In our previous functions, we’ve sometimes used the values of variables as arguments. While we could return an entirely new value, we couldn’t directly affect the variable used as the argument. An in-out parameter allows a function to reassign the value of a variable passed in as an argument.

Since a function with an inout parameter is expected to change the value of its argument, only variables can be passed in when the function is called - not constants or literals since they can not be altered.

An inout parameter is declared in the function definition after the parameter name and before its specified type:

func funcName(parameterName: inout parameterType) -> returnType {

Say we have a program that changes the color of a phone battery depending on the charge percentage of the phone. We can set up the following function that utilizes an inout parameter:

var currentColor = "green"  

func batteryInterface(percentage: Int, batteryColor: inout String) {
  if percentage < 5 {
    batteryColor = "red"
  } else if percentage < 15 {
    batteryColor = "yellow"
  } else {
    batteryColor = "green"
  }  
}

Currently, the phone’s battery color is "green". “The function holds two parameters, percentage and an inout parameter, batteryColor. Within the body of the function, we use a conditional to assign a color to batteryColor. Notice how we don’t return any values from the function.

To spring this function into action, we must call it in the following form:

batteryInterface(percentage: 12, batteryColor: &currentColor)

When a function that utilizes an inout parameter is called, an ampersand, &, must be used directly before the variable name that will act as its argument. This symbol instructs the program that the variable can be modified.

The real value for percentage is set to 12, and the if statement within the function executes. batteryColor becomes "yellow" and the variable, currentColor takes on that new value.

print(currentColor) // Prints: yellow