C++ Developer - Lesson 1

The vector is a dynamic array from the C++ Standard Library. It can grow and shrink at runtime while providing contiguous storage like a regular array. Vectors are declared with #include <vector> and are one of the most commonly used containers. This lesson covers creating vectors, adding elements with push_back, accessing items with brackets and at(), iterating with loops, modifying elements, and removing the last element. Understanding vectors is essential for writing flexible and efficient C++ programs.

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

int main() {
    vector<int> nums;
    nums.push_back(10);
    nums.push_back(20);
    nums.push_back(30);
    cout << nums.size();
    return 0;
}

• vector<int> declares a vector that holds integers.
• push_back() appends a new element to the end of the vector.
• size() returns how many elements are currently stored.
• Vectors grow automatically as elements are added.

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

int main() {
    vector<int> nums = {10, 20, 30};
    cout << nums[0] << " ";
    cout << nums.at(2);
    return 0;
}

• Initializer list syntax with curly braces creates a vector with the given elements.
• Access elements using vector[index] or the safer vector.at(index).
• The at() method performs bounds checking and throws an exception if the index is invalid.
• Indices start at 0, just like arrays.

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

int main() {
    vector<int> nums = {5, 10, 15};
    for (int i = 0; i < nums.size(); i++) {
        if (i > 0) cout << " ";
        cout << nums[i];
    }
    return 0;
}

• A for loop with size() iterates over all elements by index.
• The condition i < nums.size() ensures you stay within bounds.
• This pattern works for any sequential container that supports indexing.
• Output is formatted with spaces between elements using the if (i > 0) check.

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

int main() {
    vector<int> nums = {1, 2, 3};
    nums[1] = 99;
    for (int i = 0; i < nums.size(); i++) {
        if (i > 0) cout << " ";
        cout << nums[i];
    }
    return 0;
}

• You can modify an existing element using assignment with bracket notation.
• nums[1] = 99 changes the second element without changing the size.
• Modifying an element is O(1) because vectors provide direct access.
• After modification, iterating shows the updated values.

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

int main() {
    vector<int> nums = {1, 2, 3};
    cout << nums.size() << " " << nums[1];
    return 0;
}

• size() reflects the current number of elements.
• Accessing a valid index with operator[] retrieves the stored value.
• You can combine multiple vector operations in a single output statement.
• Always ensure the index is valid before using operator[].

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

int main() {
    vector<int> nums = {10, 20, 30};
    nums.pop_back();
    cout << nums.size() << " " << nums[1];
    return 0;
}

• pop_back() removes the last element and reduces size by one.
• Calling pop_back() on an empty vector causes undefined behaviour.
• After removal, the remaining elements are still accessible by index.
• Use empty() to check before calling pop_back() safely.