Data Warehousing, BI and Data Science

1 December 2020

Python: List comprehension

Filed under: Python — Vincent Rainardi @ 7:05 am

List comprehension means using for loop to create a list :

L = [i*2 for i in range(1,5)] #5 not included
print(L)
Output: [2,4,6,8]

We can use 2 for loops to get the words in a paragraph :

paragraph = ["This is sentence one." , 'This is sentence two.']
result  = [word for sentence in paragraph for word in sentence.split()]
print(result)
Output:
['This', 'is', 'sentence', 'one', 'This', 'is', 'sentence', 'two']

We can use 2 for loops and an if to get words beginning with certain letters :
(the example below is using the paragraph defined above)

letters = ['s','o']
result = [word for sentence in paragraph for word in sentence.split() if word[0].lower() in letters]
print(result)
Output:
['sentence', 'one.', 'sentence']

We can use an if with any conditions we like :

numbers = [100,200,300,400]
result = [n for n in numbers if n > 200]
print(result)
Output:
[300,400]

We can make a list using double for in i and j format like this:

product_list = [ i*j for i in range(1,3) for j in range(10,20,5) ] #3 and 20 are not included
print(product_list)
Output:
[10, 15, 20, 30]

We can make a dictionary too using for :

result = {i:i*3 for i in range(1,7,2)}
print(result)
Output:
{1: 3, 3: 9, 5: 15}

We can use for to access the key in a dictionary.
In this example I try to get the best Hogwart pupils in a dictionary (highest remarks):

Hogwarts = {1:['Harry',80] , 2:['Ron',70], 3:['Hermione',90], 4:['Neville',60], 5:['Seamus',50]}
best_pupils = {key:value[0] for key,value in students_data.items() if value[1] >= 70}
print(best_pupils)
Output:
{1: 'Harry', 2: 'Ron', 3: 'Hermione'}

Various examples of creating lists and dictionary using for :

#times 2 if i is divisible by 3, otherwise plus 2
result = [i*2 if i%3==0 else i+2 for i in range(1,7)]
print(result)
Output: 
[3, 4, 6, 6, 7, 12]

#create a dictionary containing cube numbers from 1 to 4
n = 4
result = {i:i**3 for i in range(1,n+1)}
print(result)
Output: 
{1: 1, 2: 8, 3: 27, 4: 64}

#A list containing combinations of letters from 2 words
result = [i+j for i in "po" for j in "he"]
print(result)
Output: 
['ph', 'pe', 'oh', 'oe']

#Create a dictionary from a word. The key is in upper case, the value is double letter.
result = {x.upper(): x*2 for x in 'potter'}
print(result)
Output: 
{'P': 'pp', 'O': 'oo', 'T': 'tt', 'E': 'ee', 'R': 'rr'}

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