Sorting lists inside nested lists

问题

I have a nested list: {{9, 8, 7}, {8, 7, 6}, {7, 6, 5}, {6, 5, 4}, {5, 4, 3}, {4, 3, 2}, {3, 2, 1}}

I need to sort the lists within the list to create:

{{7, 8, 9}, {6, 7, 8}, {5, 6, 7}, {4, 5, 6}, {3, 4, 5}, {2, 3, 4}, (1, 2, 3}}

How do I do this?

回答1:

You want the Map function, which applies a function to each element of a list.

That is, Map[f, {1, 2, 3}] gives {f[1], f[2], f[3]}.

In this case, you can use Map[Sort, list]. Map also has an infix operator, /@:

In[1]:= Map[Sort, {{9, 8, 7}, {8, 7, 6}, {7, 6, 5}, {6, 5, 4}, 
  {5, 4, 3}, {4, 3, 2}, {3, 2, 1}}]

Out[1]= {{7, 8, 9}, {6, 7, 8}, {5, 6, 7}, {4, 5, 6},
  {3, 4, 5}, {2, 3, 4}, {1, 2, 3}}

In[2]:= Sort /@ {{9, 8, 7}, {8, 7, 6}, {7, 6, 5}, {6, 5, 4}, 
  {5, 4, 3}, {4, 3, 2}, {3, 2, 1}}

Out[2]= {{7, 8, 9}, {6, 7, 8}, {5, 6, 7}, {4, 5, 6}, 
  {3, 4, 5}, {2, 3, 4}, {1, 2, 3}}

回答2:

It may be useful to be aware that Ordering will also work

lst = {{9, 8, 7}, {8, 7, 6}, {7, 6, 5}, {6, 5, 4}, {5, 4, 3}, {4, 3, 
    2}, {3, 2, 1}};
Map[#[[Ordering[#]]] &, lst]

But for this type of example the approach is slower than Map/Sort

lst2 = RandomInteger[100, {10^5, 3}];
Map[Sort, lst2]; // Timing
Map[#[[Ordering[#]]] &, lst2]; // Timing

{0.175955, Null}

{1.02087, Null}

Ordering can be useful for sorting by position

lst3 = {{a1, 4, c1}, {a2, 3, c2}, {b3, 1, d3}, {e4, 2, f4}};
lst3[[Ordering[lst3[[All, -2]]]]] // MatrixForm

alt text

来源：https://stackoverflow.com/questions/4222472/sorting-lists-inside-nested-lists

标签

wolfram-mathematica