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Suppose I have the following list:

 list = {{1,3.4},{2,6.7},{3,0},{1,2},{2,8.6},{3,7.9},{1,0},{2,7.8},{3,5}};

I will like to sort the list by the first term in the order they are presented on the list like the following:

 {{1,3.4},{1,2},{1,0},{2,6.7},{2,8.6},{2,7.8},{3,0},{3,7.9},{3,5}}. 

Finally I will like to make three different list which select the cases starting with certain number as:

list1 = {{1,3.4},{1,2},{1,0}}
list2 = {{2,6.7},{2,8.6},{2,7.8}}
list3 = {{3,0},{3,7.9},{3,5}}
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    $\begingroup$ Look up GatherBy[]. $\endgroup$ Dec 11, 2015 at 20:55
  • $\begingroup$ Thanks! It helped for the fist part. Now, how can I create a list with only the cases that start for example by 1? $\endgroup$
    – fermi
    Dec 11, 2015 at 21:11
  • $\begingroup$ Why doesn't GatherBy[data, First] not solve your problem completely? $\endgroup$
    – m_goldberg
    Dec 22, 2015 at 3:15

1 Answer 1

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list = {{1, 3.4}, {2, 6.7}, {3, 0}, {1, 2}, {2, 8.6}, {3, 7.9}, {1, 0}, {2, 7.8}, {3, 5}}

{list1, list2, list3} = GatherBy[list, First];

The more general way is

res = GatherBy[list, First];

and then get a particular part with f.e.

res[[2]]

{{2, 6.7}, {2, 8.6}, {2, 7.8}}

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  • $\begingroup$ Thanks! Is there a more general way? I have from 1 to 200. I can write list1, list2,...list200 but it would take long. $\endgroup$
    – fermi
    Dec 11, 2015 at 21:17
  • $\begingroup$ see update ------- $\endgroup$
    – eldo
    Dec 11, 2015 at 21:28

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