Why do you need vector when you have an array？

　　In C++ programming, array and vector are both common data structures, and they can both be used to store a group of data of the same type. However, there are great differences between array and vector in implementation and usage scenarios. In this paper, we will discuss the advantages and disadvantages of array and vector, and compare their usage scenarios to help readers better understand their differences and connections.

　　First, the advantages and disadvantages of arrays

　　Array is the most basic data structure, which is a group of continuous memory cells used to store the same type of data. In C++, arrays are defined as follows:

　　But there are some limitations in the use of arrays.

　　1.1 The length of the array is unchangeable.

　　In C++, the length of an array needs to be determined when it is defined, and once it is defined, it cannot be changed.

　　This will limit the flexibility of the array. If you need a larger array, you must define a new array and copy the contents.

　　1.2 Array boundary checking is cumbersome

　　When reading and writing an array, it is necessary to check whether the index is out of bounds, otherwise it will have serious consequences.

　　This requires developers to check every array access, which increases the complexity.

　　1.3 The transfer of arrays consumes resources.

　　Arrays usually need to be passed as parameters to functions, but arrays in C++ can't be passed directly, but as pointers. This will bring additional performance overhead.

　　1.4 Array memory allocation is not flexible

　　The memory allocation of the array is static and cannot be dynamically adjusted according to the demand. If the array space is insufficient, it cannot be automatically expanded.

　　Second, the advantages and disadvantages of vector

　　Vector is one of the containers in the C++ standard library, which can be used to store any type of data, including basic data types and user-defined data types. Vector is defined as follows:

　　Vector solves these limitations of arrays to a great extent.

　　2.1 the length of the vector is variable.

　　The size of the vector can be changed at will, and the capacity can be expanded and contracted through the member function.

　　Manage memory flexibly, regardless of array boundaries.

　　2.2 vector boundary check automatically

　　At function access will check the index to avoid cross-border access.

　　This can prevent many program errors.

　　2.3 vector can be passed directly.

　　Vector can be passed directly as a parameter to a function without being converted into a pointer.

　　This simplifies the code and improves efficiency.

　　2.4 vector memory dynamic allocation

　　Vector uses dynamic memory allocation internally, and automatically expands capacity as needed, without manual memory management.

　　This adapts to the needs of flexible changes.

　　Third, the use of arrays and vector

　　Because there are great differences between array and vector in implementation and usage scenarios, it is necessary to decide which data structure to use according to specific needs.

　　Arrays are the best choice when you need to store fixed-size data. For example, when you need to store a fixed-size matrix or array, array is the best choice. The access speed of array is very fast, so array is the best choice when you need to access data efficiently.

　　Vector is the best choice when you need to store dynamically sized data. For example, when you need to store an array with a dynamic size, vector is the best choice. Vector can increase or decrease its size dynamically, so it is the best choice in the scene where the data size needs to be changed dynamically.

　　IV. Conclusion

　　In this paper, we discuss the advantages and disadvantages of array and vector, and compare their usage scenarios. The main advantage of array is that it is fast, but it can't change its size dynamically, while the main advantage of vector is that it can change its size dynamically, but its access speed is relatively slow. Therefore, when choosing which data structure to use, it needs to be decided according to specific needs.

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