Only quote strings on output if needed.

Closes #116

NEWS.md

@@ -1,2 +1,3 @@
 # readr 0.1.0.9000
 
+* In `write_csv()`, only use quotes when they're actually needed (#116)

R/write.R

@@ -2,7 +2,8 @@
 #'
 #' This is about twice as fast as \code{\link{write.csv}}, and never
 #' writes row names. Non-atomic vectors are coerced to character vectors
-#' with \code{as.character}. All columns are encoded as UTF-8.
+#' with \code{as.character}. All columns are encoded as UTF-8. Values are
+#' only quoted if needed: if they contain a comma, quote or new line.
 #'
 #' @param x A data frame to write to disk
 #' @param path Path to write to. If \code{""} will return the csv file as
@@ -14,6 +15,9 @@
 #' @examples
 #' cat(write_csv(head(mtcars), ""))
 #' cat(write_csv(head(iris), ""))
+#'
+#' df <- data.frame(x = c("a", '"', ",", "\n"))
+#' read_csv(write_csv(df, ""))
 write_csv <- function(x, path, append = FALSE, col_names = !append) {
   stopifnot(is.data.frame(x))
 

man/write_csv.Rd

@@ -20,10 +20,14 @@ will append to an existing file.}
 \description{
 This is about twice as fast as \code{\link{write.csv}}, and never
 writes row names. Non-atomic vectors are coerced to character vectors
-with \code{as.character}. All columns are encoded as UTF-8.
+with \code{as.character}. All columns are encoded as UTF-8. Values are
+only quoted if needed: if they contain a comma, quote or new line.
 }
 \examples{
 cat(write_csv(head(mtcars), ""))
 cat(write_csv(head(iris), ""))
+
+df <- data.frame(x = c("a", '"', ",", "\\n"))
+read_csv(write_csv(df, ""))
 }
 

src/write_csv.cpp

@@ -18,9 +18,21 @@ void stream_csv_row(Stream& output, Rcpp::List x, int i) {
   output << '\n';
 }
 
+bool needs_quote(const char* string, char delim = ',') {
+  for (const char* cur = string; *cur != '\0'; ++cur) {
+    if (*cur == '\n' || *cur == '\r' || *cur == '"' || *cur == delim)
+      return true;
+  }
+
+  return false;
+}
+
 template <class Stream>
 void stream_csv(Stream& output, const char* string) {
-  output << '"';
+  bool quotes = needs_quote(string);
+
+  if (quotes)
+    output << '"';
 
   for (const char* cur = string; *cur != '\0'; ++cur) {
     switch (*cur) {
@@ -32,7 +44,8 @@ void stream_csv(Stream& output, const char* string) {
     }
   }
 
-  output << '"';
+  if (quotes)
+    output << '"';
 }
 
 template <class Stream>

tests/testthat/test-write.R

@@ -0,0 +1,17 @@
+context("write_csv")
+
+test_that("strings are only quoted if needed", {
+  x <- c("a", ',')
+
+  csv <- write_csv(data.frame(x), "", col_names = FALSE)
+  expect_equal(csv, 'a\n","\n')
+})
+
+test_that("read_csv and write_csv round trip special chars", {
+  x <- c("a", '"', ",", "\n")
+
+  output <- data.frame(x)
+  input <- read_csv(write_csv(output, ""))
+
+  expect_equal(input$x, x)
+})