Swapping

Swapping
A Programming Pattern (With Examples in Python)

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Motivation

Frequently:
- We need to swap the contents of two variables
A Context:
- A fantasy role playing game in which players commonly acquire items of various kinds (e.g., weapons, spells, gold, food)
An Example of a Swap:
- As the game progresses, two players meet and realize that it would benefit both of them to swap their swords

Review

The Assignment Operator:
- Conceptually - Stores the right side operand in the memory location identified by the left side operand
- Actually - Is a little more complicated than that (because references are used)
Variables:
- Can only hold one "thing"

Review (cont.)

pythonexamples/programmingpatterns/Swapping.py

(Fragment: assignment)

A Defective Algorithm (cont.)

pythonexamples/programmingpatterns/Swapping.py

(Fragment: badswap)

Thinking About the Problem

Suppose:
- You have Caladbolg in your right hand and Durendal in your left hand, and you want to swap them
To Make Progress:
- You need a place to temporarily store one of the swords

The Pattern

The Idea:
- Use another variable to temporarily hold the contents of one variable
pythonexamples/programmingpatterns/Swapping.py (Fragment: swap)
```
        
```

The Pattern (cont.)

Example

pythonexamples/programmingpatterns/Swapping.py

(Fragment: swordswap)

The Example (cont.)

A Warning

The Idea:
- Write a method that swaps the contents of its two parameters
The Issue:
- To understand what is and isn't possible one must understand parameter passing mechanisms