Why is Go's map iteration order seemingly random, and what are the exceptions?

Why are iterations over maps random?

Go's map iteration order appears random at first glance, but there's a compelling reason behind this behavior.

Traditionally, hash tables iterate deterministically based on the bucket array and bucket contents. However, in Go 1, the iteration order became unpredictable. This intentional randomization was enacted to prevent developers from relying on a specific iteration order, which could vary across platforms and releases.

TL;DR; Random iteration prevents portability bugs and enables better map balancing.

The Go Blog explains that programmers were relying on a stable iteration order, which caused portability issues. Randomizing the order resolved these issues, forcing developers to maintain separate data structures for stable iteration.

Exceptions to Map Iteration Randomness

While map iteration is generally random, there are exceptions where a sorted order is preserved:

• JSON Marshals: The encoding/json package sorts map keys before marshaling.
• fmt.Print: Starting with Go 1.12, the fmt package also prints maps with sorted keys.
• Go Templates ({{range}}): The text/template and html/template packages iterate map keys in sorted order.

By randomizing map iteration, Go prevents developers from relying on a specific order, enhancing portability and ensuring efficient map balancing.

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