TCPP2 Vector functions

Section 10.6 Vector functions

The best feature of a vector is its resizeability. 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 he inputs -1, and then display them. In such a case, we do not know the size of the vector beforehand. So we need wish 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.

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Listing 10.6.1. A program that reads in numbers, adding each to a vector, until the user inputs -1.

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

int main() {
  vector<int> values;
  int c, i;
  size_t len;
  cin >> c;

  while (c != -1) {
    values.push_back(c);
    cin >> c;
  }
  len = values.size();
  for (i = 0; i < len; i++) {
    cout << values[i] << endl;
  }
}

Note 10.6.1.

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

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Listing 10.6.2. This active code uses the push_back function to add even numbers less than or equal to 10 to the vector values.

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Checkpoint 10.6.1.

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()?

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

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Checkpoint 10.6.2.

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

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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 {                         
         vec[i] -= 1;
      } #distractor
---
   }
   return vec;
}

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Checkpoint 10.6.3.

What does the following code print?

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

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You have attempted of activities on this page.

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