Modular•13mo ago

error: expression must be mutable in assignment

I'm trying to create a fibonacci struct:

struct Fib:
    var a: Int
    var b: Int

    fn __init__(inout self):
        self.a = 0
        self.b = 1

    fn generate(self) -> Int:
        var new_value = self.a + self.b
        if self.a > self.b:
            self.b = new_value
        else:
            self.a = new_value

        return new_value

struct Fib:
    var a: Int
    var b: Int

    fn __init__(inout self):
        self.a = 0
        self.b = 1

    fn generate(self) -> Int:
        var new_value = self.a + self.b
        if self.a > self.b:
            self.b = new_value
        else:
            self.a = new_value

        return new_value

However it's failing with this error:

@eddtests ➜ /workspaces/mojo-etudes (main) $ mojo ./project-euler/0002.🔥 
./project-euler/0002.🔥:14:17: error: expression must be mutable in assignment
            self.b = new_value
            ~~~~^~
./project-euler/0002.🔥:16:17: error: expression must be mutable in assignment
            self.a = new_value
            ~~~~^~
mojo: error: failed to parse the provided Mojo

@eddtests ➜ /workspaces/mojo-etudes (main) $ mojo ./project-euler/0002.🔥 
./project-euler/0002.🔥:14:17: error: expression must be mutable in assignment
            self.b = new_value
            ~~~~^~
./project-euler/0002.🔥:16:17: error: expression must be mutable in assignment
            self.a = new_value
            ~~~~^~
mojo: error: failed to parse the provided Mojo

Any hints on what I need to do to be able to update the values in the struct? I thought inout self was enough

5 Replies

ModularBot•13mo ago

Congrats @EddTests, you just advanced to level 2!

Jack Clayton•13mo ago

Hi @EddTests just need to add inout to say the struct is mutable while running that method:

 fn generate(inout self) -> Int:

 fn generate(inout self) -> Int:

EddTests•13mo ago

So something like this? If so, I'm still seeing some errors:

struct Fib:
    var a: Int
    var b: Int

    fn __init__(inout self):
        self.a = 0
        self.b = 1

    fn generate(inout self) -> Int:
        var new_value = self.a + self.b
        if self.a > self.b:
            self.b = new_value
        else:
            self.a = new_value

        return new_value

struct Fib:
    var a: Int
    var b: Int

    fn __init__(inout self):
        self.a = 0
        self.b = 1

    fn generate(inout self) -> Int:
        var new_value = self.a + self.b
        if self.a > self.b:
            self.b = new_value
        else:
            self.a = new_value

        return new_value

Errors:

@eddtests ➜ /workspaces/mojo-etudes (main) $ mojo ./project-euler/0002.🔥 
./project-euler/0002.🔥:22:25: error: invalid call to 'generate': invalid use of mutating method on rvalue of type 'Fib'
    print(Fib().generate())
          ~~~~~~~~~~~~~~^~
./project-euler/0002.🔥:11:5: note: function declared here
    fn generate(inout self) -> Int:
    ^
mojo: error: failed to parse the provided Mojo

@eddtests ➜ /workspaces/mojo-etudes (main) $ mojo ./project-euler/0002.🔥 
./project-euler/0002.🔥:22:25: error: invalid call to 'generate': invalid use of mutating method on rvalue of type 'Fib'
    print(Fib().generate())
          ~~~~~~~~~~~~~~^~
./project-euler/0002.🔥:11:5: note: function declared here
    fn generate(inout self) -> Int:
    ^
mojo: error: failed to parse the provided Mojo

O I'm running it like

fn main() raises:
    print(Fib().generate())

fn main() raises:
    print(Fib().generate())

Jack Clayton•13mo ago

Just need to pop it in a variable first:

fn main() raises:
    var f = Fib()
    let res = f.generate()
    print(res)

fn main() raises:
    var f = Fib()
    let res = f.generate()
    print(res)

EddTests•13mo ago

Thanks Jack!

Gaming

Programming

error: expression must be mutable in assignment