W1351 Swap Function

Within these castle walls be forged Mavens of Computer Science ...

— Merlin, The Coder

Othello Board

Prerequisites

• W1301 Arrays

Introduction

A very common need in many algorithms is swapping the values in two variables. It's also very common that these two variables are contained within an array.

Swapping

The first approach that most students take when swapping is the following:

let x = 5
let y = 7

x = y
y = x

Take the time to think through what you think will occur when the above code is executed. Then, try it.

Observe

Observe, Ponder, and Journal: Section 1

1. What syntax errors do you observe in the above code?
2. What logic errors do you observe in the above code?

The primary issue that most students encounter in this scenario is overwriting the value of one of the variables, and then assigning this overwritten value to the other variable.

Observe

Observe, Ponder, and Journal: Section 2

Assuming that the syntax errors in the above code segment are resolved:

1. What will be the value of x after the code executes?
2. What will be the value of y after the code executes?

Watch the following video: Swapping Algorithm (YouTube)

Observe

Observe, Ponder, and Journal: Section 3

1. What must be done to properly swap the values of two variables?

Functions that Modify Their Arguments

If we want to modify the value of a (value-type) argument passed to a function, a special syntax is required. Consider the following code segment:

func increment(n:Int) {
    n = n + 1
}

var x = 7
increment(n:x)
print(x)

Observe

Observe, Ponder, and Journal: Section 4

1. Will the above code compile correctly? If not, why not?

Swift protects us from common errors by ensuring that a (value-type) argument passed to a function cannot be altered by a function except under specific circumstances. When (in generally rare cases) this is desirable, a special syntax is required:

1. We must ensure that the function declaration specifies that this function is enabled to modify a specific argument
2. Whenever we invoke the function, the function invocation must also specify that the specific argument may be modified

This strategy ensures that we don't accidentally modify function arguments.

An example helps to clarify:

func increment(n:inout Int) {
    n = n + 1
}

var x = 7
increment(n:&x)
print(x)

Observe

Observe, Ponder, and Journal: Section 5

1. Carefully compare the above code segment to the previous segment. What changes are required to modify the value of a (value-type) argument in a function?

Going Deeper

Technically, the inout keyword specifies that the argument will be an L-Value. See [W1038 L-Values and R-Values].

Exercises

Exercises

1.  J1351  Create a journal and answer all questions in this experience. Be sure to include all sections of the journal, properly formatted.
2.  M1351-10  Complete  Merlin Mission Manager  Mission M1351-10.