My Terribly Clever(ly Terrible) Code

March 30, 2020 #software-development

I started learning C++ in graduate school. I had written Python for five years, so I thought I was pretty good at writing code and thinking through problems.¹ Like many new programmers I enjoyed finding clever solutions to problems; sometimes too clever. This is the story of one of those times.

The Problem

I had to handle setting some state based on two variables. Each variable could take one of a few discrete values. For simplicity, think of the variables and possible values:

Variable Name	Possible Values
direction	north, east, south, west
travel_mode	bike, car, plane

All twelve combinations required doing something slightly different, so the first code I wrote looked something like this:

if (direction == "north" && travel_mode == "bike") {
  do_north_bike_stuff();
}
else if (direction == "north" && travel_mode == "car") { 
  do_north_car_stuff();
}
else if ( ... ) { 
  ... // etc.
}

This code wasn’t clever; it was boring and repetitive so I looked for a way to rewrite it! I had recently learned about a cool way to replace if/else in C++: the switch statement. I had to use it!

Making It Worse

But a switch statement needs integral values, so I had to map each state to numbers. Easy enough, but I quickly ran into a problem: I had to switch based on both values, so I had to combine the integers in some manner. Then I remembered a “useful” math fact: the product of unique primes is itself unique.² A horrible plan came together, it looked like this:

int NORTH = 2;
int EAST = 3;
int SOUTH = 5;
int WEST = 7:

int BIKE = 11;
int CAR = 13;
int PLANE = 17;

switch(direction * travel_mode) {
  case NORTH * BIKE:  do_north_bike_stuff(); break;
  case EAST  * BIKE:  do_east_bike_stuff(); break;
  ... // etc.
  case WEST  * PLANE: do_west_plane_stuff(); break;
}

My code was actually even worse; if you are morbidly curious I archived it here. I did not assign nice readable variables like NORTH but just used the numbers, so it looked like case 2 * 11: do_north_bike_stuff().

This code is way too clever; needing number theory to understand control flow is a huge warning sign. With ten years more experience, I actually prefer the verbose but understandable if/else method.

A Better Way

So how would I write it now? I think a hybrid method is actually the way to go, using enum³ and a few helper functions:

enum TravelMode { BIKE, CAR, PLANE };
enum Direction { NORTH, EAST, SOUTH, WEST };

switch(travel_mode) {
  case BIKE:  do_bike_stuff(direction); break;
  case CAR:   do_car_stuff(direction); break;
  case PLANE: do_plane_stuff(direction); break;
}

void do_bike_stuff(Direction dir) {
  switch(dir) {
    case NORTH: ...; break;
    case EAST: ...; break;
    ... // etc.
  }
}

This has a few nice advantages:

• It delegates the complexity of handling the direction to each travel mode. This is logical because it’s likely the way a car handles East and West are very similar, and very different from how a plane would.
• By using enum the compiler can check that we handle every case. If we forget case BIKE or case EAST, the compiler can warn us.

In the end, readable is better than clever, even if you have a bunch more lines to read!

---

1. Narrator: He wasn’t. ↩

2. Narrator: It was not useful. ↩

3. See Python Patterns: Enum, which covers use-cases for enum in Python. It works essentially the same in C++. ↩