Re: position of sequence of numbers in list

*To*: mathgroup at smc.vnet.net*Subject*: [mg107008] Re: position of sequence of numbers in list*From*: Bill Rowe <readnews at sbcglobal.net>*Date*: Sun, 31 Jan 2010 05:58:34 -0500 (EST)

On 1/30/10 at 7:12 AM, jkerrb at gmail.com (JB) wrote: >What is the most efficient way to find the position of the beginning >of a sequence of numbers from a list? >I found a couple of ways: >find 3,4 in list={1,2,3,4,5}; >1. pos=Intersection[Position[list,3],(Position[list,4])+1] >2. pos=Position[Partition[list,2,1],{3,4}] note this will fail if the things searched for are not sequential. That is: In[1]:= list = {1, 2, 3, 4, 5}; In[2]:= pos = Position[Partition[list, 2, 1], {1, 4}] Out[2]= {} >Are there other ways to do this? What is the best way when dealing >with large lists? Here is another way to the task In[3]:= theList = RandomInteger[100, 5] Out[3]= {94,40,22,10,40} In[4]:= Pick[Range@Length@theList, theList, Alternatives @@ {10, 22}] Out[4]= {3,4} and for non-sequential items In[5]:= Pick[Range@Length@theList, theList, Alternatives @@ {94, 22}] Out[5]= {1,3}