# setfield!

setfield!(value, name::Symbol, x)

Assign `x` to a named field in `value` of composite type. The syntax `a.b = c` calls `setfield!(a, :b, c)`, and the syntax `a.(b) = c` calls `setfield!(a, b, c)`.

## Examples

1. Set a field in a composite type:

``````julia> struct Person
name::String
age::Int
end

julia> p = Person("Alice", 25)
Person("Alice", 25)

julia> setfield!(p, :name, "Bob")
Person("Bob", 25)``````

In this example, the `setfield!` function is used to modify the `name` field of the `Person` struct.

2. Update a field using dot syntax:

``````julia> struct Point
x::Float64
y::Float64
end

julia> pt = Point(2.0, 3.5)
Point(2.0, 3.5)

julia> pt.x = 4.2  # Equivalent to setfield!(pt, :x, 4.2)
4.2

julia> pt
Point(4.2, 3.5)``````

Here, the dot syntax is used to update the `x` field of the `Point` struct.

3. Modify a field using an array of symbols:

``````julia> struct Car
make::String
model::String
year::Int
end

julia> c = Car("Toyota", "Corolla", 2020)
Car("Toyota", "Corolla", 2020)

julia> setfield!(c, [:make, :model], ["Honda", "Accord"])
Car("Honda", "Accord", 2020)``````

In this example, an array of symbols is used to specify multiple fields to be modified simultaneously.

Common mistake example:

``````julia> struct Rectangle
width::Float64
height::Float64
end

julia> rect = Rectangle(5.0, 7.0)
Rectangle(5.0, 7.0)

julia> setfield!(rect, :length, 10.0)
ERROR: type Rectangle has no field length``````

In this case, the mistake is trying to set a field (`length`) that does not exist in the `Rectangle` struct. Make sure to specify the correct field name when using `setfield!`.

## User Contributed Notes

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