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

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.

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.

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!.

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setfield!

Examples

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