Xcode – cannot pass immutable inout value as parameter: function call returns immutable value

There are errors at almost every lines of your removing function, I’ll try to address each one assuming that your function is supposed to remove every occurence of item.

func removing(_ item: Int, from array: inout [Int]) -> [Int] {

As said by @Shadowrun, you must choose between returning the modified array as the return type or updating it via the inout parameter, not both.

var array = array

Here you make a local copy of array with a shadowing name (i.e. you use the same name), which defeats the purpose of using an inout parameter as you never update it.

var arrayRes : [Int]

Using an initialized variable is very bad practice in Swift, at least you should initialize and empty array.

for i in array {
    if array[i] != item {
        notFound = false
    }
}

You tried to iterate over the array elements instead of the array indices. The fast that the compiler did not complained is by pure coincidence as array elements are also of Int type, the same as Swift array indices.
Another issue is that you don’t really need indices but only elements, here is the correct way:

for element in array {
  if element != item {
    notFound = false
  }
}

if notFound == false {
    ...
} else if array == [] {
    ...
} else {
    for element in array {
        if element == item { ... }
    }
}

What is inside the if condition of the else branch will never be executed because you previously checked that notFound == false in the first if condition. This means that notFound is necessarily true in the else branch, meaning all elements are different that item, so element == item always returns false.

array.remove(at: i)

Doing an operation that mutates the array you are currently iterating is almost always guaranteed to be an error and will certainly cause you troubles and runtime errors at some point (do it only if you know that it is safe).

return arrayRes = removing(item, from: &array[1...array.count])

This is wrong in many ways:

• you are returning the value of an assignment, i.e a code line with a = sign. This does not make sense as you probably want to return the value of removing(...) rather than the value of the assignment. And an assignment does not have any value (materialized by the Void type).
• This call (minus the previous error) is recursive, I don’t now if this is intended but I doubt you understand how this works.
• The removing(...) func accepts an array to [Int] as argument, here you are trying to pass an array Slice array[1...array.count]. Also remove the & sign if not using the inout parameter.
• I hope that you know that Swift array indices start at 0, not 1 and that the 1 is intended in your code.

All that said, the origin of your error is here:

return arrayRes = removing(item, from: &Array(array[1...array.count]))

Array(array[1...array.count]) is an immutable object because is is instantiated inline, at the call site. However, it should be mutable because removing accepts an inout parameter, which requires the parameter to be mutable.

The second error is at the same line, you are trying to return the value of an assignment (statement with an = sign) instead of the return value of the function. An assignment has a return type of Void (meaning that it does not return something useful) which is incompatible with your function return type: [Int].

I don’t know if this is homework or some kind of programming training, but if this is the case you should start with the basis, the Official Swift Book by Apple is a good starting point. If not the case you can achieve the same thing with:

var array = [1, 2, 3]

func removing(_ item: Int, from array: inout [Int]) {
  array.removeAll { $0 == item }
}

removing(2, from: &array)
print(array)