Improve documentation of 'format' conversions

* src/editfns.c (Fformat): More accurate description of %g and
effects of the various flags on it.  More accurate description of
integer conversions.

* doc/lispref/strings.texi (Formatting Strings): More accurate
description of %g and effects of the various flags on it.  More
accurate description of integer conversions.  (Bug#25557)

1 files changed, 20 insertions, 12 deletions

diff --git a/doc/lispref/strings.texi b/doc/lispref/strings.texi
index d343bcf5eff..d5980f62bd4 100644
--- a/doc/lispref/strings.texi
+++ b/doc/lispref/strings.texi
@@ -893,17 +893,18 @@ Functions}).  Thus, strings are enclosed in @samp{"} characters, and
 @item %o
 @cindex integer to octal
 Replace the specification with the base-eight representation of an
-integer.
+unsigned integer.
 
 @item %d
-Replace the specification with the base-ten representation of an
+Replace the specification with the base-ten representation of a signed
 integer.
 
 @item %x
 @itemx %X
 @cindex integer to hexadecimal
 Replace the specification with the base-sixteen representation of an
-integer.  @samp{%x} uses lower case and @samp{%X} uses upper case.
+unsigned integer.  @samp{%x} uses lower case and @samp{%X} uses upper
+case.
 
 @item %c
 Replace the specification with the character which is the value given.
@@ -918,8 +919,9 @@ floating-point number.
 
 @item %g
 Replace the specification with notation for a floating-point number,
-using either exponential notation or decimal-point notation, whichever
-is shorter.
+using either exponential notation or decimal-point notation.  The
+exponential notation is used if the exponent would be less than -4 or
+greater than or equal to the precision (default: 6).
 
 @item %%
 Replace the specification with a single @samp{%}.  This format
@@ -1000,9 +1002,11 @@ both flags are used, @samp{+} takes precedence.
   The flag @samp{#} specifies an alternate form which depends on
 the format in use.  For @samp{%o}, it ensures that the result begins
 with a @samp{0}.  For @samp{%x} and @samp{%X}, it prefixes the result
-with @samp{0x} or @samp{0X}.  For @samp{%e}, @samp{%f}, and @samp{%g},
-the @samp{#} flag means include a decimal point even if the precision
-is zero.
+with @samp{0x} or @samp{0X}.  For @samp{%e} and @samp{%f}, the
+@samp{#} flag means include a decimal point even if the precision is
+zero.  For @samp{%g}, it always includes a decimal point, and also
+forces any trailing zeros after the decimal point to be left in place
+where they would otherwise be removed.
 
   The flag @samp{0} ensures that the padding consists of @samp{0}
 characters instead of spaces.  This flag is ignored for non-numerical
@@ -1033,10 +1037,14 @@ ignored.
   All the specification characters allow an optional @dfn{precision}
 before the character (after the width, if present).  The precision is
 a decimal-point @samp{.} followed by a digit-string.  For the
-floating-point specifications (@samp{%e}, @samp{%f}, @samp{%g}), the
-precision specifies how many decimal places to show; if zero, the
-decimal-point itself is also omitted.  For @samp{%s} and @samp{%S},
-the precision truncates the string to the given width, so @samp{%.3s}
+floating-point specifications (@samp{%e} and @samp{%f}), the
+precision specifies how many digits following the decimal point to
+show; if zero, the decimal-point itself is also omitted.  For
+@samp{%g}, the precision specifies how many significant digits to show
+(significant digits are the first digit before the decimal point and
+all the digits after it).  If the precision of %g is zero or
+unspecified, it is treated as 1.  For @samp{%s} and @samp{%S}, the
+precision truncates the string to the given width, so @samp{%.3s}
 shows only the first three characters of the representation for
 @var{object}.  For other specification characters, the effect of
 precision is what the local library functions of the @code{printf}