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Combination for zero sized array #380

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Combination for zero sized array #380

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4fb0d00
Fix combination on zero-sized array
leeopop Nov 5, 2019
d0a0fd0
Add zero-sized test case for permuation
leeopop Nov 5, 2019
a9091dc
Fix combinations_with_replacement to return an empty set instead of None
leeopop Nov 11, 2019
a8aadf8
Can't choose 2 from an empty set
leeopop Nov 12, 2019
124d087
update comment
leeopop Nov 12, 2019
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5 changes: 5 additions & 0 deletions src/combinations.rs
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Expand Up @@ -50,6 +50,11 @@ impl<I> Iterator for Combinations<I>
{
type Item = Vec<I::Item>;
fn next(&mut self) -> Option<Self::Item> {
if self.first && self.k == 0 {
self.first = false;
return Some(Vec::new());
}

let mut pool_len = self.pool.len();
if self.pool.is_done() {
if pool_len == 0 || self.k > pool_len {
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12 changes: 12 additions & 0 deletions tests/test_std.rs
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Expand Up @@ -599,6 +599,13 @@ fn combinations_of_too_short() {
#[test]
fn combinations_zero() {
it::assert_equal((1..3).combinations(0), vec![vec![]]);
it::assert_equal((0..0).combinations(0), vec![vec![]]);
}

#[test]
fn permutations_zero() {
it::assert_equal((1..3).permutations(0), vec![vec![]]);
it::assert_equal((0..0).permutations(0), vec![vec![]]);
}

#[test]
Expand All @@ -622,6 +629,11 @@ fn combinations_with_replacement() {
(0..3).combinations_with_replacement(0),
<Vec<Vec<_>>>::new(),
);
// Zero size on empty pool
it::assert_equal(
(0..0).combinations_with_replacement(0),
<Vec<Vec<_>>>::new(),
);
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This looks wrong. The empty vector is a valid combination of zero elements in 0..0, so (0..0).combinations_with_replacement(0) should be vec![vec![]] (like (0..0).combinations(0) and (0..0).permutations(0)), not <Vec<Vec<_>>>::new() == vec![].

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@andersk

It seems that the original test cases of combinations_with_replacement has the same test cases, and I just made a copy of it.
Should fixing combinations_with_replacement and its test cases be included in this PR or should I make a separate PR?

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I found that python itertools implementation shows value 1 (single empty set) for following inputs.

>>> len(list(itertools.combinations_with_replacement([], 0)))
1
>>> len(list(itertools.combinations([], 0)))
1
>>>

// Empty pool
it::assert_equal(
(0..0).combinations_with_replacement(2),
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