C++ .size()

In C++, the .size() method returns the number of elements in the vector. This method is also available for other STL containers like std::string, std::map, and std::list. Knowing how to use .size() effectively helps in writing safe and efficient loops and conditionals.

Syntax

vector.size();

The vector must be defined using std::vector before the .size() method can be used.

Parameters:

The .size() method does not accept any parameters.

Return value:

Returns the number of elements in a vector.

Example 1: Basic Usage

In this example, .size() is called on the numbers vector:

#include <iostream>
#include <vector>

int main() {
  // Declaring a vector with 4 integers
  std::vector<int> numbers = {1, 2, 3, 4};

  // Print out vector size
  std::cout << numbers.size();
}

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Here is the output:

4

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Example 2: Using .size() in a Loop

This example uses the .size() method in a for loop:

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

int main() {
  vector<string> fruits = {"Apple", "Banana", "Cherry"};

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

  return 0;
}

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Here is the output:

Apple
Banana
Cherry

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Codebyte Example: Checking if a Vector is Empty

In this codebyte example, .size() is used to determine if a vector is empty:

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

int main() {
  vector<int> data;

  if (data.size() == 0) {
    cout << "The vector is empty." << endl;
  }

  return 0;
}

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Frequently Asked Questions

1. Can .size() be used with any STL container?

Yes, .size() is a member function of most STL containers like vector, string, list, map, etc. It behaves similarly across them.

2. What is the difference between .size() and .capacity()?

.size() returns the actual number of items in a vector, whereas .capacity() returns the number of elements a vector can hold before needing to allocate more memory.

3. Is .size() a costly operation?

No, .size() runs in constant time, O(1), because vectors store their size internally and update it automatically during insertions or deletions.