Resize() usage row pit in vector

resize() usage in vector

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Reference:
std::vector::resize

Requirements: The debugger has a problem that the vector is set to 0. Finally, it is located that the resize is used improperly, which is recorded.

Simplified description of the situation

1. Initialize the vector, and then want to use resize() to assign values ​​to all elements

vector test = {0,1,2,3,4};test.resize(10,0);// print the resultfor(const auto &value: test) std::cout << value << ",";std::cout << std::endl;

It is usually thought that the print result is 10 0s, but it is actually 0,1,2,3,4,0,0,0,0,0,

2. resize() function description

2.1 Function Prototype (C++11)

void resize (size_type n);void resize (size_type n, const value_type& val);

2.2 official explanation

Resizes the container so that it contains n elements.If n is smaller than the current container size, the content is reduced to its first n elements, removing those beyond (and destroying them).If n is greater than the current container size, the content is expanded by inserting at the end as many elements as needed to reach a size of n. If val is specified, the new elements are initialized as copies of val, otherwise, they arevalue-initialized.If n is also greater than the current container capacity, an automatic reallocation of the allocated storage space takes place.Notice that this function changes the actual content of the container by inserting or erasing elements from it.

2.3 Instructions

• Resize the container to contain n elements
• If n is less than the current container size, the content will be reduced to its first n elements, removing those beyond (and freeing them).
• If n is greater than the current container size, expand the content by inserting the desired number of elements at the end to reach the size of n.If val is specified, initialize new elements to copies of val, otherwise, initialize them to 0.
• If n is also greater than the current container capacity, the allocated storage is automatically reallocated.

Note that this function changes the actual content of the container by inserting or removing elements.

2.4 Practical Examples

Borrowing the official example.

// resizing vector#include #include int main(){std::vector myvector;// set some initial content:for (int i=1;i<10;i++) myvector.push_back(i);myvector.resize(5);myvector.resize(8,100);myvector.resize(12);std::cout << "myvector contains:";for (int i=0;i

Results:
myvector contains: 1 2 3 4 5 100 100 100 0 0 0 0

Explanation:

• Initialize 9 elements, from 1 to 9.
• The first operation, resize(5), deletes the last 4 elements, leaving 1 2 3 4 5.
• The second operation, resize(8,100), will insert 3 elements at the end, and the value of 3 elements is 100.
• The third operation, resize(12), will insert 12-8=4 elements at the end again. Since the element does not provide an initial value, the default value is 0.

2.5 Extensions

The real requirement in the project is to set all elements in the member variable vector to 0 at the beginning of the program, and finally use the std::fill() fill function, clear() will clear the value, but not free the memory.

std::fill(input_data_.begin(), input_data_.end(), 0); // fillinput_data_.clear(); // After clearing, traversing and printing will not see the result