C++ <vector> resize function

C++ container class <vector>

resizeis used in vector toResizefunction that can change the number of elements in the container.

resizeis a member function of container classes, used toResize the container to the specified number of elements.If the new size is greater than the current size, new elements are added; if it is less than the current size, excess elements are removed.

resizeProvides the ability to flexibly resize the container, one of vector's powerful dynamic features.

Word Definitions： resizeMeans "resize", i.e., change the number of elements in the container.

Basic syntax and parameters

resizeIs a member function of the container class that can specify a new number of elements.

Syntax format

void resize(size_type count);
void resize(size_type count, const T& value);

Parameter description

Parameter： count

  • Type:size_type(an unsigned integer type, usuallysize_t）
  • Description: the new number of elements in the container.

Parameter(Optional):value

  • Type: same as the container's element type.
  • Description: The default value used to fill new elements when elements need to be added. If not specified, value initialization is used by default (0 for built-in types).

Function description

  • Return Value： void(no return value).
  • Effect: the container'ssize()Resize to count. If elements need to be added, fill them with value; if elements need to be removed, discard the excess elements.
  • Note： resizewill change the container'ssize(), and may also changecapacity()。

Example

Let us thoroughly master through a series of examples.resizeUsage.

Example 1: Basic usage - expanding the container

Example

#include <iostream>
#include <vector>

int main() {
    std::vector<int> numbers = {1, 2, 3};

    std::cout << "Initial state - size: " << numbers.size()
              << ", capacity: " << numbers.capacity() << std::endl;
    std::cout << "Elements: ";
    for(int n : numbers) std::cout << n << " ";
    std::cout << std::endl;

    // Enlarge container to 10 elements
    numbers.resize(10);

    std::cout << "\nAfter resize(10) - size: " << numbers.size()
              << ", capacity: " << numbers.capacity() << std::endl;
    std::cout << "Elements: ";
    for(int n : numbers) std::cout << n << " ";
    std::cout << std::endl;

    return 0;
}

Expected output:

初始状态 - size: 3, capacity: 3
元素: 1 2 3

resize(10) 后 - size: 10, capacity: 10
元素: 1 2 3 0 0 0 0 0 0 0

Code analysis:

  1. Initially there are 3 elements: 1, 2, 3.
  2. resize(10)Resize to 10.
  3. The 7 newly added elements are value-initialized to 0.

Example 2: Filling new elements with a specified value

You canresizespecify a fill value when.

Example

#include <iostream>
#include <vector>
#include <string>

int main() {
    std::vector<std::string> names = {"Alice", "Bob"};

    std::cout << "Initial state - size: " << names.size() << std::endl;
    std::cout << "Elements: ";
    for(const auto& n : names) std::cout << n << " ";
    std::cout << std::endl;

    // Fill new elements with "Unknown"
    names.resize(5, "Unknown");

    std::cout << "\nresize(5, \"Unknown\"After ) - size: " << names.size() << std::endl;
    std::cout << "Elements: ";
    for(const auto& n : names) std::cout << n << " ";
    std::cout << std::endl;

    return 0;
}

Expected output:

初始状态 - size: 2
元素: Alice Bob

resize(5, "Unknown") 后 - size: 5
元素: Alice Bob Unknown Unknown Unknown

Code analysis:

  • Usageresize(5, "Unknown")Resize to 5.
  • The 3 newly added elements are all filled with "Unknown".

Example 3: Using resize to shrink the container

resizeIt can also be used to shrink the container.

Example

#include <iostream>
#include <vector>

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

    std::cout << "Initial state - size: " << numbers.size() << std::endl;
    std::cout << "Elements: ";
    for(int n : numbers) std::cout << n << " ";
    std::cout << std::endl;

    // Shrink to 5 elements
    numbers.resize(5);

    std::cout << "\nAfter resize(5) - size: " << numbers.size() << std::endl;
    std::cout << "Elements: ";
    for(int n : numbers) std::cout << n << " ";
    std::cout << std::endl;

    return 0;
}

Expected output:

初始状态 - size: 10
元素: 1 2 3 4 5 6 7 8 9 10

resize(5) 后 - size: 5
元素: 1 2 3 4 5

Code analysis:

  • resize(5)Resize to 5.
  • The last 5 elements are removed.

Example 4: resize and capacity

resizewill change simultaneouslysizeandcapacity。

Example

#include <iostream>
#include <vector>

int main() {
    std::vector<int> v;

    // pre-allocate space
    v.reserve(100);
    v.push_back(1);
    v.push_back(2);

    std::cout << "initial - size: " << v.size() << ", capacity: " << v.capacity() << std::endl;

    // resize to a larger value
    v.resize(50);
    std::cout << "after resize(50) - size: " << v.size() << ", capacity: " << v.capacity() << std::endl;

    // resize to a smaller value
    v.resize(10);
    std::cout << "after resize(10) - size: " << v.size() << ", capacity: " << v.capacity() << std::endl;

    return 0;
}

Expected output:

初始 - size: 2, capacity: 100
resize(50) 后 - size: 50, capacity: 100
resize(10) 后 - size: 10, capacity: 100

Code analysis:

  • Whenresizewhen the value is less than the current capacity,capacityremains unchanged.
  • Whenresizewhen the value is greater than the current capacity,capacitywill increase accordingly.

Example 5: Using resize to create a fixed-size container

resizecan be used to create a fixed-size container.

Example

#include <iostream>
#include <vector>

int main() {
    // create a vector containing 100 copies of -1
    std::vector<int> scores(100, -1);

    std::cout << "size: " << scores.size() << std::endl;
    std::cout << "first 10 elements: ";
    for(int i = 0; i < 10; ++i) {
        std::cout << scores[i] << " ";
    }
    std::cout << std::endl;

    // set the value at a position
    scores[0] = 100;
    scores[1] = 95;

    std::cout << "first 10 elements after modification: ";
    for(int i = 0; i < 10; ++i) {
        std::cout << scores[i] << " ";
    }
    std::cout << std::endl;

    return 0;
}

Code analysis:

  • UsageresizeCan create a container containing a fixed number of elements.
  • This is useful when you need to pre-allocate a fixed-size container.

C++ container class <vector>

other extensions