Lists in Python – the basics –
Lists in Python
– the basics –
Nick Szirbik
Sept. 2020
Week 2 of the DAPOM course (2)
First: Data structures
All advanced programming languages offer various built-in data structures or constructs that allow their development by the programmer
Typical data structures are: indexed arrays of various dimensionality, labelled records, linked lists, trees, heaps, hash tables, graphs, networks
Python has four built-in data structures (generically named collections): tuples, lists, sets, and dictionaries.
We recommend to study them using tutorials from w3Schools.com
But before collections: the string data type
To some extent, a string is a data structure,
because it is in fact an array of unicode characters
It can be use as indexed, like an array
(or mathematical vector). For example:
digits = str(‘1234567890ABCDEF’)
print(digits)
whatDigit = digits[0]
andThis = digits[9]
for count, digit in enumerate(digits):
print(“the”, count, “nth digit in the string is”, digit)
# more at https://www.w3schools.com/python/python_strings.asp
1234567890ABCDEF
the 0 nth digit in the string is 1
the 1 nth digit in the string is 2
the 2 nth digit in the string is 3
the 3 nth digit in the string is 4
the 4 nth digit in the string is 5
the 5 nth digit in the string is 6
the 6 nth digit in the string is 7
the 7 nth digit in the string is 8
the 8 nth digit in the string is 9
the 9 nth digit in the string is 0
the 10 nth digit in the string is A
the 11 nth digit in the string is B
the 12 nth digit in the string is C
the 13 nth digit in the string is D
the 14 nth digit in the string is E
the 15 nth digit in the string is F
A list is a collection which is ordered and changeable.
In Python lists are written with square brackets.
An example of a list of strings only:
fruit_list = [“apple”, “banana”, “cherry”, “orange”, “kiwi”, “melon”, “mango”]
print(fruit_list[2:5]) # display only from the third to the sixth element
How to check the length of a list – use the len() function:
print(len(fruit_list))
Insert an item (in the second position) – use the insert() function
fruit_list.insert(1, “pear”)
print(fruit_list)
Delete an item – use the remove() function:
fruit_list.remove(“banana“)
print(fruit_list)
Lists – very important for the next practical
They can be nested also (lists of lists is possible!)
https://www.w3schools.com/python/python_lists.asp
To remember:
The most important method for you: append()
Copying lists (and other collections) is done always with copy()
# some code example with a matrix (as list of lists)
###
matrix = list(([[1,2,3],[4,5,6],[7,8,9]]))
total_of_matrix_elements = sum([sum (row) for row in matrix])
print(total_of_matrix_elements)
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