In the spirit of the StackOverflow threads of "hidden" language features, we can use this thread (community wiki) to aggregate useful but little-known features or tricks of Sage. Perhaps these can be collected and added to the documentation in the future.
A very hidden feature of Python!
For faster code using itertools you may delete the reference at the end of the loop
sage: from itertools import combinations
sage: sage: timeit('for p in combinations(range(18),5): pass')
625 loops, best of 3: 615 µs per loop
sage: sage: timeit('for p in combinations(range(18),5): del p')
625 loops, best of 3: 322 µs per loop
The reason is that the iterators in itertools recycle the objects they return if they are the only one to reference it! But is it relevant to optimize Python code?
Wow, this is really cool! Is there any documentation anywhere for this?
Actually not ! You have to read the sources of Python to discover that fact. I wrote few months ago to the developer of itertools which told me that it was a minor speedup (if you want speed you will not use Python).
You can clear the screen by simply entering clear, as in a Bash shell.
The function sage_input (sometimes) alows to get the Sage code to reconstruct an object you like.
sage: m = random_matrix(ZZ,3)
sage: m
[-1 1 -1]
[-2 -2 -1]
[ 1 -5 0]
I like that one, how could i reconstruct it ?
sage: sage_input(m)
matrix(ZZ, [[-1, 1, -1], [-2, -2, -1], [1, -5, 0]])
sage: m == eval(str(sage_input(m)))
True
Another example:
sage: e = m.eigenvalues()[0]
sage: sage_input(e)
R.<x> = QQbar[]
QQbar.polynomial_root(AA.common_polynomial(x^3 + 3*x^2 + 8), CIF(RIF(-RR(3.6128878647175449), -RR(3.6128878647175444)), RIF(RR(0))))
sage: sage_input(e, preparse=False)
R = QQbar['x']
x = R.gen()
QQbar.polynomial_root(AA.common_polynomial(x**3 + 3*x**2 + 8), CIF(RIF(-RR(3.6128878647175449), -RR(3.6128878647175444)), RIF(RR(0))))
The function get_systems from sage.misc.citation will tell you the backends used to evaluate some code. For example,
sage: from sage.misc.citation import get_systems
sage: get_systems('sqrt(x)')
['MPFI', 'MPFR', 'GMP']
sage: get_systems('sqrt(3.4)')
['MPFR']
Apparently, this question disapeared, i repost it though i was not its original author. Sorry for not making this answer an anonymous wiki as it should be. In the recent versions of askbot, only one answer per user is allowed, so i had to trick by first writing a comment, and then transform it into an answer, but that way i could not make it anonymous :(
To create a tuple of variables x0,...,xk you can use var("x", n=k), e.g.
sage: var("x", n=10)
(x0, x1, x2, x3, x4, x5, x6, x7, x8, x9)
Asked: 2013-04-29 01:09:50 +0200
Seen: 3,568 times
Last updated: Oct 15 '20
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