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PRG – PROGRAMMING ESSENTIALS
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Lecture 13 – Introduction to Advanced Conceptshttps://cw.fel.cvut.cz/wiki/courses/be5b33prg/start
Tomas JenicekCzech Technical University in Prague,
Faculty of Electrical Engineering, Dept. of Cybernetics, Center for Machine Perceptionhttp://cmp.felk.cvut.cz/~jenicto2/
17/12/20 Michal Reinštein, Czech Technical University in Prague
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REFERENCES
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17/12/20 Michal Reinštein, Czech Technical University in Prague
• Lambda functions https://docs.python.org/3/tutorial/controlflow.html#lambda-expressions
• List comprehensions https://docs.python.org/3/tutorial/datastructures.html#list-comprehensions LICENSE
• Map – Filter – Reduce http://book.pythontips.com/en/latest/map_filter.html# LICENSE
• Iterators & Generators http://book.pythontips.com/en/latest/generators.html LICENSE
• Itertools by example https://realpython.com/python-itertools/
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LAMBDA FUNCTIONS
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17/12/20 Michal Reinštein, Czech Technical University in Prague
Source https://docs.python.org/3/tutorial/controlflow.html#lambda-expressions LICENSE
• Small anonymous functions can be created with the lambda keyword
• EXAMPLE: This function returns the sum of its two arguments:
lambda a, b: a+b
• Lambda functions can be used wherever function objects are required
• Syntactically restricted to a single expression • Like nested function definitions, lambda functions can
reference variables from the containing scope
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LAMBDA FUNCTIONS
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17/12/20 Michal Reinštein, Czech Technical University in Prague
Source http://book.pythontips.com/en/latest/lambdas.html LICENSE
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LAMBDA FUNCTIONS
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17/12/20 Michal Reinštein, Czech Technical University in Prague
Source http://book.pythontips.com/en/latest/lambdas.html LICENSE
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LAMBDA FUNCTIONS
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17/12/20 Michal Reinštein, Czech Technical University in Prague
Source https://docs.python.org/3/tutorial/controlflow.html#lambda-expressions LICENSE
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LIST COMPREHENSIONS
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17/12/20 Michal Reinštein, Czech Technical University in Prague
Source https://docs.python.org/3/tutorial/datastructures.html#list-comprehensions LICENSE
• The list comprehensions provide a concise way to create lists• It consists of [ ] containing an expression followed by a for
clause, then zero or more for or if clauses• The expressions can be anything (any kind of objects in lists)• The result will be a new list created by evaluating the
expression in the context of the for and if clauses• The list comprehension always returns a result list
• Common applications are to:Make new lists where each element is the result of some operations applied to each member of another sequence or iterable, or to create a subsequence of those elements that satisfy a certain condition
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LIST COMPREHENSIONS
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17/12/20 Michal Reinštein, Czech Technical University in Prague
Source https://docs.python.org/3/tutorial/datastructures.html#list-comprehensions LICENSE
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LIST COMPREHENSIONS
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17/12/20 Michal Reinštein, Czech Technical University in Prague
Source https://docs.python.org/3/tutorial/datastructures.html#list-comprehensions LICENSE
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LIST COMPREHENSIONS
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17/12/20 Michal Reinštein, Czech Technical University in Prague
Source https://docs.python.org/3/tutorial/datastructures.html#list-comprehensions LICENSE
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LIST COMPREHENSIONS
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17/12/20 Michal Reinštein, Czech Technical University in Prague
Source https://docs.python.org/3/tutorial/datastructures.html#list-comprehensions LICENSE
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MAP – FILTER – REDUCE
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17/12/20 Michal Reinštein, Czech Technical University in Prague
Source http://book.pythontips.com/en/latest/map_filter.html# LICENSE
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MAP – FILTER – REDUCE
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17/12/20 Michal Reinštein, Czech Technical University in Prague
Source http://book.pythontips.com/en/latest/map_filter.html# LICENSE
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MAP – FILTER – REDUCE
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17/12/20 Michal Reinštein, Czech Technical University in Prague
Source http://book.pythontips.com/en/latest/map_filter.html# LICENSE
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MAP – FILTER – REDUCE
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17/12/20 Michal Reinštein, Czech Technical University in Prague
Source http://book.pythontips.com/en/latest/map_filter.html# LICENSE
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MAP – FILTER – REDUCE
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17/12/20 Michal Reinštein, Czech Technical University in Prague
Source http://book.pythontips.com/en/latest/map_filter.html# LICENSE
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GENERATORS
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17/12/20 Michal Reinštein, Czech Technical University in Prague
Source https://docs.python.org/3/tutorial/datastructures.html#list-comprehensions
• Generators are “functions” that can be paused and resumed on the fly, returning an object that can be iterated over
• Unlike lists, generators are lazy and thus produce items one at a time and only when asked
• Generators are much more memory efficient when dealing with large datasets (often the only way to handle large data)
• One of the advanced python concepts is:How to create generator functions and expressions as well as why you would want to use them in the first place
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GENERATORS – TERMINOLOGY
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17/12/20 Michal Reinštein, Czech Technical University in Prague
Source http://book.pythontips.com/en/latest/generators.html LICENSE
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GENERATORS
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17/12/20 Michal Reinštein, Czech Technical University in Prague
Source http://book.pythontips.com/en/latest/generators.html LICENSE
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GENERATORS
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17/12/20 Michal Reinštein, Czech Technical University in Prague
Source http://book.pythontips.com/en/latest/generators.html LICENSE
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GENERATORS
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17/12/20 Michal Reinštein, Czech Technical University in Prague
Source http://book.pythontips.com/en/latest/generators.html LICENSE
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GENERATORS
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17/12/20 Michal Reinštein, Czech Technical University in Prague
Source http://book.pythontips.com/en/latest/generators.html LICENSE
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GENERATORS
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17/12/20 Michal Reinštein, Czech Technical University in Prague
Source http://book.pythontips.com/en/latest/generators.html LICENSE
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GENERATORS – SUMMARY
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17/12/20 Michal Reinštein, Czech Technical University in Prague
Source http://openbookproject.net/thinkcs/python/english3e/app_a.html
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REFERENCES
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17/12/20 Michal Reinštein, Czech Technical University in Prague
This lecture re-uses selected parts of the OPEN BOOK PROJECTLearning with Python 3 (RLE)
http://openbookproject.net/thinkcs/python/english3e/index.htmlavailable under GNU Free Documentation License Version 1.3)
• Version date: October 2012• by Peter Wentworth, Jeffrey Elkner, Allen B. Downey, and Chris Meyers
(based on 2nd edition by Jeffrey Elkner, Allen B. Downey, and Chris Meyers) • Source repository is at https://code.launchpad.net/~thinkcspy-rle-
team/thinkcspy/thinkcspy3-rle• For offline use, download a zip file of the html or a pdf version
from http://www.ict.ru.ac.za/Resources/cspw/thinkcspy3/
OTHER SOURCES USED:• Lambda functions https://docs.python.org/3/tutorial/controlflow.html#lambda-expressions• List comprehensions https://docs.python.org/3/tutorial/datastructures.html#list-
comprehensions LICENSE• Map – Filter – Reduce http://book.pythontips.com/en/latest/map_filter.html# LICENSE• Iterators & Generators http://book.pythontips.com/en/latest/generators.html LICENSE• Itertools by example https://realpython.com/python-itertools/
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EXAMPLE – PRIME NUMBERS
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17/12/20 Michal Reinštein, Czech Technical University in Prague
source https://hackernoon.com/prime-numbers-using-python-824ff4b3ea19
TASK: Write a program to generate a list of all prime numbers less than 20
• Before starting it is important to note what a prime number is:• A prime number has to be a positive integer• Divisible by exactly 2 integers (1 and itself)• 1 is not a prime number
• While there are many different ways to solve this problem, here are a few different approaches
SOURCE: https://hackernoon.com/prime-numbers-using-python-824ff4b3ea19
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EXAMPLE – PRIME NUMBERS
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17/12/20 Michal Reinštein, Czech Technical University in Prague
• Example of a solution
source https://hackernoon.com/prime-numbers-using-python-824ff4b3ea19
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EXAMPLE – PRIME NUMBERS
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17/12/20 Michal Reinštein, Czech Technical University in Prague
• Approach 1: notice that as soon isPrime is False, it is inefficient to keep on iterating. It would be more efficient to exit out of the loop.
source https://hackernoon.com/prime-numbers-using-python-824ff4b3ea19
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EXAMPLE – PRIME NUMBERS
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17/12/20 Michal Reinštein, Czech Technical University in Prague
• Approach 2 is more efficient than approach 1 because as soon as you find a given number isn’t a prime number you can exit out of loop using break.
source https://hackernoon.com/prime-numbers-using-python-824ff4b3ea19
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EXAMPLE – PRIME NUMBERS
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17/12/20 Michal Reinštein, Czech Technical University in Prague
• Approach 3: is similar to approach 2 except the inner range function. Notice that the inner range function is now:
range(2, int(possiblePrime ** 0.5) + 1)
source https://hackernoon.com/prime-numbers-using-python-824ff4b3ea19
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EXAMPLE – PRIME NUMBERS
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17/12/20 Michal Reinštein, Czech Technical University in Prague
• We use the properties of composite numbers• Composite number is a positive number greater than 1 that is
not prime (which has factors other than 1 and itself)• Every composite number has a factor less than or equal to its
square root (proof here).
• EXAMPLE: Factors of 15 below; the factors in red are just the reverses of the green factors so by the commutative property of multiplication 3 x 5 = 5 x 3 we just need to include the “green” pairs to be sure that we have all the factors.
source https://hackernoon.com/prime-numbers-using-python-824ff4b3ea19
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EXAMPLE – PRIME NUMBERS
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17/12/20 Michal Reinštein, Czech Technical University in Prague
• Evaluating performance
source https://hackernoon.com/prime-numbers-using-python-824ff4b3ea19
• REFERENCE: https://hackernoon.com/prime-numbers-using-python-824ff4b3ea19