Integer types and log()

asked 2015-07-21 16:57:56 +0100

Jeremy Martin
65 ●2 ●3 ●9

I am working with a bunch of lists whose lengths are all powers of 2. I'd like to be able to extract the power by taking the base-2 log of the length. However, Sage wasn't able to simplify an expression like log(len(L),2), because apparently len() returns the wrong kind of integer:

sage: A=list(range(8))
sage: len(A)
8
sage: log(len(A),2)
log(8)/log(2)
sage: log(8,2)
3
sage: type(len(A))
<type 'int'>
sage: type(8)
<type 'sage.rings.integer.Integer'>
sage: log(Integer(len(A)),2)
3

This is the first math function I've come across that seems to care about the distinction between these two kinds of integers, and it would be nice if it didn't, since it took me quite a while to figure out why Sage wouldn't simplify an expression like log(len(A),2).

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This was fixed at some point (thanks!). In Sage 9.3:

sage: a, b = log(8, 2), log(int(8), 2)
sage: a, b
(3, 3)

slelievre ( 2021-05-15 16:16:05 +0100 )edit

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answered 2015-07-22 08:10:22 +0100

rws
1149 ●2 ●20 ●44 http://google.com/+Ral...

To get the exponent in a number factor use valuation:

sage: valuation(8,2)
3
sage: valuation(65536,2)
16
sage: e=[None]*8
sage: valuation(len(e),2)
3

To get the numerical value of a symbolic expression like log(8)/log(2) use n():

sage: log(8)/log(2)
log(8)/log(2)
sage: N(log(8)/log(2))
3.00000000000000

This will be floating point however so better use the first method.

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Thanks. I did not know about the valuation() function before this.

Jeremy Martin ( 2015-07-24 23:42:01 +0100 )edit

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answered 2015-07-21 17:49:21 +0100

Nathann
1615 ●2 ●15 ●36

updated 2015-07-21 17:51:27 +0100

Could you please report this bug by sending a message to sage-devel [1]? You only need say that the following looks weird:

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

About your problem: you can easily solve it by explicitly turning len(A) into a Sage integer:

sage: e=[None]*8
sage: log(len(e),2)
log(8)/log(2)
sage: log(Integer(len(e)),2)
3

[1] https://groups.google.com/forum/#!for...

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The reported behaviour is not a bug. Rather that your method works.

rws ( 2015-07-22 08:12:44 +0100 )edit

@Nathann: Done.

@rws: Yes, it is not a bug, and I was able to do what I needed to do, but from an end-user standpoint, it would be better if the log() function did not require the user to distinguish between an integer and a Sage integer - most mathematicians would not think that there could be two kinds of integers!

Jeremy Martin ( 2015-07-24 23:41:13 +0100 )edit

I guess Sage tries to hide most integers as Sage Integer objects, but there is no easy way to mess with the Python built-in len to have it return an Integer instead of a Python int without probably disturbing a lot of other stuff, unfortunately.

kcrisman ( 2015-07-25 15:31:31 +0100 )edit

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