write_csv: Disable scientific notation for integers (#765)

Large integer numbers are not properly handled by write_csv as they are saved in scientific notation losing precision.

Fixes #845

NEWS.md

@@ -50,6 +50,7 @@ you would pass `skip = 3`, now you only need to pass `skip = 2`.
   integer values in the first 1000 (#645, #652).
 * `read_*()` now converts string `file`s to UTF-8 before parsing, which is convenient for non-UTF-8 platforms
   in most cases (#730, @yutannihilation).
+* `write_csv()` writes integers up to 10^15 without scientific notation (#765, @zeehio)
 * `read_*()` no longer throws a "length of NULL cannot be changed" warning when
   trying to resize a skipped column (#750, #833).
 * `read_*()` now handles non-ASCII paths properly with R >=3.5.0 on Windows (#838, @yutannihilation).

src/grisu3.c

@@ -360,8 +360,8 @@ int dtoa_grisu3(double v, char *dst)
 	// If grisu3 was not able to convert the number to a string, then use old sprintf (suboptimal).
 	if (!success) return sprintf(s2, "%.17g", v) + (int)(s2 - dst);
 
-	// handle whole numbers
-	if (d_exp >= 0 && d_exp <= 2) {
+	// handle whole numbers as integers if they are < 10^15
+	if (d_exp >= 0 && d_exp <= MAX(2, 15 - len)) {
 		while(d_exp-- > 0) s2[len++] = '0';
 		s2[len] = '\0';
 		return (int)(s2+len-dst);

tests/testthat/test-write-delim.R

@@ -110,3 +110,25 @@ test_that("write_csv can write to compressed files", {
   expect_true(is_bz2_file(filename))
   expect_equal(mt, read_csv(filename))
 })
+
+
+test_that("write_csv writes large integers without scientific notation #671", {
+  x <- data.frame(a = c(60150001022000, 60150001022001))
+  filename <- file.path(tempdir(), "test_large_integers.csv")
+  on.exit(unlink(filename))
+  write_csv(x, filename)
+  content <- read_file(filename)
+  expect_equal(content, "a\n60150001022000\n60150001022001\n")
+})
+
+test_that("write_csv writes large integers without scientific notation up to 1E15 #671", {
+  x <- data.frame(a = c(1E13, 1E14, 1E15, 1E16))
+  filename <- file.path(tempdir(), "test_large_integers2.csv")
+  on.exit(unlink(filename))
+  write_csv(x, filename)
+  content <- read_file(filename)
+  expect_equal(content, "a\n10000000000000\n100000000000000\n1e15\n1e16\n")
+  x_exp <- read_csv(filename, col_types = "d")
+  expect_equal(x$a, x_exp$a)
+})
+