> For the complete documentation index, see [llms.txt](https://boxlang.ortusbooks.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://boxlang.ortusbooks.com/boxlang-language/reference/built-in-functions/set/boxsetreduce.md).

# BoxSetReduce

Left-fold a Set with an accumulator function, reducing it to a single value.

The callback receives the current accumulator, the element value, its 1-based ordinal position, and the Set itself, and returns the new accumulator. Iteration follows the natural order of the underlying variant.

## Method Signature

```
BoxSetReduce(set=[set], callback=[function:BiFunction], initialValue=[any])
```

### Arguments

| Argument       | Type                  | Required | Description                                                                                 | Default |
| -------------- | --------------------- | -------- | ------------------------------------------------------------------------------------------- | ------- |
| `set`          | `set`                 | `true`   | The set to reduce.                                                                          |         |
| `callback`     | `function:BiFunction` | `true`   | Receives {@code (accumulator, value, ordinal, set)} and returns the new accumulator.        |         |
| `initialValue` | `any`                 | `false`  | The starting accumulator value. If omitted, the first element is used as the initial value. |         |

## Examples

### Sum all elements with reduce

The callback receives an accumulator and the current value. The third argument is the initial accumulator value.

```java
s = [ 1, 2, 3, 4, 5 ].toSet();
total = s.reduce( ( Any acc, Any v ) => acc + v, 0 );
writeOutput( total );

```

Result: 15

### Build a comma-separated string

```java
s = setNew( type="linked", values=[ "apple", "banana", "cherry" ] );
result = s.reduce( ( Any acc, Any v ) => listAppend( acc, v ), "" );
writeOutput( result );

```

Result: apple,banana,cherry

## Related

* [BoxSetAdd](/boxlang-language/reference/built-in-functions/set/boxsetadd.md)
* [BoxSetAddAll](/boxlang-language/reference/built-in-functions/set/boxsetaddall.md)
* [BoxSetClear](/boxlang-language/reference/built-in-functions/set/boxsetclear.md)
* [BoxSetContains](/boxlang-language/reference/built-in-functions/set/boxsetcontains.md)
* [BoxSetContainsAll](/boxlang-language/reference/built-in-functions/set/boxsetcontainsall.md)
* [BoxSetDifference](/boxlang-language/reference/built-in-functions/set/boxsetdifference.md)
* [BoxSetEach](/boxlang-language/reference/built-in-functions/set/boxseteach.md)
* [BoxSetEquals](/boxlang-language/reference/built-in-functions/set/boxsetequals.md)
* [BoxSetEvery](/boxlang-language/reference/built-in-functions/set/boxsetevery.md)
* [BoxSetFilter](/boxlang-language/reference/built-in-functions/set/boxsetfilter.md)
* [BoxSetFind](/boxlang-language/reference/built-in-functions/set/boxsetfind.md)
* [BoxSetIntersection](/boxlang-language/reference/built-in-functions/set/boxsetintersection.md)
* [BoxSetIsDisjointFrom](/boxlang-language/reference/built-in-functions/set/boxsetisdisjointfrom.md)
* [BoxSetIsEmpty](/boxlang-language/reference/built-in-functions/set/boxsetisempty.md)
* [BoxSetIsSubsetOf](/boxlang-language/reference/built-in-functions/set/boxsetissubsetof.md)
* [BoxSetIsSupersetOf](/boxlang-language/reference/built-in-functions/set/boxsetissupersetof.md)
* [BoxSetMap](/boxlang-language/reference/built-in-functions/set/boxsetmap.md)
* [BoxSetNone](/boxlang-language/reference/built-in-functions/set/boxsetnone.md)
* [BoxSetReject](/boxlang-language/reference/built-in-functions/set/boxsetreject.md)
* [BoxSetRemove](/boxlang-language/reference/built-in-functions/set/boxsetremove.md)
* [BoxSetRemoveAll](/boxlang-language/reference/built-in-functions/set/boxsetremoveall.md)
* [BoxSetRetainAll](/boxlang-language/reference/built-in-functions/set/boxsetretainall.md)
* [BoxSetSome](/boxlang-language/reference/built-in-functions/set/boxsetsome.md)
* [BoxSetSymmetricDifference](/boxlang-language/reference/built-in-functions/set/boxsetsymmetricdifference.md)
* [BoxSetToArray](/boxlang-language/reference/built-in-functions/set/boxsettoarray.md)
* [BoxSetToList](/boxlang-language/reference/built-in-functions/set/boxsettolist.md)
* [BoxSetUnion](/boxlang-language/reference/built-in-functions/set/boxsetunion.md)
* [ObjectToSet](/boxlang-language/reference/built-in-functions/set/objecttoset.md)
* [SetNew](/boxlang-language/reference/built-in-functions/set/setnew.md)
* [SetOf](/boxlang-language/reference/built-in-functions/set/setof.md)
* [StructKeySet](/boxlang-language/reference/built-in-functions/set/structkeyset.md)
* [StructValueSet](/boxlang-language/reference/built-in-functions/set/structvalueset.md)
* [ToSet](/boxlang-language/reference/built-in-functions/set/toset.md)


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