# Why am I getting a simultaneous read&write error here?

**URL:** <https://forums.swift.org/t/why-am-i-getting-a-simultaneous-read-write-error-here/27899>\
**Category:** Using Swift\
**Created:** [August 14, 2019, 5:40pm UTC](https://forums.swift.org/t/why-am-i-getting-a-simultaneous-read-write-error-here/27899 "2019-08-14T17:40:35Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![CTMacUser](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/ctmacuser/32/1802_2.png) [@CTMacUser](https://forums.swift.org/u/CTMacUser)\
**Post date:** [August 14, 2019, 5:40pm UTC](https://forums.swift.org/t/why-am-i-getting-a-simultaneous-read-write-error-here/27899/1 "2019-08-14T17:40:36Z")

</div>

When testing this extension method to post on another thread:

```
extension RangeReplaceableCollection {

    /// Removes all the elements whose indices satisfy the given predicate.
    @inlinable public mutating func removeAllAt(locations shouldBeRemoved: (Self.Index) throws -> Bool) rethrows {
        var result = Self()
        result.reserveCapacity(count)
        for i in try indices.lazy.filter({ try !shouldBeRemoved($0) }) {
            result.append(self[i])
        }
        self = result
    }

}

```

I used this in my playground to test:

```
var phrase = "The rain in Spain stays mainly in the plain."
let vowels: Set<Character> = ["a", "e", "i", "o", "u"]
let vowelIndices = Set(phrase.indices.lazy.filter({ vowels.contains(phrase[$0]) }))
phrase.removeAllAt(locations: { vowelIndices.contains($0) })
phrase

```

But this was my original code:

```
var phrase = "The rain in Spain stays mainly in the plain."
let vowels: Set<Character> = ["a", "e", "i", "o", "u"]
phrase.removeAllAt(locations: { vowels.contains(phrase[$0]) })
phrase

```

The problem was my predicate caused a crash ("`error: Execution was interrupted, reason: signal SIGABRT.`"), starting with:

```
Simultaneous accesses to 0x103fcd250, but modification requires exclusive access.
Previous access (a modification) started at (0x103ff779c).
Current access (a read) started at:
0 libswiftCore.dylib 0x00007fff73c28140 swift_beginAccess + 557
6 libswiftCore.dylib 0x00007fff73bd5db0 specialized Sequence.filter(_:) + 582
7 libswiftCore.dylib 0x00007fff73aabea0 Sequence.filter(_:) + 30
10 com.apple.dt.Xcode.PlaygroundStub-macosx 0x00000001019a41b0 main + 0
11 CoreFoundation 0x00007fff482c1040 __invoking___ + 140

```

Why is my original code wrong? Or is there a bug in Xcode 10.3 playgrounds?

---

<div class="post-metadata">

**Author:** ![Lantua](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/lantua/32/8868_2.png) [@Lantua](https://forums.swift.org/u/Lantua)\
**Post date:** [August 14, 2019, 5:53pm UTC](https://forums.swift.org/t/why-am-i-getting-a-simultaneous-read-write-error-here/27899/2 "2019-08-14T17:53:08Z")

</div>

This line

```swift
phrase.removeAllAt(locations: { vowels.contains(phrase[$0]) })

```

You're mutating `phrase`, by virtue of `removeAllAt`, and while you're not done mutating (haven't returned from `removeAllAt`), you're accessing it using subscript, `phrase[$0]`.

---

<div class="post-metadata">

**Author:** ![Andrew\_Trick](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/andrew_trick/32/323_2.png) [@Andrew\_Trick](https://forums.swift.org/u/Andrew_Trick)\
**Post date:** [August 14, 2019, 7:38pm UTC](https://forums.swift.org/t/why-am-i-getting-a-simultaneous-read-write-error-here/27899/3 "2019-08-14T19:38:23Z")

</div>

If you just want to filter characters, then `phrase.filter { /*predicate */ }` does that by passing characters to the predicate rather than indices. That way the closure doesn't need to capture the `phrase` string.

If you really want to filter indices, then you would need to first do that outside of the mutating method so that the filter closure can safely capture and read from the string.