Block 4 – Algorithms: Modifying Sequence Operations

Unlike non-modifying algorithms, these algorithms change the container’s contents or order.

Categories in this block:

  1. Generating and copying data
  2. Swapping and transforming elements
  3. Replacement and removal operations
  4. Sequence reordering algorithms

4.1 Generating and Copying Algorithms

These algorithms copy or generate values in containers.

std::copy

Purpose

Copies elements from one range to another.

Syntax

std::copy(source_begin, source_end, destination_begin);

Example

#include <vector>
#include <algorithm>
#include <iostream>int main()
{
    std::vector<int> v1 = {1,2,3};
    std::vector<int> v2(3);    std::copy(v1.begin(), v1.end(), v2.begin());    for(int x : v2)
        std::cout << x << " ";
}

Output

1 2 3

std::copy_backward

Purpose

Copies elements starting from the end of the range.

Useful when ranges overlap.

Example

std::vector<int> v = {1,2,3,4,5};std::copy_backward(v.begin(), v.begin()+3, v.end());

std::fill

Purpose

Assigns the same value to all elements in a range.

Example

std::vector<int> v(5);std::fill(v.begin(), v.end(), 10);

Result

10 10 10 10 10

std::fill_n

Purpose

Assigns a value to n elements.

Syntax

std::fill_n(iterator, count, value);

Example

std::vector<int> v(5);std::fill_n(v.begin(), 3, 7);

Result

7 7 7 0 0

std::generate

Purpose

Generates values using a function.

Example

int generateValue()
{
    static int x = 1;
    return x++;
}std::vector<int> v(5);std::generate(v.begin(), v.end(), generateValue);

Result

1 2 3 4 5

std::generate_n

Purpose

Generates n elements using a function.

Example

std::generate_n(v.begin(), 3, generateValue);

4.2 Swapping and Transformation Algorithms

These algorithms exchange values or transform data.

std::swap

Purpose

Swaps two variables.

Example

int a = 10;
int b = 20;std::swap(a, b);

Result

a = 20
b = 10

std::swap_ranges

Purpose

Swaps elements between two ranges.

Example

std::vector<int> v1 = {1,2,3};
std::vector<int> v2 = {4,5,6};std::swap_ranges(v1.begin(), v1.end(), v2.begin());

Result

v1 → 4 5 6
v2 → 1 2 3

std::iter_swap

Purpose

Swaps elements pointed to by iterators.

Example

std::vector<int> v = {1,2,3};std::iter_swap(v.begin(), v.begin()+2);

Result

3 2 1

std::transform

Purpose

Applies a function to elements and stores the result.

Syntax

std::transform(begin, end, destination, function);

Example

std::vector<int> v = {1,2,3};
std::vector<int> result(3);std::transform(v.begin(), v.end(), result.begin(),
               [](int x){ return x * 2; });

Result

2 4 6

4.3 Replacement and Removal Algorithms

These algorithms replace values or remove elements.

std::replace

Purpose

Replaces all occurrences of a value.

Example

std::vector<int> v = {1,2,3,2};std::replace(v.begin(), v.end(), 2, 9);

Result

1 9 3 9

std::remove

Purpose

Moves elements to remove a value.

Important:
It does not actually erase elements from the container.

Example

std::vector<int> v = {1,2,3,2};auto it = std::remove(v.begin(), v.end(), 2);

To fully remove:

v.erase(it, v.end());

This is called the Erase-Remove Idiom.

std::remove_if

Purpose

Removes elements based on a condition.

Example

auto it = std::remove_if(v.begin(), v.end(),
                         [](int x){ return x > 3; });v.erase(it, v.end());

std::unique

Purpose

Removes consecutive duplicate elements.

Example

std::vector<int> v = {1,1,2,2,3};auto it = std::unique(v.begin(), v.end());

Result

1 2 3

std::unique_copy

Purpose

Copies unique elements to another container.

Example

std::vector<int> src = {1,1,2,2,3};
std::vector<int> dest(5);std::unique_copy(src.begin(), src.end(), dest.begin());

4.4 Sequence Reordering Algorithms

These algorithms change the order of elements.

std::reverse

Purpose

Reverses elements in a container.

Example

std::vector<int> v = {1,2,3,4};std::reverse(v.begin(), v.end());

Result

4 3 2 1

std::reverse_copy

Purpose

Copies reversed sequence to another container.

Example

std::vector<int> v = {1,2,3};
std::vector<int> r(3);std::reverse_copy(v.begin(), v.end(), r.begin());

Result

3 2 1

std::rotate

Purpose

Rotates elements around a pivot.

Example

std::vector<int> v = {1,2,3,4,5};std::rotate(v.begin(), v.begin()+2, v.end());

Result

3 4 5 1 2

std::partition

Purpose

Reorders elements based on condition.

Example

std::vector<int> v = {1,2,3,4,5};std::partition(v.begin(), v.end(),
               [](int x){ return x % 2 == 0; });

Result

2 4 1 3 5

(Evens first)

std::stable_partition

Purpose

Same as partition but preserves order.

Example

std::stable_partition(v.begin(), v.end(),
                      [](int x){ return x % 2 == 0; });

Result

2 4 1 3 5

Order preserved within groups.

Summary Table

AlgorithmPurpose
copycopy elements
fillassign value
generategenerate values
swapexchange values
transformmodify elements
replacereplace values
removeremove elements
uniqueremove duplicates
reversereverse order
rotaterotate elements
partitionseparate elements by condition

Key Exam Points

  1. Modifying algorithms change container content or order.
  2. remove() does not erase elements, must use erase.
  3. unique() removes consecutive duplicates only.
  4. transform() applies a function to elements.
  5. partition() reorders elements based on a predicate.

✅ Block 4 Summary

Modifying algorithms allow you to:

  • Generate data
  • Copy sequences
  • Transform values
  • Remove duplicates
  • Rearrange elements

They are powerful tools for efficient container manipulation.