Bash insert a comma (,) after every third field of a variable or array?

Question

I have a variable with the following contents: "a b c d e f g h i j k l", how would you go about adding a comma sign (,) after each third member so it looks like this: "a b c, d e f, g h i, j k l".

Initially all my variable data is stored in an array so if anyone knows how to directly manipulate the array, it would be great.

Thanks in advance

Answer 1

awk

$ echo "a b c d e f g h i j k l" | awk '{for(i=1;i<NF;i++)if(i%3==0){$i=$i","}  }1'
a b c, d e f, g h i, j k l