C++ STL Algorithm Library (original) (raw)

Last Updated : 27 May, 2026

Algorithms are ready-made functions that help you perform common operations like searching, sorting, counting, and comparing on containers (like arrays, vectors, sets, etc.). These algorithms are defined in the and header files.

Type of Algorithms

We can divide the algorithms into following parts:

1. Searching Algorithms

Searching algorithms are methods used to find a specific element or information in a data structure like an array, vector, or list.
They help locate an element efficiently without manually checking each item.

Some of the most frequently used searching algorithms are:

• find() : Searches for an element and returns its iterator if present.
• find_if() : Finds the first element satisfying a given condition.
• binary_search() : Checks if an element exists in a sorted container.
• lower_bound() : Returns an iterator to the first element not less than a given value.
• upper_bound() : Returns an iterator to the first element greater than a given value.
• find_end() : Finds the last occurrences of a subsequence in a range.

2. Sorting and Rearranging Algorithms

These change the order or position of elements.

• sort() : Arranges elements in a given range into ascending order by default.
• stable_sort() : Sorts elements while preserving the relative order of equal elements.
• partial_sort() : Sorts only the first N elements in order, leaving the rest unsorted.
• nth_element() : Rearranges elements so the nth position has the correct element as in sorted order, with smaller ones before it and larger ones after it (but unordered).
• is_sorted() : Checks if the given range is already sorted in ascending order (or by custom comparator).
• is_sorted_until() : Returns an iterator to the first element where the sorted order breaks.

3. Manipulation Algorithms

These change or update values in the container without necessarily sorting or searching them.

• copy() : Copies element from one range into another range, preserving their order.
• copy_if() : Copies only the elements that satisfy a given condition to another range.
• copy_n() : Copies exactly n elements from a source range to a destination range.
• copy_backward() : Copies elements from a source range to a destination range in reverse order, starting from the end.
• move() : Transfers elements from a source range to a destination range, leaving the source elements in a valid but unspecified state.
• swap() : Exchanges the values of two elements.
• replace() : Replaces all occurrences of a specific value in a range with a new value.
• replace_if() : Replaces all elements that satisfy a given condition with a new value.
• remove() : moves all elements not equal to a given value to the front of the range and returns an iterator to the new logical end (does not actually reduce container size).
• fill() : Assigns a specified value to every element in a given range.
• transform() : Applies a function to each element in a range and stores the result in another range.
• generate() : Assigns values to a range by repeatedly calling a given function or generator.
• shuffle() : Randomly rearranges the elements in a given range.

4. Counting & Comparing Algorithms

These check or count elements based on conditions.

• count() : Returns the number of elements in a range that are equal to a specified value.
• count_if() : Returns the number of elements in a range that satisfy a given condition.
• equal() : Checks if two ranges have the same elements in the same order.
• mismatch() : Finds the first position where two ranges differ and returns iterators to the mismatched elements.
• lexicographical_compare() : Checks if one range is lexicographically less than another (like dictionary order).
• is_permutation() : Checks if one range is a permutation of another, i.e., contains the same elements in any order.

5. Functions

Include common functions like:

• accumulate() → sum of elements
• iota() → fills range with sequential values
• partial_sum() → cumulative sum
• inner_product() → dot product