Generate a list using given frequency list

Last Updated : 01 May, 2019

Given a list, the task is to generate another list containing numbers from 0 to n, from given list whose each element is frequency of numbers in the generated list.

Input: freq = [1, 4, 3, 5]
Output: [0, 1, 1, 1, 1, 2, 2, 2, 3, 3, 3, 3, 3]

Explanation:
Number = 0
Input[0] = 1 Output = [0]

Number =1
Input[1] = 4 Output = [0, 1, 1, 1, 1] (repeats 1 four times)

Number =2
Input[2] = 3  Output = [0, 1, 1, 1, 1, 2, 2, 2] (repeats 2 three times)

Number =3
Input[3] = 5  Output = [0, 1, 1, 1, 1, 2, 2, 2, 3, 3, 3, 3, 3] (repeats 3 five times)

Below are some ways to achieve above task.

Method #1 : Using Iteration

# Python code to generate list containing numbers from 0 to 'n' 
# having frequency of no from another list 
  
# List initialisation 
Input = [1, 4, 3, 5] 
Output = [] 
  
# Number initialisation 
no = 0
  
# using iteration 
for rep in Input: 
    for elem in range(rep): 
        Output.append(no) 
    no += 1
  
# printing output 
print(Output)  

Output:

[0, 1, 1, 1, 1, 2, 2, 2, 3, 3, 3, 3, 3]

Method #2 : Using enumerate and list comprehension

# Python code to generate list containing numbers from 0 to 'n' 
# having frequency of no from another list 
  
# List initialisation 
Input = [1, 4, 3, 5] 
  
# Using enumerate and list comprehension 
Output = [no for no, rep in enumerate(Input) 
                     for elem in range(rep)] 
  
# Printing output 
print(Output)  

Output:

[0, 1, 1, 1, 1, 2, 2, 2, 3, 3, 3, 3, 3]

Method #3 : Using enumerate and chain

# Python code to generate list containing numbers from 0 to 'n' 
# having frequency of no from another list 
  
# List initialisation 
Input = [1, 4, 3, 5] 
  
# Importing 
from itertools import repeat, chain 
  
# Using chain and enumerate 
Output = list(chain.from_iterable((repeat(x, y)) 
                  for x, y in enumerate(Input))) 
  
# Printing output 
print(Output)  

Output:

[0, 1, 1, 1, 1, 2, 2, 2, 3, 3, 3, 3, 3]

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