String methods

Python strings come with a rich set of built-in methods. Since strings are immutable, every method returns a new string — the original is never modified.

Case Methods

Useful in CP when problems say "case-insensitive comparison" — just lowercase both strings before comparing:

Checking String Content

These methods return True or False:

Checking character type inside a loop:

Searching

find(sub) and rfind(sub)

Returns the index of the first occurrence of sub, or -1 if not found:

You can also specify a search range find(sub, start, end):

count(sub)

Counts non-overlapping occurrences:

startswith(prefix) and endswith(suffix)

Frequently used in CP to check format or conditions on strings.

Modifying (Returns New String)

replace(old, new)

Replaces all occurrences of old with new:

Replace only the first N occurrences with the third argument:

strip(), lstrip(), rstrip()

Removes whitespace (or specified characters) from the edges:

You can also pass specific characters to strip:

Very useful when reading input that might have trailing spaces or newlines.

Splitting and Joining

These two methods are the most important ones for competitive programming.

split(sep)

Splits a string into a list of substrings:

With no argument, split() splits on any whitespace and ignores leading/trailing spaces. With an argument, it splits on that specific separator:

You have been using split() since Module 3 for reading input — now you know exactly what it does.

join(iterable)

The opposite of split() — joins a list of strings into one, placing the calling string between each element:

This is the efficient way to build a string from many pieces. Instead of using += in a loop (which creates a new string each time), collect into a list and join once at the end:

A very common CP pattern — filter or transform characters and join:

Note: A more compact way to write the above using generators will be covered in a later module.

Formatting

The cleanest way to embed variables inside strings is using f-strings — prefix the string with and place expressions inside :