@Pieter_Goosen to me the scenario is:
post1 - key1
post2 - key2
post3 - -
post4 - key2
post5 - key2
post6 - -
post7 - key1
...
output from my function: post1,post2,post4,post5,post7
post1 - key1
post2 - key2
post3 - -
post4 - key2
post5 - key2
post6 - -
post7 - key1
...
output from my function: post1,post2,post4,post5,post7