How to convert Apfloat to BigDecimal?

Question

I am using the following code where k is a BigDecimal:

Apfloat power = ApfloatMath.pow(new Apfloat(-1), new Apfloat(k));

Apfloat seems to have methods like intValue(), doubleValue(), floatValue() but I can't see how I can convert Apfloat to BigDecimal.

Oh I forgot to mention the docs are at: http://www.apfloat.org/apfloat_java/docs/org/apfloat/Apfloat.html — user2948708, Nov 02 '13 at 23:35
The only trivial way of doing this I can find is converting the `Apfloat` to a string (since it is `Formattable`) and then constructing a new `BigDecimal` from that string. Dunno if the string you get is usable, though. — , Nov 02 '13 at 23:35

score 2 · Accepted Answer · answered Nov 03 '13 at 00:09

BigDecimals are best generated from Strings or using the BigDecimal.valueOf( double) static constructor.

For your specific requirement, start with the following:

public static asBigDecimal (Apfloat val) {
    return BigDecimal.valueOf( val.doubleValue());
}

Calling new BigDecimal(double) is specifically to be avoided, since it generates exact decimal representations of the floating-point bits & can somewhat unpredictably require vast numbers of decimal digits to do so.

On the other hand, BigDecimal.valueOf( double) and Double.toString( double) have specific logic to represent doubles -> decimal without spurious deep decimalization.

See:

java.math.BigDecimal.BigDecimal(double)

Quoting from the Javadoc:

The results of this constructor can be somewhat unpredictable. One might assume that writing new BigDecimal(0.1) in Java creates a BigDecimal which is exactly equal to 0.1 (an unscaled value of 1, with a scale of 1), but it is actually equal to 0.1000000000000000055511151231257827021181583404541015625. This is because 0.1 cannot be represented exactly as a double (or, for that matter, as a binary fraction of any finite length). Thus, the value that is being passed in to the constructor is not exactly equal to 0.1, appearances notwithstanding.

How to convert Apfloat to BigDecimal?

1 Answers1