C++: List

A STL list in C++ provides the features of a doubly-linked list. Using the list is similar to using any other STL container, with iterators. Here is a simple example that illustrates the common list operations:

#include <list>
using namespace std;

typedef list<int>               IntList;
typedef IntList::iterator       IListIter;
typedef IntList::const_iterator	IListCIter;

int main()
{
    IntList ilist;                                  // Empty list
    IntList ilist1( 100 );                          // List with 100 elements of 0
    IntList ilist2( 100, 3 );                       // List with 100 elements of 3
    IntList ilist3( ilist2.begin(), ilist2.end() ); // List copied from previous list

    // Create list: 10->20->30
    ilist.push_back( 10 );
    ilist.push_back( 20 );
    ilist.push_back( 30 );

    ilist.size();  // 3
    ilist.empty(); // false

    ilist.front(); // 10
    ilist.back();  // 30

    ilist.insert( ilist.begin(), 99 );          // 99 added, 99->10->20->30
    ilist.erase( ilist.begin() );               // 99 erased, 10->20->30
    ilist.erase( ilist.begin(), ilist.end() );	// Entire list erased

    return 0;
}

Python: __lt__()

If you try to sort a list of objects of your own class …

class Foobar:
    def __init__(self, idx):
        self.idx = idx
        return

fooList = [Foobar(10), Foobar(2)]
fooList.sort()

… you get a TypeError error:

TypeError: unorderable types: Foobar() < Foobar()

Python is reporting that it does not know how to compare Foobar objects.

Define a __lt__() method in the Foobar class. This rich comparison method is preferred over the generic __cmp__() method.

class Foobar:
    def __lt__(self, other):
    """Less-than comparison."""
    return self.idx < other.idx

Python: Sorting a List

alist = [10, 2, 8]

# Returns sorted list of input list
blist = sorted(alist)

# In-place sort of list
alist.sort()

Python: Creating empty list, tuple, set & dict

alist  = []
atuple = ()
aset   = set()  # Sadly, no literal for sets!
adict  = {}

Python: Checking if nested list is essentially empty

# Also see: http://stackoverflow.com/questions/1593564
# Note: all([]) returns True

def isListEmpty(inList):
    if isinstance(inList, list):    # Is a list
        return all( map(isListEmpty, inList) )
    return False # Not a list

Python: Making a completely unshared copy of list

# Also see: http://stackoverflow.com/questions/1601269
def unsharedListCopy(inList):
    if isinstance(inList, list):
        return list( map(unsharedListCopy, inList) )
    return inList

Python: List & dictionary comprehension

#  Lists
alist = [1, 2, 3]
blist = [x*x for x in alist]    # [1, 4, 9]

# Dictionaries
adict = {3:"three", 5:"five", 1:"one"}
bdict = {num:None for num in adict} # {1: None, 3: None, 5: None}

Python: Deleting part of list

alist = [10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
del alist[2:6] # [10, 11, 16, 17, 18, 19]

Python: Picking out part of list

alist = [10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
alist[2:6] # [12, 13, 14, 15]

Python: Enumerate list using index & values

alist = [10, 11, 12, 13, 14, 15]
for idx, val in enumerate(alist):
    print(i, j) # 0 10, 1 11, 2 12, 3 13, 4 14, 5 15