10.5. Vector functions

The best feature of a vector is its ability to resize. A vector, once declared, can be resized from anywhere within the program. Suppose we have a situation where we input numbers from the user and store them in a vector till the input is -1, and then display them. In such a case, we do not know the size of the vector beforehand. So we need a way add new values to the end of a vector as the user inputs them. We can use then vector function push_back for that purpose.

Note

push_back adds a specified element to the end of the vector, pop_back removes element from the end of a vector.

#include <iostream>
#include <vector>

int main() {
  std::vector<int> values;
  int c;
  cin >> c;

  while (c != -1) {
    values.push_back(c);
    cin >> c;
  }
  std::size_t len = values.size();
  for (std::size_t i = 0; i < len; i++) {
    cout << values[i] << '\n';
  }
}

The active code below uses the push_back function to add even numbers less than or equal to 10 to the vector values.

Example vector_functions_AC_1
 1#include <iostream>
 2#include <vector>
 3
 4void print_vec(std::vector<int> nums);
 5
 6int main() {
 7    std::vector<int> values;
 8    int i = 0;
 9
10    while (i <= 10) {
11        values.push_back(i);
12        i += 2;
13    }
14    print_vec(values);
15}

Q1

Let nums be the vector { 0, 1, 2, 3, 4 }. If we run the command nums.push_back(3), what will be returned by nums.size()?

Q2

Construct the <code>make_even</code> function that loops through vec, adds 1 to any elements that are odd, and returns the new vector.

  1.    i += 1;
}
  2.    vec[i] += 1;
}
  3.    }
}
  4. else {                         #distractor
   vec[i] -= 1;
}
  5. for (std::size_t i = 0; i &#60; vec.size(); i++) {
  6. for (std::size_t i = 0; i &#60; vec.size(); i++) {
  7. if (i % 2 == 1) {
  8. if (vec[i] % 2 == 1) {
  9. return vec;
  10. vector&#60;int&#62; make_even(vector&#60;int&#62; vec) {
  11. void make_even(vector&#60;int&#62; vec) {

Q3

What does the following code print?

 1vector<int> numbers(5);
 2int size = 5;
 3for (int i = 0; i < size; i++){
 4   numbers[i] = i;
 5}
 6
 7int end = 4;
 8
 9for (int i = 0; i < size; i++){
10   numbers[i] = numbers[end];
11   end--;
12}
13
14for (int i = 0; i < size; i++){
15   cout << numbers[i] << "  ";
16}
17
18cout << '\n';