Kay Schluehr wrote:
> Steven Bethard wrote:
>>"The slice of s from i to j with step k is defined as the sequence of
>>items with index x = i + n*k such that 0 <= n < (j-i)/k."
>>
>>This seems to contradict list behavior though.
>> range(10)[9:-1:-2] == []
>
>
> No, both is correct. But we don't have to interpret the second slice
> argument m as the limit j of the above definition.
Even if "we don't have to," it sure reads like we should.
> For positive values
> of m the identity
> m==j holds. For negative values of m we have j = max(0,i+m).
First, the definition from the doc is still ambiguous: Is the
division in
0 <= n < (j-i)/k
real division, or is it Python integer (truncating) division? It
matters.
Second, the rule Kay Schluehr states is wrong for either type
of division. Look at:
range(5)[4 : -6 : -2]
Since Python is so programmer-friendly, I wrote some code to
make the "look at" task easy:
slice_definition = """"
The slice of s from i to j with step k is defined as the sequence of
items with index x = i + n*k such that 0 <= n < (j-i)/k.
"""
Kay_Schluehr_rule = """
For positive values of m the identity m==j holds. For negative values
of m we have j = max(0,i+m).
"""
def m_to_j(i, m):
""" Compute slice_definition's 'j' according to Kay_Schluehr_rule
when the slice of sequence is specified as,
sequence[i : m : k].
"""
if m > 0:
j = m
else:
j = max(0, i + m)
return j
def extract_slice(sequence, i, m, k, div_type='i'):
""" Apply the slice definition with Kay Schluehr's rule to find
what the slice should be. Pass div_type of 'i' to use integer
division, or 'f' for float (~real) division, in the
slice_definition expression,
(j-i)/k.
"""
j = m_to_j(i, m)
result = []
n = 0
if div_type == 'i':
end_bound = (j - i) / k
else:
assert div_type == 'f', "div_type must be 'i' or 'f'."
end_bound = float(j - i) / k
while n < end_bound:
result.append(sequence[i + n * k])
n += 1
return result
def show(sequence, i, m, k):
""" Print what happens, both actually and according to stated rules.
"""
print "Checking: %s[%d : %d : %d]" % (sequence, i, m, k)
print "actual :", sequence[i : m : k]
print "Kay's rule, int division :", extract_slice(sequence, i, m, k)
print "Kay's rule, real division:", extract_slice(sequence, i, m,
k, 'f')
print
show(range(5), 4, -6, -2)
--
--Bryan
--
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