zsh associative array key iteration order is nondeterministic across runs

Answer to the Core Questions

1. Does the printed order of keys differ between successive runs of the same script?

• When you run a script that creates an associative array and iterates over $(k)assoc within the same shell session, the order will usually be identical each time you iterate. The hash table layout remains constant until the array is modified.
• Across separate invocations of zsh (i.e., a new shell process each time), the order is not guaranteed to be the same. The internal hash table may be rebuilt differently, leading to a different traversal order.

2. Is the order independent of the zsh version used?

• No. Different zsh releases can change the hash function, table size, or collision resolution strategy. As a result, the key order can shift when you upgrade or downgrade the shell.

3. Can explicit sorting of keys be relied upon without incurring significant performance loss for large arrays?

• Sorting is the only reliable way to guarantee deterministic key order. It is an O(n log n) operation, which is acceptable for most use‑cases. However, if you iterate over a huge array millions of times, you should benchmark and consider caching the sorted list.

Why the Order Is Unstable

Associative arrays in zsh are implemented as hash tables. The shell’s iterator walks the table in the order the buckets are laid out in memory. That layout depends on the hash function and the number of buckets, both of which can vary between builds and between runs if the hash seed changes.

Practical Verification Steps

1. Save the following script as check-order.zsh:

   #!/usr/bin/env zsh
   typeset -A a
   for i in {1..10}; do
     a["key${i}"]=value
   end
   for k in ${(k)a}; do
     print -l $k
   end
   

   Run it twice in the same terminal and compare the output.

2. Run the script in two different terminals or on two machines with different zsh versions, and diff the outputs. Any difference indicates a version‑dependent reordering.

3. Benchmark sorting vs unsorted iteration on a large array:

   time for k in ${(k)a}; do :; done
   time for k in ${(k)a} | sort; do :; done
   

   Typical results show sorting adds a small overhead (≈10–20 % for 100 k keys). If you need higher performance, cache the sorted list.

When to Cache Sorted Keys

If you iterate over the same associative array many times without modification, store the sorted list in a separate array:

sortedKeys=(${(k)a})
sortedKeys=(${sortedKeys:#}) # optional: remove duplicates
for k in $sortedKeys; do ...; done

Rebuild sortedKeys only when the array changes.

Missing Diagnostic Detail

To fine‑tune performance recommendations, it would help to know how large your arrays typically are and how frequently you need deterministic order. If you plan to sort millions of keys on a hot path, consider profiling or using a custom comparison function.