.assign()

In C++, the .assign() method is used with vectors to replace existing elements or add new values. This method can either fill the vector with a specified number of copies of a value or replace the vector’s contents with elements from a range, modifying its size if necessary.

Note: To use vectors, including the vector library is necessary.

Syntax

vector.assign(size, value);

• vector: The instance of the std::vector class where the elements will be assigned.
• count: The number of elements to assign to the vector.
• value: The value to be assigned to each of the count elements.

Alternatively, the other syntax is:

vector.assign(first, last);

• first: The iterator pointing to the beginning of the range to be copied.
• last: The iterator pointing just past the end of the range to be copied. This element is excluded from the range.

Example

In the following example, the .assign() method is demonstrated in two distinct ways. First, it fills the vector with a specified number of copies of a given value. Second, it replaces the vector’s elements with values from a specified range:

#include <iostream>
#include <vector>
using namespace std;

int main() {
  vector<int> numbers{1, 2, 3};
  vector<int> newNumbers{4, 5, 6, 7};

  // Replacing current elements with 4 copies of the value 8
  numbers.assign(4, 8);
  cout << "After assigning 4 copies of 8: ";
  for (int num : numbers) {
      cout << num << " ";
  }

  cout << endl;

  // Replacing current elements with elements from newNumbers
  numbers.assign(newNumbers.begin(), newNumbers.end());
  cout << "After assigning elements from newNumbers: ";
  for (int num : numbers) {
      cout << num << " ";
  }

  cout << endl;

  return 0;
}

The output for the above code is:

After assigning 4 copies of 8: 8 8 8 8
After assigning elements from newNumbers: 4 5 6 7

Note: The .assign() method completely replaces the existing contents of the vector, resizing it if necessary. This method is particularly useful for resetting a vector or filling it with specific values or elements from another collection.