Useful built-in functions

Python's always-available functions most useful in competitive programming: enumerate, zip, map, filter, sorted, any, all, min, max, abs, pow, and sum.

enumerate

When iterating over a list, enumerate gives you both the index and the value at the same time:

You can start the index from any number:

CP use — finding the index of the maximum element:

zip

zip pairs up elements from two or more iterables, stopping at the shortest one:

Building a dict from two lists:

Iterating over two arrays in parallel — very common in CP:

Transposing a matrix using zip:

zip with more than two iterables works the same way — elements are grouped by position across all of them.

map

map applies a function to every element of an iterable. It returns a lazy iterator, so wrap it in list() if you need a list:

You have seen this pattern constantly throughout the course:

map with a lambda for custom transformations:

In most cases a list comprehension is more readable than map + lambda. Use map when you already have a named function to pass:

filter

filter keeps only the elements for which the function returns True. Also lazy — wrap in list() to get a list:

Again, a list comprehension is usually more readable:

filter is most useful when you already have a named predicate function:

sorted with key=

You have seen .sort() and sorted() before. The real power comes from the key= parameter — a function that extracts a comparison value from each element:

Sorting tuples by a specific element:

Sorting by multiple criteria — return a tuple from key=. Python compares tuples element by element:

This pattern — negating the primary key to simulate descending order — is extremely common in CP.

lambda

Lambda functions were introduced in Module 9. As a reminder, a lambda is an anonymous one-line function most commonly used as a key= argument:

For anything more complex than a single expression, define a regular function with def instead.

any and all

any returns True if at least one element is truthy. all returns True if every element is truthy. Both short-circuit — they stop as soon as the result is determined:

Common CP patterns: