splitting a list into files

Question

Given is a list with x, y, z coordinates:

{{100, 94, -1.112}, {100.05, 94, -1.112}, {100.1, 94, -0.982}, ..., {100, 94.25, -1.112}, {100.05, 94.25, -0.855}, {100.1, 94.25, -0.728}, ..., {139.95, 114.75, 1.184},{140, 114.75, 1.184}}

I want to split this list into files with y = constant, y should moreover form the name of the new file (see the code below) and only the (x, z) values should be written into the file.

Getting the name for the first file I use this code:

liste = ReadList[inffile, {Number, Number, Number}];
y = liste[[All, 2]];
fil = IntegerString[IntegerPart[First[y]*100], 10, 5];
newfile = ToString[fil] <> ".dat";
wfile = OpenWrite[newfile, FormatType -> TraditionalForm];

Which is the best way to extract now all (x,y,z) with the same y (y = 94) from this list, to write all of them into the new file, and to continue with the next new file with the next y-value (y = 94.25)? Thanks a lot

Answer 1

GatherBy is a very convenient function you should look into (Performing operations on subgroups of data).

data = {{100, 94, -1.112}, {100.05, 94, -1.112}, {100.1, 
   94, -0.982}, {100, 94.25, -1.112}, {100.05, 94.25, -0.855}, {100.1,
   94.25, -0.728}, {139.95, 114.75, 1.184}, {140, 114.75, 1.184}};

Here you group all the cases that have the identical second value:

groups = GatherBy[data, #[[2]] &];

And, then you can just select each group:

groups[[1]]
(* {{100, 94, -1.112}, {100.05, 94, -1.112}, {100.1, 94, -0.982}} *)

To export x,z values into different files named after y value:

Table[Export[IntegerString[IntegerPart[groups[[i, 1, 2]]*100], 10, 5] <> ".dat", 
  groups[[i, All, {1, 3}]], "TSV"], {i, Length[groups]}]

or, more concisely, as suggested by Verde:

Export[IntegerString[IntegerPart[#[[1, 2]]*100], 10, 5] <> 
    ".dat", #[[All, {1, 3}]], "TSV"] & /@ groups