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perldata: Add details to hex, etc floats #23988

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40 changes: 34 additions & 6 deletions pod/perldata.pod
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Original file line number Diff line number Diff line change
Expand Up @@ -448,12 +448,14 @@ integer formats:
0b011011 # binary
0x1.999ap-4 # hexadecimal floating point (the 'p' is required)
07.65p2 # octal floating point (the 'p' is required)
0o7.65p2 # alternative octal floating point
0o7.65p2 # alternative octal floating point ('p' still
# required)
0b101.01p-1 # binary floating point (the 'p' is required)

You are allowed to use underscores (underbars) in numeric literals
between digits for legibility (but not multiple underscores in a row:
C<23__500> is not legal; C<23_500> is).
between digits for legibility (multiple underscores in a row are
tolerated but warn:
C<23__500> warns; C<23_500> is fine, doesn't warn).
You could, for example, group binary
digits by threes (as for a Unix-style mode argument such as 0b110_100_100)
or by fours (to represent nibbles, as in 0b1010_0110) or in other groups.
Expand All @@ -468,14 +470,20 @@ characters such as newline, tab, etc., as well as some more exotic
forms. See L<perlop/"Quote and Quote-like Operators"> for a list.
X<string, literal>

Hexadecimal, octal, or binary, representations in string literals
Hexadecimal, octal, or binary representations in string literals
(e.g. '0xff') are not automatically converted to their integer
representation. The hex() and oct() functions make these conversions
for you. See L<perlfunc/hex> and L<perlfunc/oct> for more details.

Hexadecimal floating point can start just like a hexadecimal literal,
and it can be followed by an optional fractional hexadecimal part,
but it must be followed by C<p>, an optional sign, and a power of two.
but it must be followed by C<p>, an optional sign, and an exponent. The
digits in the exponent are interpreted as being base 10 (digits 0-9 are
accepted), but the actual value is two raised to that power. Thus

5e2 == 5 * 10-squared == 500
0x5p2 == 5 * 2-squared == 20
Comment on lines +480 to +485
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@tonycoz tonycoz Dec 7, 2025

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An example with a 2 digit exponent for one of the bitwise bases (hex etc) would better illustrate the base-10-ness of the exponent number vs the base-2-ness of the power.

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@khwilliamson khwilliamson Dec 15, 2025

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Good point; I've added two. The problem is to choose something that can be calculated in one's head. And I think that means only binary, and that is not introduced immediately

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@tonycoz tonycoz Dec 15, 2025

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The exponent is always $2^N$, so the significand doesn't need to be binary:

$ perl -le 'print 0x1p16'
65536

though perhaps I got to know my powers of two too well.

Binary formats describing the values might be more readable:

$ perl -e 'printf "%#x\n", 0x3p16'
0x30000
$ perl -e 'printf "%#o\n", 03p12'
030000


The format is useful for accurately presenting floating point values,
avoiding conversions to or from decimal floating point, and therefore
avoiding possible loss in precision. Notice that while most current
Expand All @@ -485,7 +493,27 @@ rounding modes, which can differ between CPUs, operating systems,
and compilers, and which Perl doesn't control.

Octal and binary floating point numbers use the same format as hexadecimal
floating point numbers, but limited to binary and octal digits, respectively.
floating point numbers, but the digits in their integer and fractional
components are limited to binary and octal, respectively.

Capital 'P' may be used to signify a binary-style exponent, besides
lowercase 'p'; just as capital 'E' can be used instead of 'e' for
10-based. Here are some examples:

05p2 # 20
05.0P2 # 20
05.P2 # 20
0b101p2 # 20
0b1p10 # 1024
0B1p11 # 2048
0x5e2 # Illegal
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@t-a-k t-a-k Dec 15, 2025

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I think this is misleading; 0x5e2 is not illegal (valid integer, 150610).

05e2 # Illegal
5p2 # Illegal

Like the corresponding integers, hex, octal, and binary, float
representations in string literals are not automatically converted to
their floating point. Use C<eval> for this. (This limitation is due to
ambiguity with the dot being interpreted as the concatenation operator.)

You can also embed newlines directly in your strings, i.e., they can end
on a different line than they begin. This is nice, but if you forget
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