Handle 'NA' in factors written as strings. [fixed #35]

src/R-yyjson-serialize.c

@@ -261,6 +261,8 @@ yyjson_mut_val *scalar_factor_to_json_val(SEXP factor_, R_xlen_t idx,  yyjson_mu
 
   if (opt->factor == FACTOR_AS_INT) {
     val =  scalar_integer_to_json_val(factor, doc, opt);
+  } else if (factor == NA_INTEGER) {
+    val = yyjson_mut_null(doc);
   } else {
     SEXP nms_ = getAttrib(factor_, R_LevelsSymbol);
     const char *nm = CHAR(STRING_ELT(nms_, factor - 1));

tests/testthat/test-factors.R

@@ -0,0 +1,20 @@
+
+test_that("multiplication works", {
+
+  # issue #35
+  color <- c("red", "blue", NA, "red", "white")
+  color <- factor(color, levels=c("red", "blue"))
+  color
+
+  expect_no_error({
+    color_str <- yyjsonr::write_json_str(color, factor = 'integer')
+  })
+  expect_identical('[1,2,null,1,null]', color_str)
+
+  expect_no_error({
+    color_str <- yyjsonr::write_json_str(color, factor = 'string')
+  })
+  expect_identical('["red","blue",null,"red",null]', color_str)
+
+})
+