bash - how to concatenate array with given delimiter

Let's declare array

 a=(aa bb cc) 

Now we can display the content with

$echo ${a[@]}
aa bb cc

First, give some naive solutions a try

$for el in ${a[@]}; do echo -n ,$el; done
,aa,bb,cc

$printf ",%s" ${a[@]}
,aa,bb,cc

Both work well, right? Hmm.. almost.

The last thing to do is get rid of the first comma

$for ab in ${a[@]}; do echo -n ,${ab}; done | cut -c2-
aa,bb,cc