How to Create Vector in C++: Syntax and Examples

In C++, you create a vector by including the <vector> header and declaring a vector with std::vector<Type> name;. This creates a dynamic array that can grow or shrink as needed.

Syntax

To create a vector in C++, you first include the <vector> header. Then declare a vector with the syntax:

• std::vector<Type> name; - creates an empty vector of the specified type.
• std::vector<Type> name(size); - creates a vector with a fixed size, initialized with default values.
• std::vector<Type> name(size, value); - creates a vector with a fixed size, initialized with the given value.

cpp

#include <vector>

// Create an empty vector of integers
std::vector<int> numbers;

// Create a vector of 5 integers, default initialized to 0
std::vector<int> zeros(5);

// Create a vector of 3 integers, each initialized to 7
std::vector<int> sevens(3, 7);

Example

This example shows how to create a vector, add elements, and print them.

cpp

#include <iostream>
#include <vector>

int main() {
    std::vector<int> numbers; // empty vector
    numbers.push_back(10);    // add 10
    numbers.push_back(20);    // add 20
    numbers.push_back(30);    // add 30

    std::cout << "Vector elements:" << std::endl;
    for (int num : numbers) {
        std::cout << num << std::endl;
    }

    return 0;
}

Output

Vector elements:
10
20
30

Common Pitfalls

Common mistakes when creating vectors include:

• Forgetting to include <vector> header.
• Not using std:: prefix or missing using namespace std;.
• Trying to access elements without adding any, causing out-of-range errors.
• Confusing vector size with capacity.

cpp

#include <vector>
#include <iostream>

int main() {
    // Wrong: no elements added, accessing index 0 causes error
    std::vector<int> v;
    // std::cout << v[0]; // This is unsafe and may crash

    // Right: add elements before accessing
    v.push_back(5);
    std::cout << v[0] << std::endl; // prints 5

    return 0;
}

Output

5

Quick Reference

Syntax	Description
std::vector name;	Create empty vector of Type
std::vector name(size);	Create vector with size, default values
std::vector name(size, value);	Create vector with size, initialized to value
name.push_back(value);	Add value to end of vector
name.size();	Get number of elements in vector
name[index];	Access element at index (0-based)

Key Takeaways

Include and use std::vector to create vectors in C++.

Vectors are dynamic arrays that can grow with push_back().

Always add elements before accessing to avoid errors.

Use vector.size() to get the current number of elements.

Remember to use the std:: prefix or a using directive.