How to Use zip() for Parallel Iteration

Table Of Contents

• Problem
• Solution
• Explanation

Problem

You need to iterate over multiple lists or sequences at the same time, accessing corresponding elements from each sequence.

Solution

# Basic zip usage
names = ['Alice', 'Bob', 'Charlie']
ages = [25, 30, 35]
cities = ['NYC', 'LA', 'Chicago']

for name, age, city in zip(names, ages, cities):
    print(f"{name} is {age} years old and lives in {city}")

# Create dictionary from two lists
keys = ['name', 'age', 'occupation']
values = ['John', 28, 'Engineer']
person = dict(zip(keys, values))
print(person)  # {'name': 'John', 'age': 28, 'occupation': 'Engineer'}

# Transpose matrix (swap rows and columns)
matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]
transposed = list(zip(*matrix))
print(transposed)  # [(1, 4, 7), (2, 5, 8), (3, 6, 9)]

# Handle unequal length sequences (zip stops at shortest)
list1 = [1, 2, 3, 4, 5]
list2 = ['a', 'b', 'c']
result = list(zip(list1, list2))
print(result)  # [(1, 'a'), (2, 'b'), (3, 'c')]

# Use itertools.zip_longest for unequal lengths
from itertools import zip_longest

result = list(zip_longest(list1, list2, fillvalue='N/A'))
print(result)  # [(1, 'a'), (2, 'b'), (3, 'c'), (4, 'N/A'), (5, 'N/A')]

# Enumerate with multiple sequences
scores = [85, 92, 78, 96]
subjects = ['Math', 'Science', 'English', 'History']

for i, (score, subject) in enumerate(zip(scores, subjects)):
    print(f"{i+1}. {subject}: {score}%")

# Unzip (separate combined data)
paired_data = [(1, 'a'), (2, 'b'), (3, 'c')]
numbers, letters = zip(*paired_data)
print(numbers)  # (1, 2, 3)
print(letters)  # ('a', 'b', 'c')

# Calculate element-wise operations
x = [1, 2, 3, 4]
y = [5, 6, 7, 8]

# Add corresponding elements
sums = [a + b for a, b in zip(x, y)]
print(sums)  # [6, 8, 10, 12]

# Find differences
differences = [abs(a - b) for a, b in zip(x, y)]
print(differences)  # [4, 4, 4, 4]

# Compare sequences element by element
def compare_lists(list1, list2):
    matches = []
    for a, b in zip(list1, list2):
        matches.append(a == b)
    return matches

result = compare_lists([1, 2, 3], [1, 5, 3])
print(result)  # [True, False, True]

# Process multiple files simultaneously
filenames = ['data1.txt', 'data2.txt', 'data3.txt']
headers = ['Name', 'Age', 'City']

for filename, header in zip(filenames, headers):
    print(f"Processing {filename} with header: {header}")

Explanation

zip() combines multiple iterables into tuples, stopping when the shortest sequence is exhausted. Use zip(*sequence) to unzip data back into separate sequences.

For sequences of different lengths, use itertools.zip_longest() with a fillvalue. This pattern is essential for parallel data processing and matrix operations.