converting from symbolic ring to int is broken

[syazdani77]

Here is simple example:

sage: h = 3^64;
sage: int(h)==int(SR(h))
FALSE

Looking a bit deeper into this, it seems that the first 100 bits are correct, and after that int(SR(h)) is just zeroes. (As a side note, the conversion to ZZ works without a problem.)

CC: @katestange @orlitzky

Component: symbolics

Reviewer: Burcin Erocal

Issue created by migration from https://trac.sagemath.org/ticket/9627

[tornaria]

comment:1

Yikes!

    def __int__(self):
        """
        EXAMPLES::
        
            sage: int(sin(2)*100)
            90
            sage: int(log(8)/log(2))
            3
        """
        #FIXME: can we do better?
        return int(self.n(prec=100))

Can we just change that to

        return int(self._integer_())

?

[tornaria]

comment:2

OTOH,

sage: ZZ(log(8)/log(2))
...
TypeError: unable to convert x (=log(8)/log(2)) to an integer

but

sage: int(log(8)/log(2))  
3

OTOH, int(sin(2)*100) is not equal to 90... not an integer anyway... What's the expected meaning of int(x)?

[sagetrac-cwitty]

comment:3

Pure Python implements int() of a float as truncating toward zero:

>>> int(3.14159)
3
>>> int(-3.14159)
-3
>>> int(2.71828)
2
>>> int(-2.71828)
-2

I think in general we've implemented int() on real numbers of various types as truncating toward zero to follow this precedent.

[tornaria]

comment:4

Something like this may work:

def SR_int(x):
    """
    sage: SR_int(sin(2)*100)
    90
    sage: SR_int(-sin(2)*100)
    -90
    sage: SR_int(log(8)/log(2))
    3
    sage: SR_int(-log(8)/log(2))
    -3
    sage: SR_int(SR(3^64)) == 3^64
    True
    sage: SR_int(SR(10^100)) == 10^100
    True
    sage: SR_int(SR(10^100+10^-100)) == 10^100
    True
    sage: SR_int(SR(10^100-10^-100)) == 10^100 - 1
    True

    sage: SR_int(sqrt(-1))
    ...
    ValueError: math domain error
    sage: SR_int(x)
    ...
    ValueError: math domain error
    """
    try:
        if x in RR:
            ## truncate toward zero
            y = floor(abs(x))
            if parent(y) is ZZ:
                return int(ZZ(sign(x) * y))
    except:
        pass
    raise ValueError, "math domain error"

Note that floor(log(8)/log(2)) takes about 1s to complete, which means SR_int(log(8)/log(2)) is waaay slower than int(log(8)/log(2). But this is at the cost of int(x) being incorrect when x is very close to an integer (cf. int(SR(10<sup>20-10</sup>-20))).

OTOH, (log(8)/log(2)).full_simplify() takes 35ms to give 3, so it may be worth revisiting the floor() strategy. Opens a can of worms, I guess...

---

I won't turn the snippet above into a patch, if somebody likes it and wants to produce a patch, go ahead.

[burcin]

Reviewer: Burcin Erocal

[burcin]

comment:7

#12968 has a patch with a positive review which fixes this. We should close this as a duplicate.