Well, zsh has a :| parameter expansion operator for array subtraction - so a klunky way to do it would be to define a second array consisting of only an empty element

b=('')

and then do

 % echo ${(j./.)a:|b}

a/b/c

However it feels like there ought to be a way to do it without the additional array.

Well, zsh has a :| parameter expansion operator for array subtraction - so a klunky way to do it would be to define a second array consisting of only an empty element

b=('')

and then do

 % echo ${(j./.)a:|b}

a/b/c

However it feels like there ought to be a way to do it without the additional array

... and indeed there is.