C6 - Data structures

Sequence

The most basic data structure in Python is the sequence (lists, tuples and strings). Every element in a sequence is assigned a number, which is its position or index. The first index is always zero, the second is one, and so forth.

Indexes are numbered from 0 to n-1 where n is the number of items (or characters):

 H  e  l  l  o  ,  _  w  o  r  l  d  !
 0  1  2  3  4  5  6  7  8  9 10 11 12

There are certain things you can do with all sequence types. These operations include indexing, slicing, adding, multiplying, and checking for membership.

Python Lists

The list is versatile data structure that is available in Python, it is a mutable data structure, which can be written as a list of comma-separated values (items) between square brackets.

Creating a list is as simple as putting different comma-separated values between square brackets. For example −

list1 = ['physics', 'chemistry', 1997, 2000];
list2 = [1, 2, 3, 4, 5 ];
list3 = ["a", "b", "c", "d"]

Similar to string indices, list indices start at 0, and lists can be sliced, concatenated and so on.

Click on the link below to go through a python tutorial on lists. By the end you should have a basic understanding of:

Exercises

Exercise 1

Take a list, say for example this one:

	a = [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]

and write a program that prints out all the elements of the list that are less than 5.

Click the button below for a sample solution that solves the exercise, including extras 1 and 2.

a = [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]

num = int(input("Choose a number: "))

new_list = []

for i in a:
	if i < num:
		new_list.append(i)

print (new_list)

Arrays

An Array is a 'sequence' type data structure, which is a series of memory locations each of which holds a single data type. All data in an array must be of the same data type.

You can think of an array like a row of cells, like the ones found in a table. Each cell represents an element:

Exercises

Exercise 1

Research the differences between an array and a list:

Exercise 2

Write a function that returns an array of all of the positive odd elements that are less than n, where n is the user input.

So if a users enters the number: 10 your funtion will return :

 [1, 3, 5, 7, 9]

Click the button below for a solution.

num = int(input("Enter a number: "))

def odd_less(n):
  return [x for x in range(0, n) if x % 2 == 1 ] #single line list comprehension

new_list = odd_less(num)

print (new_list)

Multi-Dimensional Arrays

Watch the video below for a brief introduction on how to create multi-dimensional arrays

Exercise

Copy the following code into python and then run it to see if you can understand how it is manipulating a two dimensional array/list.

       
a = [[1, 2, 3], [4, 5, 6]]
print(a[0])
print(a[1])
b = a[0]
print(b)
print(a[0][2])
a[0][1] = 7
print(a)
print(b)
b[2] = 9
print(a[0])
print(b)

To process a 2-dimensional array/list, you will most often use nested loops. The first loop iterates through the row number, the second loop runs through the elements inside of a row, which you can think of like a column in a table.

a = [[1, 2, 3, 4], [5, 6]]
b = [[3, 7, 2, 7], [5, 4]]

Click the button below for a solution to task 1.

for row in range(len(a)):
    for column in range(len(a[row])):
        print(a[row][column], end=' ')
    print()

Sets

Sets are unordered collections of simple objects. These are used when the existence of an object in a collection is more important than the order or how many times it occurs.

Using sets, you can test for membership, whether it is a subset of another set, find the intersection between two sets, and so on.

Exercise

Exercise 1

Given a list of integers e.g. 1 2 3 2 1.

Determine how many distinct numbers there are. This task can be solved in one line of code.

Dictionaries

A dictionary is like an address-book where you can find the address or contact details of a person by knowing only his/her name i.e. we associate keys (name) with values (details).

Note that the key must be unique just like you cannot find out the correct information if you have two persons with the exact same name.

Exercise

Given the following dictionary

A = {'ab' : 'ba', 'aa' : 'aa', 'bb' : 'bb', 'ba' : 'ab'}

Write a loop which iterates through the keys of all items in the dictionary: