contextFraction interaction with changing Context()->{format}{number}

[somiaj]

Consider the following example problem (I have a problem that mixes fractions and decimals, and I wanted the decimals to all be rounded to three decimal digits that this came from).

DOCUMENT();
loadMacros(qw(PGstandard.pl contextFraction.pl PGML.pl));

Context('Fraction');
Context()->{format}{number} = "%.3g";
$x = Fraction('27375/17');
BEGIN_PGML
[`[$x]`]
END_PGML
ENDDOCUMENT();

This problem gives the error:

ERRORS from evaluating PG file:
Fraction numerators must be integers at line 1 of (eval 1729)
   from within Value::Error called at line 855 of [PG]/macros/contexts/contextFraction.pl
   from within context::Fraction::Value::Fraction::new called at line 1 of (eval 1729)
   from within main::Fraction called at line 6 of undefined

The issue I ran into was a little different, where fractions where being printed using SI notation, as in this example.

DOCUMENT();
loadMacros(qw(PGstandard.pl contextFraction.pl PGML.pl));

Context('Fraction');
Context()->{format}{number} = "%.3g";
$x = Fraction('11/70') / Fraction('3/55') * 40;
BEGIN_PGML
[`[$x]`]
END_PGML
ENDDOCUMENT();

The faction should have been 2420/21 and if I create the fraction with Fraction('2420') / Fraction('21') I don't see an issue, but creating it using fraction operations did cause the fraction to be displayed using SI notation, 2.42E+3/21.

[drgrice1]

This seems to have to do with the change to line 927 of contextFraction.pl made in #1208. If I change that line back to return $a / $b, then both of the examples you gave work as expected. However, that causes issue #1204 that was fixed by that pull request to recur.

In general it seems that this is a rounding issue. The context::Fraction::Value::Fraction::new method calls toFraction to convert the number into a fraction. However, at that point the value it is passed is rounded to two decimal places (even without Context()->{format}{number} = "%.3g";). As a result the returned fraction is incorrect.

[dpvc]

A couple of things going on, here. First, it looks like when you pass a string to Fraction(), then the resulting fraction's data array is getting Real objects rather than the perl reals that it is supposed to have. That is part of what is causing the error message in the first problem, and also what is getting the exponential notation in the second. This patch should resolve the issue (I hope without introducing other problems).

diff --git a/macros/contexts/contextFraction.pl b/macros/contexts/contextFraction.pl
index af047f86..2c662a7e 100644
--- a/macros/contexts/contextFraction.pl
+++ b/macros/contexts/contextFraction.pl
@@ -813,7 +813,7 @@ sub make {
        my $context = (Value::isContext($_[0]) ? shift : $self->context);
        my $x       = shift;
        $x = $context->Package("Formula")->new($context, $x)->eval if !ref($x) && $x =~ m!/!;
-       $x = $x->eval                                              if @_ == 0  && Value::classMatch($x, 'Fraction');
+       return $x->eval                                            if @_ == 0  && Value::classMatch($x, 'Fraction');
        return $self->mutate($context)->make($context, $x, @_);
 }
 
@@ -852,10 +852,10 @@ sub new {
        return $x->[0]                            if Value::classMatch($x->[0], 'Fraction') && @_ == 0;
        $x = $context->toFraction($x->[0]->value) if Value::isReal($x->[0]) && @_ == 0;
        return $self->formula($x)                 if Value::isFormula($x->[0]) || Value::isFormula($x->[1]);
-       Value::Error("Fraction numerators must be integers")   unless isInteger($x->[0]);
-       Value::Error("Fraction denominators must be integers") unless isInteger($x->[1]);
        my ($a, $b) = ($x->[0]->value, $x->[1]->value);
        ($a, $b) = (-$a, -$b) if $b < 0;
+       Value::Error("Fraction numerators must be integers")   unless isInteger($a);
+       Value::Error("Fraction denominators must be integers") unless isInteger($b);
        Value::Error("Denominator can't be zero") if $b == 0;
        ($a, $b) = context::Fraction::reduce($a, $b) if $context->flag("reduceFractions");
        bless { data => [ $a, $b ], context => $context }, $class;
@@ -872,6 +872,8 @@ sub make {
        push(@_, 0) if @_ == 0;
        push(@_, 1) if @_ == 1;
        my ($a, $b) = @_;
+       $a = $a->value if Value::isReal($a);
+       $b = $b->value if Value::isReal($b);
        ($a, $b) = (-$a, -$b) if $b < 0;
        return $context->Package("Real")->make($context, $a / $b) unless isInteger($a) && isInteger($b);
        ($a, $b) = context::Fraction::reduce($a, $b) if $context->flag("reduceFractions");

That should make sure that the number format isn't used in printing the fractions or in the isInteger() checks (which doesn't match the exponential notation).

As for you use of %.3g, I'n not sure that is really what you want. That actually means you will get no more than three significant figures (and will use exponential notation for numbers bigger than 999 in order to enforce that. For example, with that number pattern, Compute("12345") will produce 1.23E+04, while Compute(".00012345") produces .000123.

If you want three decimal places, then you probably want to use %.3f, or perhaps %.3f#. The latter will remove trailing zeros in the decimal places.

[somiaj]

@dpvc thanks, I did misstate my goal (as it has been a while since I wrote this problem and am updating it). I did want sigfigs, as I do have decimals like 0.000123456 I wanted to only show three sigfigs, 0.000123, and not just "three decimal digits". I just forgot that this would also cause numbers to use SI notation, since outside of fraction numerators, none of the other numbers were large enough to notice.

[dpvc]

I did misstate my goal

No problem. I just wanted to make sure you were getting what you wanted.