On Wed, Apr 1, 2009 at 3:30 AM, zieglerk <[email protected]> wrote:
>
> Hi,
>
>>
>> Even better, of course, is [a..b] and (a...b).
>>
>
> this seems quite intuitive and handy, but I haven't found the
> documentation for both anywhere. Neither among the topic "lists" nor
> when searching for "intervals".
>
> Can you give me a hint.
This is a Sage-specific feature that isn't in Python. It was proposed
in a Python PEP but that didn't get into Python for some reason.
Below is the very minimal docstring and the actual source code. I
recommend you just try it out. In short [a..b] makes a list from a to
b,
[a,a+eps,..,b] makes a list in steps of eps, and (a..b),
(a,a+eps,..,b) are similar, but give a generator (lazy list) instead
of a list.
def parse_ellipsis(code, preparse_step=True):
"""
Preparse [0,2,..,n] notation.
EXAMPLES:
sage: from sage.misc.preparser import parse_ellipsis
sage: parse_ellipsis("[1,2,..,n]")
'(ellipsis_range(1,2,Ellipsis,n))'
sage: parse_ellipsis("for i in (f(x) .. L[10]):")
'for i in (ellipsis_iter(f(x) ,Ellipsis, L[10])):'
"""
ix = code.find('..')
while ix != -1:
if ix == 0:
raise SyntaxError, "Cannot start line with ellipsis."
elif code[ix-1]=='.':
# '...' be valid Python in index slices
code = code[:ix-1] + "Ellipsis" + code[ix+2:]
elif len(code) >= ix+3 and code[ix+2]=='.':
# '...' be valid Python in index slices
code = code[:ix] + "Ellipsis" + code[ix+3:]
else:
start_list, end_list = containing_block(code, ix, ['()','[]'])
arguments = code[start_list+1:end_list-1].replace('...',
',Ellipsis,').replace('..', ',Ellipsis,')
arguments = re.sub(r',\s*,', ',', arguments)
if preparse_step:
arguments = arguments.replace(';', ', step=')
range_or_iter = 'range' if code[start_list]=='[' else 'iter'
code = "%s(ellipsis_%s(%s))%s" % (code[:start_list],
range_or_iter,
arguments,
code[end_list:])
ix = code.find('..')
return code
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