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;
  }
  size_t len = values.size();
  for (size_t i = 0; i < len; i++) {
    cout << values[i] << endl;
  }
}

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

#include <iostream> #include <vector> void print_vec(std::vector<int> nums); int main() { std::vector<int> values; int i = 0; while (i <= 10) { values.push_back(i); i += 2; } print_vec(values); } ==== void print_vec(std::vector<int> nums) { std::cout << '['; for (const auto& n: vec) { cout << n << ','; } cout << "]\n"; }

Q-2: 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()?

• 5
• Incorrect! This is the size of the vector before we ran the command.
• 6
• Correct!
• 7
• Incorrect!
• 8
• Incorrect! We are adding the element 3 to the end of the vector, not 3 elements!

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

        vector<int> make_even(vector<int> vec) {
---
void make_even(vector<int> vec) {                         #paired
---
   for (size_t i = 0; i < vec.size(); i++) {
---
   for (int i = 0; i < vec.size(); i++) {                         #paired
---
      if (vec[i] % 2 == 1) {
---
      if (i % 2 == 1) {                         #paired
---
         vec[i] += 1;
      }
---
         i += 1;                         #paired
      }
---
      else {                         #distractor
         vec[i] -= 1;
      }
---
   return vec;
---
   }
}
        

Q-4: What does the following code print?

vector<int> numbers(5);
int size = 5;
for (int i = 0; i < size; ++i){
   numbers[i] = i;
}

int end = 4;

for (int i = 0; i < size; ++i){
   numbers[i] = numbers[end];
   end--;
}

for (int i = 0; i < size; ++i){
   cout << numbers[i] << "  ";
}

cout << endl;

• 4 3 2 1 0
• we change the numbers in the first half of the vector before we copy them to the second half
• 4 3 2 3 4
• when i is 3 we copy from end = 1 copying the values we already changed.
• 0 1 2 3 4
• we change values in the second loop.

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