我輩豈是蓬蒿人！

(一)

One of the basic classes implemented by the Standard Template Library is the vector class. A vector is, essentially, a resizeable array; the vector class allows random access via the [] operator, but adding an element anywhere but to the end of a vector causes some overhead as all of the elements are shuffled around to fit them correctly into memory. Fortunately, the memory requirements are equivalent to those of a normal array. The header file for the STL vector library is vector. (Note that when using C++, header files drop the .h; for C header files - e.g. stdlib.h - you should still include the .h.) Moreover, the vector class is part of the std namespace, so you must either prefix all references to the vector template with std:: or include "using namespace std;" at the top of your program.

Vectors are more powerful than arrays because the number of functions that are available for accessing and modifying vectors. Unfortunately, the [] operator still does not provide bounds checking. There is an alternative way of accessing the vector, using the function at, which does provide bounds checking at an additional cost. Let's take a look at several functions provided by the vector class:

also, there are several basic operators defined for the vector class:

=           Assignment replaces a vector's contents with the contents of another
==          An element by element comparison of two vectors
[]          Random access to an element of a vector (usage is similar to that
            of the operator with arrays.) Keep in mind that it does not provide
            bounds checking.

Let's take a look at an example program using the vector class:

				

						?1?#include?<iostream>
?2?#include?<vector>
?3?using?namespace?std;
?4?
?5?int?main()
?6?{
?7?????vector?<int>?example;?????????//Vector?to?store?integers
?8?????example.push_back(3);?????????//Add?3? onto?the?vector
?9?????example.push_back(10);????????//Add?10?to?the?end
10?????example.push_back(33);????????//Add?33?to?the?end

11?????for(int?x=0;?x<example.size();?x++)?
12?????{
13?????????cout<<example[x]<<"?";????//Should?output:?3?10?33
14?????}

15?????if(!example.empty())??????????//Checks?if?empty
16?????????example.clear();??????????//Clears?vector
17?????vector?<int>?another_vector;??//Creates?another?vector?to?store?integers
18?????another_vector.push_back(10);?//Adds?to?end?of?vector
19?????example.push_back(10);????????//Same
20?????if(example==another_vector)???//To?show?testing?equality
21?????{
22?????????example.push_back(20);?
23?????}
24?????for(int?y=0;?y<example.size();?y++)
25?????{
26?????????cout<<example[y]<<"?";????//Should?output?10?20
27?????}
28?????return?0;
29?}
		

Summary of Vector Benefits

Vectors are somewhat easier to use than regular arrays. At the very least, they get around having to be resized constantly using new and delete. Furthermore, their immense flexibility - support for any datatype and support for automatic resizing when adding elements - and the other helpful included functions give them clear advantages to arrays.

Another argument for using vectors are that they help avoid memory leaks--you don't have to remember to free vectors, or worry about how to handle freeing a vector in the case of an exception. This simplifies program flow and helps you write tighter code. Finally, if you use the at() function to access the vector, you get bounds checking at the cost of a slight performance penalty.

(二)

?C++ vector is a container template available with Standard Template Library pack. This C++ vector can be used to store similar typed objects sequentially, which can be accessed at a later point of time. As this is a template, all types of data including user defined data types like struct and class can be stored in this container.

This article explains briefly about how to insert, delete, access the data with respect to the C++ vector. The type stl::string is used for the purposes of explaining the sample code. Using stl::string is only for sample purposes. Any other type can be used with the C++ vector.

The values can be added to the c++ vector at the end of the sequence. The function push_back should be used for this purpose. The <vector> header file should be included in all the header files in order to access the C++ vector and its functions.

The above code adds 4 strings of std::string type to the str_Vector. This uses std:: vector.push_back function. This function takes 1 parameter of the designated type and adds it to the end of the c++ vector.

Accessing Elements of C++ Vector:

?? The elements after being added can be accessed in two ways. One way is our legacy way of accessing the data with vector.at(position_in_integer) function. The position of the data element is passed as the single parameter to access the data.

Using our normal logic for accessing elements stored in C++ Vector:

If we want to access all the data, we can use a for loop and display them as follows.

The std:: vector .size() function returns the number of elements stored in the vector.

Using C++ vector iterator provided by STL:

The next way is to use the iterator object provided by the STL. These iterators are general purpose pointers allowing c++ programmers to forget the intricacies of object types and access the data of any type.

Removing elements from C++ vector:

Removing elements one by one from specified positions in c++ vector is achieved with the erase function.

The following code demonstrates how to use the erase function to remove an element from position 2 in the str_Vector from our sample.

?The following sample demonstrates how to use the erase() function for removing elements 2,3 in the str_Vector used in the above sample.?

If one wants to remove all the elements at once from the c++ vector, the vector.clear() function can be used.