Submitter: Fred Tydeman (USA)
Submission Date: 2005-04-04
Source: WG 14
Reference Document: ISO/IEC
WG14 N1094
Version: 1.3
Date: 2006-04-04
Subject: printf("%a", 1.0) and trailing zeros
Summary
Given that FLT_RADIX is 2, what is the output of:
double x = 1.0;
printf("%a", x);
In particular, are trailing zeros removed or kept? Some choices that occur to me are:
Some implementations that I have seen do 1, others do 2, and one does both 1 and 2 (value and format dependent). I believe choice 1 is the intended behaviour.
Another way to look at this is: should %a act like %e (keep trailing zeros) or %g (remove trailing zeros) with respect to trailing zeros? Should this behaviour depend upon the user specifing a precision?
Some parts of 7.19.6.1 The fprintf function that are relavent are:
Paragraph 6 on the '#' flag has: "For g and G conversions, trailing zeros are not removed from the result."
Paragraph 8, section e,E, has: "... if the precision is zero and the # flag is not specified, no decimal-point character appears."
Paragraph 8, section g,G, has: "Trailing zeros are removed from the fractional portion of the result unless the # flag is specified; a decimal-point character appears only if it is followed by a digit."
Paragraph 8, section a,A, has: "... if the precision is missing and FLT_RADIX is a power of 2, then the precision is sufficient for an exact representation of the value; ..."
Paragraph 8, section a,A, has: "... if the precision is missing and FLT_RADIX is not a power of 2, then the precision is sufficient to distinguish values of type double, except that trailing zeros may be omitted; ..."
There are corresponding sections for the wide character versions of the functions in 7.24.2.1 The fwprintf function.
Suggested Technical Corrigendum
Change 7.19.6.1 The fprintf function sections as follows.
Paragraph 6 on the '#' flag, change the above to: "For a, A, g and G conversions, trailing zeros are not removed from the result."
Paragraph 8, section a,A, change the above 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 (removal of trailing zeros depends upon the # flag); ..."
Paragraph 8, section a,A, change the above 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 (removal of trailing zeros depends upon the # flag); ..."
Also, update the corresponding sections for the wide character versions of the functions in 7.24.2.1 The fwprintf function.
Add to the Rationale in section 7.19.6.1: %a (without an explicit precision) acts like %g (removes trailing zeros), while %.*a (with an explicit precision) acts like %e (keeps trailing zeros). This was done to allow two forms of behaviour while using only one conversion specifier.
Committee Response
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.