Better fix fread's check.names using make.names, #1027.

R/fread.R

@@ -97,8 +97,9 @@ fread <- function(input="",sep="auto",sep2="auto",nrows=-1L,header="auto",na.str
     } else {
         setattr(ans, "class", "data.frame")
     }
+    # #1027, make.unique -> make.names as spotted by @DavidArenberg
     if (isTRUE(as.logical(check.names))) {
-        setattr(ans, 'names', make.unique(names(ans)))
+        setattr(ans, 'names', make.names(names(ans), unique=TRUE))
     }
     as_factor <- function(x) {
         lev = forderv(x, retGrp = TRUE, na.last = NA)

README.md

@@ -95,6 +95,8 @@
 
   6. `dim.data.table` is now implemented in C. Thanks to Andrey Riabushenko.
 
+  7. Better fix to `fread`'s `check.names` argument using `make.names()`, [#1027](https://github.com/Rdatatable/data.table/issues/1027). Thanks to @DavidArenberg for spotting the issue with the previous fix using `make.unique()`.
+
 ### Changes in v1.9.6  (on CRAN 19 Sep 2015)
 
 #### NEW FEATURES

inst/tests/tests.Rraw

@@ -6490,11 +6490,12 @@ text = "x,y\n1,a\n2,a\n3,b\n4,b\n5,a\n"
 test(1527.3, dt[, y := factor(y)], fread(text, stringsAsFactors=TRUE))
 
 # #1027, check.names argument to fread
-nm1 = names(data.table(a=1:2, a=3:4))
+nm1 = names(fread("a,a\n1,2\n3,4", check.names=FALSE))
 nm2 = names(fread("a,a\n1,2\n3,4", check.names=TRUE))
-nm3 = names(fread("a,a\n1,2\n3,4", check.names=FALSE))
-test(1528.1, make.unique(nm1), nm2)
-test(1528.2, nm1, nm3)
+nm3 = names(fread("a b,a b\n1,2\n3,4", check.names=TRUE))
+test(1528.1, c("a", "a"), nm1)
+test(1528.2, c("a", "a.1"), nm2)
+test(1528.3, c("a.b", "a.b.1"), nm3)
 
 # add tests for between
 x = sample(10, 20, TRUE)