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Added correct indexing for negative steps and check for 0-index #263

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Added correct indexing for negative steps and check for 0-index #263

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47 changes: 36 additions & 11 deletions mathics/builtin/lists.py
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Original file line number Diff line number Diff line change
Expand Up @@ -276,10 +276,18 @@ def replace_item(all, item, new):
return False
if len(index.leaves) > 2:
step = index.leaves[2].get_int_value()

if start == 0 or stop == 0:
# index 0 is undefined
evaluation.message('Part', 'span', 0)
return False

if start is None or step is None:
evaluation.message('Part', 'span', index)
return False
start, stop = python_seq(start, stop)

start, stop = python_seq(start, stop, step)

for inner in inner_list:
if inner.is_atom():
evaluation.message('Part', 'partd')
Expand Down Expand Up @@ -533,14 +541,31 @@ def test(self, ls):
return False


def python_seq(start, stop):
if start > 0:
start -= 1
if stop is not None and stop < 0:
stop += 1
if stop == 0:
def python_seq(start, stop, step=+1):
if not step: step = +1
if step == +1:
if start > 0:
start -= 1
if stop is not None and stop < 0:
stop += 1
if stop == 0:
stop = None
return start, stop
else:
if stop and stop > 0:
stop -= 2
elif stop and stop < 0:
stop -= 1
if start and start > 0:
start -= 1
if not start:
start = None
if stop == -1:
stop = None
return start, stop
elif stop == None:
# No stop equals len(list)!
stop = -1
return start, stop


def convert_seq(seq):
Expand Down Expand Up @@ -972,7 +997,7 @@ def apply(self, list, seqs, evaluation):
evaluation.message('Take', 'seqs', seq)
return
start, stop, step = seq_tuple
py_start, py_stop = python_seq(start, stop)
py_start, py_stop = python_seq(start, stop, step)
for inner in inner_list:
if (inner.is_atom() or # noqa
abs(start) > len(inner.leaves) or
Expand Down Expand Up @@ -1023,7 +1048,7 @@ def apply(self, list, seqs, evaluation):
evaluation.message('Drop', 'seqs', seq)
return
start, stop, step = seq_tuple
py_start, py_stop = python_seq(start, stop)
py_start, py_stop = python_seq(start, stop, step)
for inner in inner_list:
if (inner.is_atom() or # noqa
abs(start) > len(inner.leaves) or
Expand Down Expand Up @@ -2011,4 +2036,4 @@ def get_call():
*self.complement(all.leaves, others_leaves,
evaluation, same_test))
result.sort()
return result
return result