Hi,

I ran the following script with raku: 
my $d_1 = 0.1e0; # double   0.1 ( == 3602879701896397/3602879701896396
+8 )
my $r_1 = 0.1;   # rational 1/10

my $d_2 = 0.10000000000000001e0;# double, same value as $d_1
my $r_2 = 0.10000000000000001;  # rational 10000000000000001/100000000
+000000000

# check that $d_1 == 3602879701896397/36028797018963968
say "not ok 0" if $d_1 != 3602879701896397/36028797018963968;

# Check that $d_1 != 1 / 10;
say "not ok 1" if $d_1 == 1 / 10;

# Check that $d_1 and $d_2 are assigned to exactly the same value:
say "not ok 2" if $d_1 != $d_2;

# Check that $r_ and $r_2 are assigned different values:
say "not ok 3" if $r_1 == $r_2;

# Since $r_1 and $r_2 are unequal, $d_1 should not be equal to both $r
+_1 && $r_2.
# We check this, interchanging lhs and rhs operands in case that makes
+ a
# difference:
say "not ok 4" if ($d_1 == $r_1 && $d_1 == $r_2);
say "not ok 5" if ($r_1 == $d_1 && $r_2 == $d_1);

# Similarly $d_2 should not be equal to both $r_1 and $r_2:
say "not ok 6" if ($d_2 == $r_1 && $d_2 == $r_2);
say "not ok 7" if ($r_1 == $d_2 && $r_2 == $d_2);

say Rat($d_1);
say Rat($d_2);

[download]
I was surprised that it output: 
not ok 1
not ok 4
not ok 5
not ok 6
not ok 7
0.1
0.1

[download]
However, I think I understand what's happening.
I had thought that when raku compared a double with a rational, it would convert the double to its exact rational value, and then compare the 2 rationals.
Instead it apparently converts the rational to a double (rounding if necessary), and then compares the 2 doubles.

I can see some sense in performing the comparison in the way they've chosen:
1) It's probably much cheaper to convert a rational to a double than vice-versa;
2) If the rounded rational is equivalent to the double then, under certain circumstances, you might be happy enough to think of the 2 values as being equivalent.

Doing it that way might be a useful comparison to be making, but I don't think it's a comparison that the numeric comparison operators (==, <, >, etc.) should be performing.

Anyway - the first of my questions (and there probably will be more):
Is there a way to get raku to explicitly display the fact that the (double precision) values held by $d_1 and $d_2 are exactly equivalent to the rational value 3602879701896397/36028797018963968 ?

Cheers,
Rob

In reply to [Raku] Arcane treatment of doubles and rationals by syphilis

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