Access object by index in a pipeline

I can use For loop to calculate array elements as below:

$a = 12,34,56,78,99

For ($i=0;$i -lt $a.count-1;$i++) { write-host ($a[$i]-$a[$i+1]) }

Is it a way to do it by using pipeline? $a | % { ... }

how about if array is [pscustomobject]@{ ... } ?

Thanks.