting sep2='' otherwise quote='auto' would quote fields containing sep2.\n"));
     args.sep2 = sep2 = '\0';
   } else {