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Allow BigDecimal accept Float without precision #314

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Merged
merged 4 commits into from
Jun 10, 2025
Merged

Allow BigDecimal accept Float without precision #314

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8cdf5cd
Allow BigDecimal accept Float without precision
mrzasa Feb 4, 2025
13506dd
Merge branch 'master' into float-without-precision
mrzasa Jun 9, 2025
2b0946c
Fix typo in BigDecimal#scale comment (#348)
timcraft Jun 9, 2025
867cac3
Added tests for float
mrzasa Jun 9, 2025
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16 changes: 6 additions & 10 deletions ext/bigdecimal/bigdecimal.c
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Original file line number Diff line number Diff line change
Expand Up @@ -664,8 +664,8 @@ BigDecimal_precision(VALUE self)
* BigDecimal("1").scale # => 0
* BigDecimal("1.1").scale # => 1
* BigDecimal("3.1415").scale # => 4
* BigDecimal("-1e20").precision # => 0
* BigDecimal("1e-20").precision # => 20
* BigDecimal("-1e20").scale # => 0
* BigDecimal("1e-20").scale # => 20
* BigDecimal("Infinity").scale # => 0
* BigDecimal("-Infinity").scale # => 0
* BigDecimal("NaN").scale # => 0
Expand Down Expand Up @@ -3428,11 +3428,7 @@ rb_float_convert_to_BigDecimal(VALUE val, size_t digs, int raise_exception)
}

if (digs == SIZE_MAX) {
if (!raise_exception)
return Qnil;
rb_raise(rb_eArgError,
"can't omit precision for a %"PRIsVALUE".",
CLASS_OF(val));
digs = 0;
}
else if (digs > BIGDECIMAL_DOUBLE_FIGURES) {
if (!raise_exception)
Expand Down Expand Up @@ -3682,12 +3678,12 @@ rb_convert_to_BigDecimal(VALUE val, size_t digs, int raise_exception)
*
* - Integer, Float, Rational, Complex, or BigDecimal: converted directly:
*
* # Integer, Complex, or BigDecimal value does not require ndigits; ignored if given.
* # Integer, Complex, Float, or BigDecimal value does not require ndigits; ignored if given.
* BigDecimal(2) # => 0.2e1
* BigDecimal(Complex(2, 0)) # => 0.2e1
* BigDecimal(BigDecimal(2)) # => 0.2e1
* # Float or Rational value requires ndigits.
* BigDecimal(2.0, 0) # => 0.2e1
* BigDecimal(2.0) # => 0.2e1
* # Rational value requires ndigits.
* BigDecimal(Rational(2, 1), 0) # => 0.2e1
*
* - String: converted by parsing if it contains an integer or floating-point literal;
Expand Down
9 changes: 6 additions & 3 deletions test/bigdecimal/test_bigdecimal.rb
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Expand Up @@ -166,7 +166,10 @@ def test_BigDecimal_with_float
assert_equal(BigDecimal("0.1235"), BigDecimal(0.1234567, 4))
assert_equal(BigDecimal("-0.1235"), BigDecimal(-0.1234567, 4))
assert_equal(BigDecimal("0.01"), BigDecimal(0.01, Float::DIG + 1))
assert_raise_with_message(ArgumentError, "can't omit precision for a Float.") { BigDecimal(4.2) }
assert_nothing_raised { BigDecimal(4.2) }
assert_equal(BigDecimal(4.2), BigDecimal('4.2'))
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@mrzasa Could you add more assertions to check many cases, especially for numbers that don't have accurate representations in Float?
Moreover, we must compare two values by assert_in_delta and pass an appropriate value to the delta argument for the original Float value; it should be 0.5ulp.

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I reconsidered how to check here.

I suggest comparing the result of to_f with the original Float value, like this: assert_equal(4.2, BigDecimal(4.2).to_f).
This approach is preferable because it doesn’t depend on the specific decimal representation for a given Float value. I’d like to avoid relying on the particular algorithm used for converting a Float to a BigDecimal.

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Adding a same test case as BigDecimal(float, 0) is enough.
But since test for BigDecimal(float, 0) is currently missing, adding both will be nice.

assert_equal(BigDecimal("string representation"), BigDecimal(float_literal_here, 0))
assert_equal(BigDecimal("string representation"), BigDecimal(float_literal_here))

# Just like this one on line 166-168
assert_equal(BigDecimal("0.1235"), BigDecimal(0.1234567, 4))

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Thanks! I've added the floats test cases

assert_equal(BigDecimal("0.12345"), BigDecimal(0.12345, 0))
assert_equal(BigDecimal("0.12345"), BigDecimal(0.12345))
assert_raise(ArgumentError) { BigDecimal(0.1, Float::DIG + 2) }
assert_nothing_raised { BigDecimal(0.1, Float::DIG + 1) }

Expand Down Expand Up @@ -242,10 +245,10 @@ def test_BigDecimal_with_exception_keyword
assert_equal(nil, BigDecimal(42.quo(7), exception: false))
}
assert_raise(ArgumentError) {
BigDecimal(4.2, exception: true)
BigDecimal(4.2, Float::DIG + 2, exception: true)
}
assert_nothing_raised(ArgumentError) {
assert_equal(nil, BigDecimal(4.2, exception: false))
assert_equal(nil, BigDecimal(4.2, Float::DIG + 2, exception: false))
}
# TODO: support conversion from complex
# assert_raise(RangeError) {
Expand Down