Document: WG14 N1492

Fix for DR 319 (printf("%a", 1.0) and trailing zeros)

Submitter: Fred J. Tydeman (USA)
Submission Date: 2010-06-17
Related documents: DR 319
Subject: Fix for DR 319 (printf("%a", 1.0) and trailing zeros)

Defect Report 319 has a suggested change to which the committee said: The Committee does not believe this is a defect, however the Committee may consider establishing a rule for removing or not removing trailing zeros at some point in the future.

The issue:

Given that FLT_RADIX is 2, what is the output of:

  double x = 1.0;
  printf("%a", x);

The fix:

Change 7.20.6.1 The fprintf function sections as follows.

Paragraph 6 on the '#' flag, change to: "For a, A, g and G conversions, trailing zeros are not removed from the result."

The above adds a and A to the existing g and G.

Paragraph 8, section a,A, change to: "... if the precision is missing and FLT_RADIX is a power of 2, then the precision is the minimum sufficient for an exact representation of all values of type double (unless the # flag is used, any trailing zeros are removed from the fraction portion of the result); ..."

The above adds the phrase in ().

Paragraph 8, section a,A, change to: "... if the precision is missing and FLT_RADIX is not a power of 2, then the precision is the minimum sufficient to distinguish values of type double (unless the # flag is used, any trailing zeros are removed from the fraction portion of the result); ..."

The above adds the phrase in () in place of "except that trailing zeros may be omitted".

Also, update the corresponding sections for the wide character versions of the functions in 7.27.2.1 The fwprintf function.

Add to the Rationale in section 7.19.6.1: %a (without an explicit precision) acts like %g (removes trailing zeros or not, depending upon #), while %.*a (with an explicit precision) acts like %e (keeps trailing zeros). This was done to make implementations have same behavior.

Update DR 319 to point to this document.