Re: select a range of elements from a nested list

*To*: mathgroup at smc.vnet.net*Subject*: [mg5427] Re: select a range of elements from a nested list*From*: haberndt at dnai.com (Harald Berndt)*Date*: Sat, 7 Dec 1996 00:25:59 -0500*Organization*: DNAI ( Direct Network Access )*Sender*: owner-wri-mathgroup at wolfram.com

In article <57ush1$3cb at dragonfly.wolfram.com>, Oliver Schurr <schurro at gusun.acc.georgetown.edu> wrote: > Hi MathGroup > > Say you have a nested list like that {{x1,y1},{x2,y2},{x3,y3},...{xn,yn}} > basically containing X and Y values for a XY plot in each element. The X > values span a range from say 0 to 0.4 and you ONLY want to select the > elements (x,y values) from the list for which the x value spans say 0 to > 0.3. BUT you don't want to COUNT the elements (visually on the screen) and > figure out the position at which the x value is just greater than 0.3 and > then subtract 1 from this position. > > I still want do define the list by using > > Table[list[[x,1]],list[[x,2]],{x, 1, ZZZ}] > > here ZZZ means the number for which the x value <= 0.3, this number needs > to be found by MATHEMATICA! > > I like to know if there is a general procedure to select elements from a > nested list based on the values of one member of the elements of that > nested list. There is, and it's quite simple. A quick look into the documentation turns up the function Select[], which even sounds like the right thing to try. So, let's say you have a list In[14]:= t1 Out[14]= {{0.,0.00498942},{0.02,0.00637976},{0.04,0.00853591},{0.06,0.00774966},{0.08, 0.0146887},{0.1,0.0159799},{0.12,0.010505},{0.14,0.0218018},{0.16, 0.0158617},{0.18,0.0377771},{0.2,0.036134},{0.22,0.0565887},{0.24, 0.0579106},{0.26,0.0711923},{0.28,0.0787898},{0.3,0.0801005},{0.32, 0.0984567},{0.34,0.123357},{0.36,0.137414},{0.38,0.139577},{0.4,0.152203}} Simply select all pairs whose first part has the right properties: In[15]:= Select[t1, First[#]<=0.3&] Out[15]= {{0.,0.00498942},{0.02,0.00637976},{0.04,0.00853591},{0.06,0.00774966},{0.08, 0.0146887},{0.1,0.0159799},{0.12,0.010505},{0.14,0.0218018},{0.16, 0.0158617},{0.18,0.0377771},{0.2,0.036134},{0.22,0.0565887},{0.24, 0.0579106},{0.26,0.0711923},{0.28,0.0787898},{0.3,0.0801005}} It doesn't even matter whether the list is ordered or not. It's reaonably fast, too ... > > I hope I made myself clear enough to you all. > Well, and I hope I got it. Best, -- Harald Berndt, Ph.D. Research Specialist, Voice: 510-652-5974 Consultant FAX: 510-215-4299