# mapfoldl(f, op, itr)

``````..  mapfoldl(f, op, itr)

Like ``mapfoldl(f, op, v0, itr)``, but using the first element of
``itr`` as ``v0``. In general, this cannot be used with empty
collections (see ``reduce(op, itr)``).``````

## Examples

In the Julia programming language, the function `mapfoldl(f, op, v0, itr)` is similar to `mapreduce`, but it guarantees left associativity. The initial value `v0` will be used exactly once.

1. Perform a left-associative map and fold operation:

``````julia> add(x, y) = x + y
julia> mapfoldl(add, +, 0, [1, 2, 3, 4, 5])
15``````

This example applies the `add` function element-wise to the array `[1, 2, 3, 4, 5]` and then folds the resulting values using the `+` operator in a left-associative manner. The initial value `0` is used as the starting point for the fold operation.

2. Apply a custom function and operator to a range of values:
``````julia> square(x) = x^2
julia> multiply(x, y) = x * y
julia> mapfoldl(square, multiply, 1, 1:5)
14400``````

Here, the `square` function is applied to each element in the range `1:5`, and the resulting values are folded using the `multiply` function in a left-associative manner. The initial value `1` is used as the starting point for the fold operation.

Common mistake example:

``````julia> mapfoldl(+, *, 0, [1, 2, 3, 4, 5])
ERROR: MethodError: no method matching +(::Int64, ::Int64)``````

In this example, the provided functions and operator (`+` and `*`) do not match the element type of the input collection (`Int64`). It's important to ensure that the functions and operator are compatible with the element type of the collection to avoid such errors.