How to Count Consecutive Elements in a Python List: 5 Clever Solutions

Introduction

A follower asked how to count consecutive identical values in a Python list. The original data is shown below.

Solution 1

list1=[1,1,1,0,0,0,1,1,0,1,1,1,1,1,1]
result=[0]
flag=0
for i in range(1,len(list1)):
    if list1[i]==list1[i-1]:
        flag+=1
    else:
        flag=0
    result.append(flag)
print(result)

Solution 2

list1 = [1,1,1,0,0,0,1,1,0,1,1,1,1,1,1]
list2 = []
l = 0
for i in range(len(list1)):
    l = l + 1 if list1[i] == list1[i-1] and i != 0 else 0
    list2.append(l)
print(list2)

Solution 3

list1 = [1,1,1,0,0,0,1,1,0,1,1,1,1,1,1]
result = [0] * len(list1)
result[0] = 0
for i in range(1, len(list1)):
    if list1[i] != list1[i-1]:
        result[i] = 0
    else:
        result[i] = result[i-1] + 1
print(result)

Solution 4

list1 = [1,1,1,0,0,0,1,1,0,1,1,1,1,1,1]
result = [0] * len(list1)
pre_num = 0
for num in range(len(list1)):
    print(num, pre_num)
    if list1[num] != list1[pre_num]:
        pre_num = num
    result[num] = num - pre_num
    print(result)
print(result)

Solution 5

from functools import reduce
from itertools import groupby

list1 = [1,1,1,0,0,0,1,1,0,1,1,1,1,1,1]
ret = reduce(lambda x, y: x + y, [[i for i in range(len(list(num)))] for item, num in groupby(list1)])
print(ret)

Conclusion

The article presents five different Python implementations for counting consecutive identical elements in a list, demonstrating techniques such as flag counters, list comprehensions, pre‑number tracking, and functional tools like reduce and groupby. Each solution includes full source code and a screenshot of its output.