Dynamic Arrays using std::vector

Removing Objects from std::vector

How can I remove objects from a std::vector?

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Removing objects from a std::vector is a common operation, and C++ provides several ways to do this depending on your specific needs. Let's explore the main approaches.

Using erase()

The erase() function is the most straightforward way to remove elements from a std::vector. It can remove a single element or a range of elements:

#include <iostream>
#include <vector>

int main(){
  std::vector<int> numbers{1, 2, 3, 4, 5};

  // Remove the third element (index 2)
  numbers.erase(numbers.begin() + 2); 

  // Print the vector
  for (int num : numbers) {
    std::cout << num << ' ';
  }
  std::cout << '\n';

  // Remove a range (first two elements)
  numbers.erase( 
    numbers.begin(), 
    numbers.begin() + 2); 

  // Print the vector again
  for (int num : numbers) {
    std::cout << num << ' ';
  }
}
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4 5

Using remove() and erase()

For more complex removal conditions, you can use the remove() algorithm along with erase(). This is known as the erase-remove idiom:

#include <iostream>
#include <vector>
#include <algorithm>

int main(){
  std::vector<int> numbers{
    1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

  // Remove all even numbers
  numbers.erase(
    std::remove_if(
      numbers.begin(), numbers.end(),
      [](int n){ return n % 2 == 0; }), 
    numbers.end());

  // Print the vector
  for (int num : numbers) {
    std::cout << num << ' ';
  }
}
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Using pop_back()

If you only need to remove the last element, use pop_back():

#include <iostream>
#include <vector>

int main(){
  std::vector<int> numbers{1, 2, 3, 4, 5};

  numbers.pop_back(); 

  // Print the vector
  for (int num : numbers) {
    std::cout << num << ' ';
  }
}
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Remember that removing elements from the middle of a std::vector can be expensive, as it requires shifting all subsequent elements. If you frequently need to remove elements from arbitrary positions, consider using std::list instead.

Also, be cautious when removing elements while iterating over a vector. Removing an element invalidates iterators at or after the point of removal. Here's a safe way to remove elements while iterating:

#include <iostream>
#include <vector>

int main(){
  std::vector<int> numbers{
    1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

  for (auto it = numbers.begin();
    it != numbers.end();) {
    if (*it % 2 == 0) {
      it = numbers.erase(it); 
    } else { ++it; }
  }

  // Print the vector
  for (int num : numbers) {
    std::cout << num << ' ';
  }
}
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By using these techniques, you can effectively remove objects from a std::vector in various scenarios, whether you're removing single elements, ranges, or elements that meet specific criteria.

Answers to questions are automatically generated and may not have been reviewed.

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