Re: How Do You Reduce Multiple Elements in a List?

*To*: mathgroup at smc.vnet.net*Subject*: [mg33871] Re: [mg33863] How Do You Reduce Multiple Elements in a List?*From*: BobHanlon at aol.com*Date*: Sun, 21 Apr 2002 06:12:17 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

In a message dated 4/20/02 4:25:28 AM, nitlion at mindspring.com writes: >This seems like such a simple thing, yet I've search through the big book >trying to figure it out. Perhaps someone can help me... > >Given a list of integers that generally contains the same number many times, >I'd like to reduce it to a list that just gives me the number/count pairs. >For example, I'd like to convert the list > {1, 1, 1, 1, 1, 2, 2, 2, 4, 4, 4, 4, 4, 4} >into > {{1,5}, {2,3}, (4,6}} > >Is there a built-in function to do this? Seems like a histogramming >operation, but I get no useful matches from ?*Hist*. > lst = Table[Random[Integer,{1,4}],{10}]; Look in the index under run length encoding. You will be directed to the function Split {First[#],Length[#]}&/@Split[Sort[lst]] or {#,Count[lst, #]}& /@ Union[lst] or Needs["Statistics`DataManipulation`"]; Reverse /@ Frequencies[lst] Bob Hanlon Chantilly, VA USA