A Way to Remember compare/compareTo

##Introduction

This is a memo I wish I had when I started programming. It's about how to remember the meaning of return values for comparison functions like compare, comp, and compareTo that exist in most languages.

Learning to compare two values is one of the first things in programming. At first you learn operators like <, >, =, <=, >=, but in some languages you cannot compare non-primitive types with these operators. In those cases you pass values to a comparison function or method and judge ordering based on its return value.

In languages like Haskell and Rust, the result is expressed as an ADT like GT/Greater, EQ/Equal, LT/Less ^{1 2}, but in languages like C, Java, JavaScript, and PHP, comparison functions return a negative value, zero, or a positive value. I'm not well-versed in C, so I don't know which document to cite for it; I'll include docs for the other languages.

• Java
• JavaScript
• PHP

There is a simple way to remember these functions, but I haven't seen it mentioned in their docs. Beginner sites mention that they return -1, 0, or 1 when values are greater/equal/less, but they don't explain how to remember it. If you read the definition you'll notice it quickly, but I only learned it after seeing it mentioned somewhere. So this is a memo for beginners, written decades later.

If you can instantly tell when each if condition below is true for x and y, then this article is unnecessary for you!³

1
if (Integer.compare(x, y) < 0) {
2
  // ...
3
}
4

5
if (Integer.compare(x, y) == 0) {
6
  // ...
7
}
8

9
if (Integer.compare(x, y) > 0) {
10
  // ...
11
}

##Specification of compare

First, let's check the specification of functions and methods named compare. Let the values be x and y, and consider compare(x, y) or x.compareTo(y) where x is the reference. In most languages, the specification is as follows.

If x is less than y, return a negative value. If x equals y, return zero. If x is greater than y, return a positive value.

Even if the spec is described this way, implementations often return -1 for smaller, 0 for equal, and 1 for larger.

##How to remember compare

For any language (basically) where the comparison function returns an integer value, it follows these rules.

It's obvious in a table, but the inequality direction matches between the function form (left column) and the operator form. This holds regardless of the types of x and y. More precisely, if the earlier spec holds, then the table follows.

If x is less than y, return a negative value. If x equals y, return zero. If x is greater than y, return a positive value.

So when you want to write code that compares x and y,

1
if (x < y) {
2
  // ...
3
}

but x and y don't support < (e.g. Java's Integer, Date, etc.), you can keep the same inequality and compare the result of compare with 0.

1
if (Integer.compare(x, y) < 0) {
2
  // ...
3
}

Rather than remembering that smaller returns negative and larger returns positive and then mapping to < and >, first think about the inequality you want to test, then call compare and keep the inequality direction the same. This way you don't have to worry about the specific return values. It's simple but it helps you read and write code without pausing or looking things up. When reading, think in reverse: Integer.compare(x, y) < 0 means it's checking x < y.

##Conclusion

While looking for how existing sites explain this, I found a Qiita article that mentions the same idea.

• [Java] Notes on Comparable and Comparator #Java - Qiita

There are also posts about not being able to remember the sign of Java's compareTo, which I understand.

• I can't remember Java's compareTo, so I wrote a memo #Java - Qiita

Before I learned this trick, I used to google or read docs every time.

Beginner sites keep saying "it returns 1" or "a positive value," but you don't need to memorize that 😄

• What is Java compareTo? How to use compareTo for sorting freely
• 【Java Beginner】How to compare sizes with compareTo (strings/dates) | Samurai Engineer Blog
• What is Java compareTo? How to use compareTo for sorting freely
• Easy explanation! How to use compareTo in Java | TechAcademy Magazine

Because if x > y, then > 0 is easy enough to remember, right?