Data Structures Akshay Singh

 Lists in python can contain any data type  Declaring a list:  a = [‘random’,’variable’, 1, 2]

 List.append(x) – adds “x” to the end of the list  List.extend(L) – adds the list “L” to the end of the list  List.insert(I,x) – adds the “I” item to position “x” in the list.  List.remove(x) – removes the first occurrence of “x” from the list.

 List.pop(I) – remove the element at the “I” position and return it. Removes last element if left blank.  List.index(x) – Returns the position of the first occurrence of “x”  List.count(x) – Returns the number of times “x” appears in the list.

 List.sort() – sorts the list.  List.reverse() – reverses the arrangement of elements in the list.

 Last element in is the first element out.  Use append and pop functions.

 First element in the list is the first the leave.  Use list.append(x) to add to the list and list.popleft() to remove the first element.

 Filter() – returns items from a sequence where the specified function is satisfied.  def f(x): return x % 2 != 0 and x % 3 != 0  filter(f, range(2, 25)) Output: [5, 7, 11, 13, 17, 19, 23]

 Map() – uses the sequence as arguments for the function and returns the results.  def cube(x): return x*x*x  map(cube, range(1, 11)) Output: [1, 8, 27, 64, 125, 216, 343, 512, 729, 1000]

 Reduce() – Sends the first two elements of the sequence to the function, then the result and the third element and so on.  def add(x,y): return x+y  reduce(add, range(1, 11)) Output: 55

 Easy way to work with lists.  Similar to loops.  vec = [2, 4, 6]  [3*x for x in vec] Output: [6, 12, 18]

 The del() statement can be used to remove an element from a list.  It does not return a value.  del a[0] : deletes first element in list “a”  del a[2:4] : deletes third and fourth element  del a[:] : deletes all elements in list “a”

 Tuples are a standard sequence data type.  Tuples are a number of values seperated by commas.  Tuples may be nested.  t = 12345, 54321, 'hello!'  u = t, (1, 2, 3, 4, 5)  ((12345, 54321, 'hello!'), (1, 2, 3, 4, 5))

 Collection of elements with no duplicates  Can test for membership of elements  basket = [‘a', ‘o', 'a’, ‘p', ‘o', ‘b']  fruit = set(basket)  fruit  >>>set([‘o', ‘p', ‘a', ‘b'])  'orange' in fruit  >>>True

 Similar to hash tables.  A set of key:value pairs.  tel = {'jack': 4098, 'sape': 4139}  tel.keys()  >>> ['jack‘,’sape’]

 Retrieves keys and values from a dictionary.  knights = {'gallahad': 'the pure', 'robin': 'the brave'}  for k, v in knights.iteritems():  print k, v  >>>gallahad the pure robin the brave

 While looping over a sequence, provides the position of elements.  for i, v in enumerate(['tic', 'tac', 'toe']):  print i, v  >>>0 tic  >>>1 tac  >>>2 toe

 Zip(list1, list2) : loops 2 or possibly more sequences at the same time.  Reversed(list) : loops over the list in a reverse order.  Sorted(list) : returns the list in a sorted order, however leaves the original list unsorted.