Enumerating every other array element in loop

Question

I have a dynamic array with key-pair values and I’d like to enumerate every other value. In this case, just the names in the array. Since I’m not using bash v4, I have been able to use seq to step through an array using echo.

$ array=(Guido yes Francesca no Balthazar yes)
for x in $(seq 0 2 ${#array[@]});do echo $x;done
0
2
4
6

This returns one more element than pairs I have (due to 0 offset) but I can account for that using simple bash math:

for x in $(seq 0 2 $((${#array[@]}-1)));do echo $x;done
0
2
4

This works just fine. But as soon as I try to pull elements out of $array, I get an error that I cannot explain. I've tried with and without the "math"

$ for x in $(seq 0 2 $((${#array[@]}-1)));do echo ${array[$x]};done
-bash: 0: syntax error: operand expected (error token is "0")

$ for x in $(seq 0 2 ${#array[@]});do echo ${array[$x]};done
-bash: 0: syntax error: operand expected (error token is "0")

How can I get just the names out of this dynamic array?

EDIT: my $BASH_VERSION is 3.2.57(1)-release (Mac OS X 10.10.2)

EDIT #2: I’ve come to the conclusion this is possibly a bug in this version. Since I manage a number of AWS instances, I decided to check them and they all have bash 4.1.2(1). I upgraded my Mac to this version and this now works. Thanks for looking but I'm chalking this up to an anomaly.

Answer 1

If want to loop over alternate elements of the array, this one should work on bash 3.2:

while IFS= read -d '' -r name; do
  # do something with "$name"
done < <(printf %s%.0s\\0 "${array[@]}")

You could do similar things with xargs -0 if that was more convenient.

If none of your names includes a colon (for example), you could extract the names into a new array like this:

IFS=: read -a names -r < <(printf %s%.0s: "${array[@]}")

(Unfortunately you can't set the field separator to NUL.)

All of these are dependent on the fact that printf repeats its format string as often as is necessary to use all its arguments (Posix behaviour, so it should work on any bash version, or with the non-builtin printf). The occasionally useful format %.0s uses an argument and prints at most 0 characters from it, which is to say drops it into the bitbucket.