for example, I have

a1=c(1,3,5)
a2=c(2,4,6)
a3=c(7,8)
a4=c(9,10)

now if I have i=5, so i in a1, then I get a feedback tag[5]=1
                   i=8,  so i in a3, then can get tag[8]=3

in there any function to do this to check the element belongs to which
group?

thank you!!!

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