Getting a sublist of a Python list, with the given indices?

Question

I have a Python list, say a = [0,1,2,3,4,5,6]. I also have a list of indices, say b = [0,2,4,5]. How can I get the list of elements of a with indices in b?

Answer 1

You can use list comprehension to get that list:

c = [a[index] for index in b] print c 

This is equivalent to:

c= [] for index in b:     c.append(a[index]) print c 

Output:

[0,2,4,5] 

Note:

Remember that some_list[index] is the notation used to access to an element of a list in a specific index.

Answer 2

Something different...

>>> a = range(7) >>> b = [0,2,4,5] >>> import operator >>> operator.itemgetter(*b)(a) (0, 2, 4, 5) 

The itemgetter function takes one or more keys as arguments, and returns a function which will return the items at the given keys in its argument. So in the above, we create a function which will return the items at index 0, index 2, index 4, and index 5, then apply that function to a.

It appears to be quite a bit faster than the equivalent list comprehension

In [1]: import operator  In [2]: a = range(7)  In [3]: b = [0,2,4,5]  In [4]: %timeit operator.itemgetter(*b)(a) 1000000 loops, best of 3: 388 ns per loop  In [5]: %timeit [ a[i] for i in b ] 1000000 loops, best of 3: 415 ns per loop  In [6]: f = operator.itemgetter(*b)  In [7]: %timeit f(a) 10000000 loops, best of 3: 183 ns per loop 

---

As for why itemgetter is faster, the comprehension has to execute extra Python byte codes.

In [3]: def f(a,b): return [a[i] for i in b]  In [4]: def g(a,b): return operator.itemgetter(*b)(a)  In [5]: dis.dis(f)   1           0 BUILD_LIST               0               3 LOAD_FAST                1 (b)               6 GET_ITER         >>    7 FOR_ITER                16 (to 26)              10 STORE_FAST               2 (i)              13 LOAD_FAST                0 (a)              16 LOAD_FAST                2 (i)              19 BINARY_SUBSCR              20 LIST_APPEND              2              23 JUMP_ABSOLUTE            7         >>   26 RETURN_VALUE 

While itemgetter is a single call implemented in C:

In [6]: dis.dis(g)   1           0 LOAD_GLOBAL              0 (operator)               3 LOAD_ATTR                1 (itemgetter)               6 LOAD_FAST                1 (b)               9 CALL_FUNCTION_VAR        0              12 LOAD_FAST                0 (a)              15 CALL_FUNCTION            1              18 RETURN_VALUE