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Comprehensions

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List comprehensions are used to create a new list from existing sequences. It is a tool for transforming a given list into another list

1. Without Comprehensions

Suppose to create a list to store five different numbers such as 10,20,30,40 and 50. Using the for loop, add number five to the existing elements of the original list.

a=[10,20,30,40,50]
for i in range(0,len(a)):
    a[i]=a[i]+5
print(a)

#Output
[15,25,35,45,55]

Above code is workable but not an optimal code or the best way to write a code in Python.  Using list comprehension, we can replace the loop with a single expression that produces the same result.

 

2. With Comprehension

The syntax for comprehension is

[    for   in      if ]

The syntax is designed to read a  "Compute the expression for each element in the sequence, if the conditional is true".

#Using List Comprehension
a=[10,20,30,40,50]
a=[x+5  for x in a]
print(a)

#Output
[15,25,35,45,55]

With reference to the above example we can say that, list comprehension contains

a) An Input Sequence

b) A Variable referencing the input sequence

c) An Optimal Expression

e) An Output Expression or Output Variable.

 

Example 1:

Write a program to create a list with elements 1,2,3,4 and 5.  Display even elements of the list using list comprehension.

a=[1,2,3,4,5]
print("Content or original List : ",a)
a=[x  for x in a if x%2==0]
print("Even Elements from the List : ",a)

#Output
Content or original List :  [1, 2, 3, 4, 5]
Even Elements from the List :  [2, 4]

Example 2:

Consider a list with five different Celsius values. Convert all the Celsius values into Fahrenheit with and without using comprehension.

list=[10,20,31.3,40,39.2]
print("Celsius  Values : ",list)
celsius=[0,0,0,0,0]

#Without Using Comprehension
for i in range(0,len(list)):
    f=((float(9)/5)*list[i]+32)
    celsius[i]=f
print("Fahrenheit Values(Without Comprehension) : ",celsius)

#With Comprehension

fahrenheit=[((float)(9)/5)*x+32 for x in list]
print("Fahrenheit Values(With Comprehension) : ",fahrenheit)


#Output
Celsius  Values :  [10, 20, 31.3, 40, 39.2]
Fahrenheit Values(Without Comprehension) :  [50.0, 68.0, 88.34, 104.0, 102.56]
Fahrenheit Values(With Comprehension) :  [50.0, 68.0, 88.34, 104.0, 102.56]