Python | Multiply each element in a sublist by its index

Last Updated : 29 May, 2019

Given a list of lists, the task is to multiply each element in a sublist by its index and return a summed list.

Given below are a few methods to solve the problem.

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Method #1: Using Naive Method

# Python3 code to demonstrate
# to multiply numbers with position
# and add them to return num
  
import numpy as np
  
# initialising list
ini_list = [[3, 4, 7], [ 6, 7, 8], [ 10, 7, 5], [ 11, 12, 13]]
  
# printing initial_list
print ("initial_list ", ini_list)
  
res = []
# Using Naive Method
for sub_list in ini_list:
    sublistsum = 0
  
    for i, value in enumerate(sub_list):
        sublistsum = sublistsum + i * value
  
    res.append(sublistsum)
  
# printing result
print ("result", res)

Output:

initial_list  [[3, 4, 7], [6, 7, 8], [10, 7, 5], [11, 12, 13]]
result [18, 23, 17, 38]

Method #2: Using List comprehension

# Python3 code to demonstrate
# to multiply numbers with position
# and add them to return num
  
  
# initialising list
ini_list = [[3, 4, 7], [ 6, 7, 8], [ 10, 7, 5], [ 11, 12, 13]]
  
# printing initial_list
print ("initial_list ", ini_list)
  
# Using list comprehension
res = [sum(i * j for i, j in enumerate(sublist)) 
                         for sublist in ini_list]
  
# printing result
print ("result", res)
             

Output:

initial_list  [[3, 4, 7], [6, 7, 8], [10, 7, 5], [11, 12, 13]]
result [18, 23, 17, 38]

Method #3: Using numpy

# Python3 code to demonstrate
# to multiply numbers with position
# and add them to return num
  
import numpy as np
  
# initialising list
ini_list = [[3, 4, 7], [ 6, 7, 8], [ 10, 7, 5], [ 11, 12, 13]]
  
# printing initial_list
print ("initial_list ", ini_list)
  
# Using numpy
res = [np.arange(len(sublist)).dot(sublist) for sublist in ini_list]
  
# printing result
print ("result", res)
              
             

Output:

initial_list  [[3, 4, 7], [6, 7, 8], [10, 7, 5], [11, 12, 13]]
result [18, 23, 17, 38]

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Python | Multiply each element in a sublist by its index