Python List Comprehension

Presentation given at Mempy meeting on May 18, 2015 on list comprehension in Python.

Transcript

1. List Comprehension >> [x**2 for x in (1,2,3 ....)]
2. About me: James Sheppard • Graduate Student at University of Memphis, Dept of Biomedical Eng. • Studying 'electrochemical sensors' • introduced to programming via Matlab • Python as a hobby • ... used to process and plot test results.
3. Intro: 'Set Notation' and For Loops • 'Set Notation' in Math • For loops (in Python) Given a radius r and center (a,b), a circle ... >>> S = [] >>> for x in range(10): ... S.append(x**2) >>> S [0,1,4,9,16,25,36,49,64,81] for loop use cases making lists
4. Mathematics 'Set' Notation For Loops List Comprehension
5. List comprehension • Multiple Statements • 'Matrices' • If statements • Advantages of List Comprehension: More human readable; faster. >>> [x+y+z for x in (1,2) ... for y in (3,5) ... for z in (10,20)] [14,24,16,26,15,25,17,27] >>> S = [x**2 for x in range(10)] >>> S [0,1,4,9,16,25,36,49,64,81] >>> [[y+z for x in (10,10,20)] for y in (1,2,3)] [[11, 11, 21], [12, 12, 22], [13, 13, 23]] >>> [x**2 for x in range(10) if x>5] [36,49,64,81]
6. >>> [x**2 for x in range(10) if x>5 ...] Generator Func. for statement (iteration var x) if statement (optional) Sequence Statement (iteration var x) Grammar Rules . . .
7. ... more properties • Strings • Other Generator Functions • Dictionaries >>> [x.upper() for x in 'hello'] ['H','E','L','L','O'] >>> [x for x in 'hello'] ['h','e','l','l','o'] >>> fruit = {'apples':2, 'pears':7, 'bananas':4, 'kiwi':1} >>> [x for x in fruit] ['apples','pears','bananas','kiwi'] >>> [x for x in fruit.values()] [1, 2, 7, 4] >>> [x for x in fruit.items()] [('kiwi', 1), ('apples', 2), ('pears', 7), ('bananas', 4)] fruit.keys()
8. >>> S = [] >>> for x in range(10): ... S.append(x**2) >>> S [0,1,4,9,16,25,36,49,64,81] for loop use cases making lists next time you want to use .... • For loops (in Python) try "List Comprehension"!