Re: extracting elements from a list of lists

*To*: mathgroup at smc.vnet.net*Subject*: [mg5827] Re: [mg5811] extracting elements from a list of lists*From*: "w.meeussen" <w.meeussen at solair4b.eunet.be>*Date*: Thu, 23 Jan 1997 03:19:30 -0500*Sender*: owner-wri-mathgroup at wolfram.com

At 00:44 22-01-97 -0500, you wrote: >Hi, > >Suppose I have lists within a list, like: > > myList = {{a,b}, {c,d}} > >and I want to extract some element (let's say the d). >I can request: > > myList[[2,2]]. > >Now suppose what I have the index of the element as >the list {2,2}. Then I can do: > > Fold[Part, myList, {2,2}] > >Is it correct to suppose that it will first return the >2nd sublist, and then extract the second element from it? >How efficient is it? This may be a crucial point >if I have long lists, I think. Then isn't there a >better way of doing this? hi Olivier, your guess is ok : In[3]:= Fold[Part, myList, {2,2}] Out[3]= d as you can see by checking the steps: In[4]:= FoldList[Part, myList, {2,2}] Out[4]= {{{a,b},{c,d}} ,{c,d} ,d} but The Book as good documentation on myList [[2,2]]: In[5]:= Part[myList,2,2] Out[5]= d your general case : In[18]:= target={1,3,2} Out[18]= {1,3,2} In[19]:= longer= { {{a,b,c},{d,e,f},{g,h,i}},{{j,k,l},{m,n,o},{p,q,r}} } Out[19]= {{{a,b,c},{d,e,f},{g,h,i}},{{j,k,l},{m,n,o},{p,q,r}}} In[20]:= Part[longer,Sequence@@target] Out[20]= h the trick is in Sequence[] Dr. Wouter L. J. MEEUSSEN w.meeussen at solair4b.eunet.be