Cast expression to rational number

sagenibble
23 ●2 ●2 ●4

Hello,

I am trying to perform the ceil() function on the result of a square root operation. My code is as follows:

x = 10
x = x.sqrt()
x = x.ceil()

However, I get the following error:

AttributeError: 'sage.symbolic.expression.Expression' object has no attribute 'ceil'

I have tried searching the the documentation, but I can't seem to find a straight forward way to "flatten" this expression to a rational number. How can I go about this?

Thanks,

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2 Answers

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answered 9 years ago

vdelecroix
7655 ●20 ●85 ●168 http://www.labri.fr/pe...

updated 9 years ago

slelievre
17779 ●22 ●161 ●352 http://carva.org/samue...

Note that if you are taking the square root of an integer, the fastest might be the method sqrtrem that returns the floor of the square root and the remainder:

sage: x = 10
sage: s,r = x.sqrtrem()
sage: print s
3
sage: print r
1
sage: x == s^2 + r
True

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Then you could get the ceiling as follows.

def sqrtceil(x):
    """
    Return the ceiling of the square root of this integer
    """"
    s, r = x.sqrtrem()
    if r:   # meaning if r nonzero, then sqrtceil = sqrtfloor + 1
        return s + 1
    else:  # meaning r is zero, ie x is a square, so sqrtfloor = sqrtceil
        return s

slelievre ( 9 years ago )

Or condensed ;-)

def sqrtceil(x):
    s, r = x.sqrtrem()
    return s + bool(r)

vdelecroix ( 9 years ago )

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answered 9 years ago

slelievre
17779 ●22 ●161 ●352 http://carva.org/samue...

When you take the square root of an integer number that is not a square, you get an object in Sage's "Symbolic Ring", where the ceil method is not available.

You could start with a floating-point 10 rather than an integer 10:

sage: x = RDF(10)
sage: x
10.0
sage: x.parent()
Real Double Field
sage: x = x.sqrt()
sage: x
3.1622776601683795
sage: x.parent()
Real Double Field
sage: x = x.ceil()
sage: x
4
sage: x.parent()
Integer Ring

Compare with your original computations:

sage: x = 10
sage: x
10
sage: x.parent()
Integer Ring
sage: x = x.sqrt()
sage: x
sqrt(10)
sage: x.parent()
Symbolic Ring
sage: x = x.ceil()
...
AttributeError: 'sage.symbolic.expression.Expression' object has no attribute 'ceil'

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Interesting, but is there no way to force the Symbolic Ring to produce a flattened number?

sagenibble ( 9 years ago )

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