Can't test whether key exists in associative zsh array

Question

This is my script (zsh 5.8):

# Consider parameter expansion of an unset variable to be an error
set -u  

# Define and populate associative array
typeset -A arr 
arr=(x xxx y yyy)

echo $arr[x] # Outputs xxx

# Test whether key x is present
((${arr[(Ie)x]})) && echo true

The last line gives me the error zsh: x: parameter not set, and from this message I conclude that zsh treats the x not literally, but as a variable which zsh wants to expand. Why do I have a variable expansion here, and how can I write test (without violating set -u) for checking that the key x is present in the array?

I know that I can of course write an explicit loop over the keys (i.e. for k in ${(k)arr}, but I suspect that there must be an easier way to accomplish this.

Answer 1

Since the key x is present in arr, ${arr[(Ie)x]} expands to the list of keys that match x, which is just x. This results in the arithmetic expression x, which evaluates to the numerical value of x. But x is unset, hence the error that you see.

Generally, be careful with associative arrays in arithmetic expressions.

[[ $arr[(Ie)x] ]] would work for a non-empty key. Alternatively, ((${+arr[x]})) is more common, but I don't see any advantage to it: it also doesn't work with empty keys. I think that the $+ approach works for arbitrary keys if you use an intermediate variable:

key=x # or '' or ']' …
(($+arr[$key]))