enumerate and zip

Introduction

Two built-in functions make loops over lists cleaner: enumerate adds a counter, and zip pairs up items from several lists.

Index arithmetic such as names[i] and marks[i] is a common source of small mistakes. These two functions let the loop hand you exactly what you need, which is both shorter and safer.

How it works

A counter with enumerate

names = ["Rahim", "Nadia", "Karim"]
for position, name in enumerate(names, start=1):
    print(position, name)

enumerate(names, start=1) gives pairs: (1, 'Rahim'), (2, 'Nadia'), (3, 'Karim'). The loop unpacks each pair into position and name. Without start=1 the count begins at 0.

The older way

names = ["Rahim", "Nadia"]
for i in range(len(names)):
    print(i, names[i])

for i in range(len(names)) with names[i] works, but it is longer and easier to get wrong. Prefer enumerate when you need the position, and plain for name in names when you do not.

Syntax

for i, name in enumerate(names, start=1):
    print(i, name)
for name, mark in zip(names, marks):
    print(name, mark)

Example

names = ["Rahim", "Nadia", "Karim"]
marks = [72, 91, 85]
for name, mark in zip(names, marks):
    print(name, mark)
best = max(zip(marks, names))
print("Top:", best[1])

zip(names, marks) pairs the first name with the first mark, the second with the second, and so on. max(zip(marks, names)) compares the pairs by mark first, so it finds the top student: Nadia, with 91.

Line by line

1. names and marks are two lists in matching order.
2. zip(names, marks) makes pairs, and the for line unpacks each one.
3. Each pass prints one name and its mark.
4. max(zip(marks, names)) compares the pairs by mark first and finds the largest.
5. best[1] is the name part of that pair.

Expected result

Rahim 72
Nadia 91
Karim 85
Top: Nadia

In the real world

Data often arrives as matching columns, such as dates and prices, and zip joins them back up. Numbered lists and menus are the everyday use of enumerate.

Common mistakes

• Lists of different lengths: zip stops at the shortest one and silently drops the rest.
• Forgetting to unpack: for pair in enumerate(names) gives tuples like (0, 'Rahim').
• Expecting zip(...) to print as a list: it is lazy; use list(zip(...)) to see it.

Best practices

• Use for x in items when you do not need the position.
• Use enumerate when you need the position, and zip for parallel lists.
• Check that lists have the same length before relying on zip.

Summary

enumerate adds a counter to a loop, and zip pairs up several lists and stops at the shortest. Unpacking in the for line gives each part a name.

Practice

Given names and marks, print a numbered list like 1. Rahim: 72 using enumerate and zip together.

Mark lesson complete