Merge Counters

A medium Python interview practice problem on DataDriven. Write the Python and run it against test cases, with instant feedback.

Domain
Python
Difficulty
medium
Seniority
L3

Problem

Given two dicts d1 and d2 mapping keys to numbers, return a new dict combining all keys. For keys present in both, sum the two values. For keys in only one, keep that value.

Two tallies. Combine them.

Example 1

Input:

d1 = {"a":1,"b":2}, d2 = {"b":3,"c":4}

Output:

{"a":1,"b":5,"c":4}

Example 2

Input:

d1 = {}, d2 = {}

Output:

{}

Worked solution and explanation

Why this problem exists in real interviews

Merging two counter dicts by summing shared keys is a reduce operation common in MapReduce-style pipelines. It tests whether you handle keys unique to each dict correctly.

Break down the requirements

Step 1: Start with all keys from both dicts

The merged result contains every key that appears in either dict.

Step 2: Sum values for shared keys

When both dicts have the same key, add their values.

Step 3: Pass through unique keys unchanged

Keys in only one dict appear with their original value.

The solution

Union-key merge with value summation
def merge_dicts(d1, d2):
    result = {}
    for key in d1:
        result[key] = d1[key]
    for key in d2:
        if key in result:
            result[key] += d2[key]
        else:
            result[key] = d2[key]
    return result

Common follow-up questions