---
author: "Devoalda"
authorEmoji: 🐺
title: "Rust - Swap Numbers"
date: 2023-04-11T09:12:58+08:00
description: Swapping 2 numbers without a temp variable
draft: false
hideToc: false
enableToc: true
enableTocContent: true
tocPosition: inner
tocLevels: ["h1", "h2", "h3"]
tags:
- Algorithm
- rust
series:
- rust
categories:
- rust
image:
libraries:
  - mathjax
math: true
---

# Introduction
This is a number swapping operation without using a temp variable written in Rust.

A swap with a temp variable would look like this:

```rust
fn swap_with_temp(mut a:i32, mut b:i32) -> (i32, i32) {
    let temp = a;
    a = b;
    b = temp;
    (a, b)
}
```
This creates another temp variable to contain either values in `a` or `b`, followed by doing the actual swap, and reassigning the value in the temp variable to the second variable.

This causes a slight waste of space as a temp variable of greater or equal size to the larger of the 2 variables is needed to do the swap.

A space efficient is to use mathematical operations `+`, `-`, `*`, `/` or Bitwise `XOR` to do the swap.

# Code
```rust
fn main() {
    let a:i32 = 20;
    let b:i32 = 10;
    println!("Before swap a = {} and b = {}", a, b);
    let mut result = swap(a, b);
    println!("After swap1: a = {} and b = {}", result.0, result.1);
    result = swap2(result.0, result.1);
    println!("After swap2: a = {} and b = {}", result.0, result.1);
    result = match result{
        (0, 0) => (a, b),
        (0, _) => (a, b),
        (_, 0) => (a, b),
        _ => swap3(result.0, result.1)
    };
    println!("After swap3: a = {} and b = {}", result.0, result.1);
    result = swap_with_temp(result.0, result.1);
    println!("After swap_with_temp: a = {} and b = {}", result.0, result.1);

}

fn swap(mut a: i32, mut b: i32) -> (i32, i32) {
    a = a + b;
    b = a - b;
    a = a - b;
    (a, b)
}

fn swap2(mut a:i32, mut b:i32) -> (i32, i32) {
    a = a ^ b; // A XOR B
    b = a ^ b; // A XOR B XOR B = A
    a = a ^ b; // A XOR B XOR A = B
    (a, b)
}

fn swap3(mut a:i32, mut b:i32) -> (i32, i32) {
    a = a * b;
    b = a / b;
    a = a / b;
    (a, b)
}

fn swap_with_temp(mut a:i32, mut b:i32) -> (i32, i32) {
    let temp = a;
    a = b;
    b = temp;
    (a, b)
}
```

```bash
# Output
Before swap a = 10 and b = 20
After swap1: a = 20 and b = 10
After swap2: a = 10 and b = 20
After swap3: a = 20 and b = 10
After swap_with_temp: a = 10 and b = 20
```

The above uses multiple methods to do a swap between 2 variables. However, this is not foolproof. 

## Potential Issues

### Overflow
If either of the variables are larger than the specified type (`i32`), there will be an **overflow** of variables:

```rust
fn main() {
    let a:i32 = 1000000000;
    let b:i32 = 2000000000;
}
```

```python
Before swap a = 1000000000 and b = 2000000000
thread 'main' panicked at 'attempt to add with overflow', src/main.rs:17:9
```

### Divide by zero

`swap3` uses multiplication and division for the swap. Exception handling is required to prevent a `divide by zero` error, if either of `a` or `b` is zero.

```rust
let a:i32 = 0;
let b:i32 = 10;
```

```python
thread 'main' panicked at 'attempt to divide by zero', src/main.rs:33:9
```

A `match` is done to prevent the use of the function if either numbers are zero:

```rust
result = match result{
    (0, 0) => (a, b),
    (0, _) => (a, b),
    (_, 0) => (a, b),
    _ => swap3(result.0, result.1)
};
println!("After swap3: a = {} and b = {}", result.0, result.1);
```