Text representation of a float formatted using given options.

This BIF solves a problem of float_to_list/1 that doesn't allow
specifying the number of digits after the decimal point when
formatting floats.

    float_to_list(Float, Options) -> string()

    Float = float()
    Options = [Option]
    Option = {decimals, Decimals::0..249} |
             {scientific, Decimals::0..249} |
             compact

Returns a string which corresponds to the text representation of
a `Float` formatted using given options.

When decimals option is specified the returned value will contain
at most `Decimals` number of digits past the decimal point.
When `compact` option is provided the trailing zeros at the end
of the list are truncated (this option is only meaningful together
with the `decimals` option). When `scientific` option is provided,
the float will be formatted using scientific notation with
`Decimals` digits of precision. If `Options` is `[]` the function
behaves like `float_to_list/1`. When using `decimals` option and
the number doesn't fit in the static internal buffer of 256 bytes
the function throws `badarg`.

1 files changed, 5 insertions, 4 deletions

diff --git a/erts/emulator/sys/unix/sys_float.c b/erts/emulator/sys/unix/sys_float.c
index 3fcb4d88dc..6875c17a75 100644
--- a/erts/emulator/sys/unix/sys_float.c
+++ b/erts/emulator/sys/unix/sys_float.c
@@ -735,7 +735,7 @@ void erts_sys_unblock_fpe(int unmasked)
 
 /* 
  ** Convert a double to ascii format 0.dddde[+|-]ddd
- ** return number of characters converted
+ ** return number of characters converted or -1 if error.
  **
  ** These two functions should maybe use localeconv() to pick up
  ** the current radix character, but since it is uncertain how
@@ -745,11 +745,12 @@ void erts_sys_unblock_fpe(int unmasked)
  */
 
 int
-sys_double_to_chars(double fp, char *buffer, size_t buffer_size)
+sys_double_to_chars_ext(double fp, char *buffer, size_t buffer_size, size_t decimals)
 {
     char *s = buffer;
-    
-    (void) erts_snprintf(buffer, buffer_size, "%.20e", fp);
+
+    if (erts_snprintf(buffer, buffer_size, "%.*e", decimals, fp) >= buffer_size)
+        return -1;
     /* Search upto decimal point */
     if (*s == '+' || *s == '-') s++;
     while (ISDIGIT(*s)) s++;