Intersection of lists of lists based on the first term

*To*: mathgroup at smc.vnet.net*Subject*: [mg87485] Intersection of lists of lists based on the first term*From*: Stern <nycstern at gmail.com>*Date*: Fri, 11 Apr 2008 01:47:51 -0400 (EDT)

There is something I've been doing inefficiently that I think might be done much better, and was hoping folks here could help. I am working with time series data in the form seriesA={{timestamp1A,data1A},{timestamp2A,data2A}....{timestampNA,dataNA}}; seriesB={{timestamp1B,data1A},{timestamp2B,data2B}....{timestampNB,dataNB}}; etc. where the many of the timestamps will be in common between the series, but not all. I frequently need to work with only those data which are available in all the time series, and I have written a function that does this, in the simplest case, by first working out the timestamps that all the series share in common timestampsincommon=Intersection @@ Map[Transpose[#][[1]] &, {seriesA,seriesB....seriesN}]; then running Select based on MemberQ Map[Select[#, MemberQ[timestampsincommon, #[[1]]] &] &, {seriesA,seriesB....seriesN}]; This works, but the completion time increases roughly linearly with the number of datapoints involved, and is slow when the number of points gets large, and it seems inelegant. I was hoping somebody on the list might suggest a better way. Thank you, Michael