PythonData Structures

Akshay Singh

Lists in python can contain any data type

Declaring a list:

a = [‘random’,’variable’, 1, 2]

List

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 methods

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 Methods

List.sort() – sorts the list.

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

List Methods

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

Lists as Stacks

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.

Lists as Queues

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]

Functional Programming Tools – filter()

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]

Functional Programming Tools – map()

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

Functional Programming Tools – reduce()

Easy way to work with lists. Similar to loops.

vec = [2, 4, 6] [3*x for x in vec]

Output:[6, 12, 18]

List Comprehensions

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”

Function – del()

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))

Tuples

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

Sets

Similar to hash tables. A set of key:value pairs.

tel = {'jack': 4098, 'sape': 4139} tel.keys() >>> ['jack‘,’sape’]

Dictionaries

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

Looping: iteritems()

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

Looping: enumerate()

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.

Looping (contd.)

Questions?