[ruby-core:119822] [Ruby master Bug#20880] Hash allows array-type key duplicates

[ruby-core:119822] [Ruby master Bug#20880] Hash allows array-type key duplicates

[alexfrstnberg (Yana Prystopchuk) via ruby-core]

Issue #20880 has been reported by alexfrstnberg (Yana Prystopchuk).

----------------------------------------
Bug #20880: Hash allows array-type key duplicates
https://bugs.ruby-lang.org/issues/20880

* Author: alexfrstnberg (Yana Prystopchuk)
* Status: Open
* ruby -v: 3.3.5
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
I was solving small tasks on Leetcode and encountered an issue with Hash which looked strange to me:

There's an array saved into the variable and a hash with a previously created array as a key
``` ruby
arr = [1,2,3]
hsh = {arr => 1}
```
``` shell
=> {[1, 2, 3]=>1}
```

Modified the array. Added the same array to the same hash.
``` ruby
arr << 4
hsh[arr] = 2
```
``` shell
=> {[1, 2, 3, 4]=>1, [1, 2, 3, 4]=>2}
```
Added another duplicate
``` ruby
arr << 5
hsh[arr] = 3
```
``` shell
=> {[1, 2, 3, 4, 5]=>1, [1, 2, 3, 4, 5]=>2, [1, 2, 3, 4, 5]=>3}
```


### First issue:
As a user, I'd expect to have the already existing key reassigned with the new value. Since the key is duplicated, now it's not obvious which value *1*, *2*, or *3* should be returned for the key (*[1, 2, 3, 4, 5]* or *arr*).


### Second issue:
When trying to actually retrieve a value, different methods behave differently:
``` ruby
hsh[[1,2,3,4,5]]
```
``` shell
=> 3
```
``` ruby
hsh[arr]
```
``` shell
=> 3
```
**But**
``` ruby
hsh.assoc(arr)
```
``` shell
=> 1
```


### Third issue:
Another example is created:
``` ruby
arr = [1]
hsh = {arr => 1}
```
And modified with the same logic using a loop:
``` ruby
incr = 1
10.times do
  arr << (incr += 1)
  hsh[arr] = incr
end
```
The received hsh:
``` shell
=> 
{[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]=>9,
 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]=>2,
 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]=>3,
 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]=>4,
 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]=>5,
 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]=>6,
 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]=>7,
 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]=>8,
 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]=>10,
 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]=>11}
```
As you can see, the first key eventually got reassigned (there's no value 1 anymore, it was replaced by 9)
I tried to add each next key manually and got the same result: 1 is replaced by 9.






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[ruby-core:119823] [Ruby master Bug#20880] Hash allows array-type key duplicates

[jeremyevans0 (Jeremy Evans) via ruby-core]

Issue #20880 has been updated by jeremyevans0 (Jeremy Evans).

Status changed from Open to Rejected

This is not a bug, it is expected.  It's best not to mutate objects used as hash keys, but Ruby doesn't stop you from doing it.  If you do mutate an object used as a hash key, you need to call rehash on the hash to update it (https://docs.ruby-lang.org/en/master/Hash.html#method-i-rehash):

```ruby
arr = [1,2,3]
hsh = {arr => 1}
arr << 4
hsh.rehash
hsh[arr] = 2
hsh
# => {[1, 2, 3, 4]=>2}
```

----------------------------------------
Bug #20880: Hash allows array-type key duplicates
https://bugs.ruby-lang.org/issues/20880#change-110511

* Author: alexfrstnberg (Yana Prystopchuk)
* Status: Rejected
* ruby -v: ruby 3.3.5 (2024-09-03 revision ef084cc8f4) [arm64-darwin23]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
I was solving small tasks on Leetcode and encountered an issue with Hash which looked strange to me:

There's an array saved into the variable and a hash with a previously created array as a key
``` ruby
arr = [1,2,3]
hsh = {arr => 1}
```
``` shell
=> {[1, 2, 3]=>1}
```

Modified the array. Added the same array to the same hash.
``` ruby
arr << 4
hsh[arr] = 2
```
``` shell
=> {[1, 2, 3, 4]=>1, [1, 2, 3, 4]=>2}
```
Added another duplicate
``` ruby
arr << 5
hsh[arr] = 3
```
``` shell
=> {[1, 2, 3, 4, 5]=>1, [1, 2, 3, 4, 5]=>2, [1, 2, 3, 4, 5]=>3}
```


### First issue:
As a user, I'd expect to have the already existing key reassigned with the new value. Since the key is duplicated, now it's not obvious which value *1*, *2*, or *3* should be returned for the key (*[1, 2, 3, 4, 5]* or *arr*).


### Second issue:
When trying to actually retrieve a value, different methods behave differently:
``` ruby
hsh[[1,2,3,4,5]]
```
``` shell
=> 3
```
``` ruby
hsh[arr]
```
``` shell
=> 3
```
**But**
``` ruby
hsh.assoc(arr)
```
``` shell
=> 1
```


### Third issue:
Another example is created:
``` ruby
arr = [1]
hsh = {arr => 1}
```
And modified with the same logic using a loop:
``` ruby
incr = 1
10.times do
  arr << (incr += 1)
  hsh[arr] = incr
end
```
The received hsh:
``` shell
=> 
{[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]=>9,
 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]=>2,
 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]=>3,
 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]=>4,
 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]=>5,
 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]=>6,
 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]=>7,
 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]=>8,
 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]=>10,
 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]=>11}
```
As you can see, the first key eventually got reassigned (there's no value 1 anymore, it was replaced by 9)
I tried to add each next key manually and got the same result: 1 is replaced by 9.






-- 
https://bugs.ruby-lang.org/
 ______________________________________________
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 To unsubscribe send an email to ruby-core-leave@ml.ruby-lang.org
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