Python Data Structures

Learning Python data structures. this document is generated by code itself I hope you enjoy it.

String

A string is a sequence of characters.

oneString = "hello" #>>> hello
anotherString = "world!" #>>> world!

The contatenate operator it the + symbol.

gretting = oneString + " " + anotherString #>>> hello world!

The len function take a string argument and returns interger representing the length of a string.

len(gretting) #>>> 12

Looping through strings

index = 0
while index < len(greetting):
    letter = fruit[index]
    index = index + 1
    print(letter) 

for letter in greetting:
    print(letter)

#both loops will print: 
#>> h
#>> e
#>> l
#>> l
#>> o
#>> 
#>> w
#>> o
#>> r
#>> l
#>> d
#>> !

String methods

split

The split method divide the string by the white spaces, returning a list with the resulting strings. It takes an optional string argument in order to change the delimiter.

sentence = "With three words."
words = sentence.split() # returns ["With", "three", "words."]

Files

In Python you can obtain the instance of a file by using the open built-in function.

File is a path-like object giving the pathname (absolute or relative to the current working directory) of the file to be opened or an integer file descriptor of the file to be wrapped. (If a file descriptor is given, it is closed when the returned I/O object is closed unless closefd is set to False.).

open function

Open file and return a corresponding file object. If the file cannot be opened, an OSError is raised.

Open Syntax

open(file, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=True, opener=None)

Mode is an optional string that specifies the mode in which the file is opened. It defaults to 'r' which means open for reading in text mode.

Char	Meaning
'r'	open for reading (default)
'w'	open for writing, truncating the file first
'x'	open for exclusive creation, failing if the file already exists
'a'	open for writing, appending to the end of file if it exists
'b'	binary mode
't'	text mode (default)
'+'	open for updating (reading and writing)

Example of the code that writes this document:

try:
    file = open("README.md", "w")
    file.write(content)
    print("File writed.")
except:
    print("Error: can't read or write file.")

Reading files

File.read() Reads a whole file returning a string.

File.readlines() Reads the file returning an array of a string per line.

fHanddler = open(fileName, "r")
lines = fHanddler.readlines()
for line in lines:
    print(line)

Here's the output of a function that reads the first 10 lines of this very file:

# Python Data Structures

Learning Python data structures. this document is generated by code itself I hope you enjoy it.

## String

A string is a sequence of characters.
```python
oneString = "hello" #>>> hello

Lists

List are collection of data.

leonidas = ["This", "is", "Sparta!"]
len(stringList) # returns 3

The range function an integer representing a size and returns a list ordered numbers starting from 0.

for i in range(len(stringList)):
    print(i, stringList[i])

Operators

The contateantion operator + concatenates lists returning a new list.

result = ""
messenger = ["\nMessenger:", "This", "is", "madness..."]
leonidas = ["\nLeonidas:", "Madness?", "This", "is", "Sparta!"]
dialog = messenger + leonidas
for word in dialog:
    result += word + " "
print(result)
'''
Messenger: This is madness...
Leonidas: Madness? This is Sparta!
'''

The in operator returns a boolean if a value belongs to a list.

leonidas = ["This", "is", "Sparta!"]
if("Sparta!" in leonidas):
    return True

List Methods

append

Adds an element to de end of the list.

ages = [18, 22, 30, 25]
ages.append(27) # now: [18, 22, 30, 25, 27]

extend

Extend the list by appending all the items from the iterable.

ages = [18, 22, 30, 25]
ages.extend([27, 19, 37]) # now: [18, 22, 30, 25, 27, 19, 37]

insert

Insert an item at a given position. The first argument is the index of the element before which to insert.

ages = [18, 22, 30, 25]
ages.insert(0, 27) # now: [27, 18, 22, 30, 25]

remove

Removes the first item from the list whose value is equal to x. It raises a ValueError if there is no such item.

ages = [18, 22, 30, 25]
ages.remove(25) # now: [18, 22, 30]

pop

Removes the item at the given position in the list, and return it. If no index is specified, a.pop() removes and returns the last item in the list.

ages = [18, 22, 30, 25]
ages.pop() # now: [18, 22, 30]
ages.pop(0) # now: [22, 30]

clear

Remove all items from the list.

ages = [18, 22, 30, 25]
ages.clear() # now: []

index

Return zero-based index in the list of the first item whose is equal to the argument passed in the first position. Can take 2 optional arguments start and end are interpreted as in the slice notation and are used to limit the search to a particular subsequence of the list. The returned index is computed relative to the beginning of the full sequence rather than the start argument. Raises a ValueError if there is no such item.

ages = [18, 22, 30, 25]
ages.index(30) # returns: 2

count

Return the number of times given value appears in the list.

ages = [18, 22, 30, 25]
ages.count(18) # returns: 1

sort

Sort the items of the list in place.

ages = [18, 22, 30, 25]
ages.sort() # now: [18, 22, 25, 30]

reverse

Reverse the elements of the list in place.

ages = [18, 22, 30, 25]
ages.reverse() # now: [25, 30, 22, 18]

copy

Return a shallow copy of the list.

ages = [18, 22, 30, 25]
ages.copy() # returns: [18, 22, 30, 25]

Dictionaries

Dictonaries are colection of data strored in key-value paris.

Operators

The in operator:

count = dict()
names = ["Ismael", "Ruben", "Sohany", "Sohany", "Ismael", "Ismael"]
for name in names:
    if name not in count:
        conunt[name] = 1
    else:
        count[name] += 1

for key, value in count: # supports 2 iteration variables.
    print(key, value)
# Prints the following: 
# Ismael 3
# Ruben 1 
# Sohany 2

Functions

The list function takes a dictionary instance and returns a list of it.

keyArray = list(count.keys()) # ["Ismael", "Ruben", "Sohany"]
valueArray = list(count.values()) # [3, 1, 2]
itemArray = list(count.items()) # [("Ismael", 3), ("Ruben", 1), ("Sohany", 2)]

Methods

get

cesarCount = count.get("Cesar", 0) # There no key Cesar in count so cesarCount = 0
ismaelCount = count.get("Ismael", 0) # ismaelCount = 3

Tuples

Tuples are immutable sequences, typically used to store collections of heterogeneous data (such as the 2-tuples produced by the enumerate() built-in). Tuples are also used for cases where an immutable sequence of homogeneous data is needed (such as allowing storage in a set or dict instance).

tuple('abc') # returns ('a', 'b', 'c') 
tuple( [1, 2, 3] ) # returns (1, 2, 3)

Tuples assignment:

person = ("fred", 21)
name, age = person
print(name) # prints: fred
print(age) # prints: 21

The > & < operators

(0, 23) > (-1, 24) # returns True
(0, 23) < (0, 24) # returns True
("Zod", "apple") > ("angel", "Ponking") # returns false
("Bob", "joe") < ("Bob", "tim") # returns false

Example of a program that return the 10 most used words in this document.

import files

def wordCount(lineList: List[str])-> dict:
    result = dict()
    for line in lineList:
        for word in line.split():
            result[word] = result.get(word, 0) + 1
    return result    

def sortDictByValue(dictionary: dict[str, int])-> List[tuple[int, str]]:
    result: List[tuple[int, str]] = []
    for word, count in dictionary:
        result.append(tuple(count, word))
    return sorted(result, reverse=True)


sortedList = tuples.sortDictByValue(tuples.wordCount(files.readFile(fileName)))

Here the result:

Word	Times
the	59
=	45
#	34
```	32
```python	30
a	28
of	26
in	26
is	22
###	22

Credits

Ismael Varela, Fulstack Developer