> For the complete documentation index, see [llms.txt](https://gabriel-schade-cardoso.gitbook.io/tango/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://gabriel-schade-cardoso.gitbook.io/tango/modules/collection/fold.md).

# Fold

Applies a function to each element of the collection, threading an accumulator argument through the computation. Take the second argument, and apply the function to it and the first element of the collection. Then feed this result into the function along with the second element and so on.

This method is very similar to [`Reduce`](https://github.com/gabrielschade/tango/tree/379cc4a38ae47796971eb875ec66e7dc053a9081/Modules/Collection/reduce.html), but in this case is considered a initial `state`.

| Parameters                                                                             | Returns |
| -------------------------------------------------------------------------------------- | ------- |
| <p>Func\<TState, T, TState> folder</p><p>TState state</p><p>IEnumerable\<T> source</p> | TState  |

## Usage

&#x20;**Accumulating an individual property of each element through a collection**&#x20;

```csharp
//IEnumerable<Animal> source = 
//    { ("Cats",4), ("Dogs",5), ("Mice",3), ("Elephants",2) }

int result = source.Fold( 6, (_state, element) => _state + element.Item2);

//result = 20
```

When the type of elements in your collection are: `int`, `decimal`, `double`, `string` or `bool` you can also use this function combined with the `Operations` described in [operations section](https://github.com/gabrielschade/tango/tree/379cc4a38ae47796971eb875ec66e7dc053a9081/Operations/Introduction.html) as folder functions.

&#x20;**Using an operation as a folder**&#x20;

```csharp
//IEnumerable<int> source = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }
int result = source.Fold(15, IntegerOperations.Add);

//result = 15 + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10
//result = 70
```


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