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

1. Introduction

A follower asked how to process a Python list to count consecutive identical values. The original data were:

a = [1, 1, 1, 0, 0, 0, 1, 1, 0, 1, 1, 1, 1, 1, 1]
b = [0, 1, 2, 0, 1, 2, 0, 1, 0, 0, 1, 2, 3, 4, 5]

The goal is to generate a result list where each element records how many times the current value has appeared consecutively up to that position.

2. Implementation

Five community members shared their solutions.

Method 1 (by 瑜亮老师)

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)

This approach uses a flag that increments when the current element matches the previous one and resets otherwise.

Method 2 (by 绅)

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)

This version condenses the logic into a single line inside the loop.

Method 3 (by 逸)

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)

This method pre‑allocates the result list and updates each position based on the previous count.

Method 4 (by 月神)

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)

Here the algorithm tracks the index of the last change (pre_num) and computes the consecutive count as the distance from that index.

Method 5 (by 布达佩斯的永恒)

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)

This solution leverages itertools.groupby and functools.reduce to build the consecutive counts in a functional style.

3. Summary

The article presented five practical ways to compute consecutive element counts in a Python list, ranging from straightforward loops with a flag variable to more compact one‑liners and functional programming techniques, giving readers multiple tools to solve similar list‑processing challenges.