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Vector Operations

Vector Operations. and the Standard Template Library (STL). 13. 21. 08. 14. 19. 0. 1. 2. 3. 4. Review. The standard template library (STL) provides the vector, a container for storing multiple values of the same type. A vector is an indexed container, like a string.

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Vector Operations

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  1. Vector Operations and the Standard Template Library (STL)

  2. 13 21 08 14 19 0 1 2 3 4 Review The standard template library (STL) provides the vector, a container for storing multiple values of the same type. A vector is an indexed container, like a string. The values in a vector can be accessed using the subscript operator ([]).

  3. 5 8 13 21 08 14 19 size capacity data 0 1 2 3 4 5 6 7 Vector Attributes A vector has several attributes, including its • data -- the values it contains. • size -- the number of values it contains; and • capacity -- the number of values it can store before having to grow.

  4. 5 8 13 21 08 14 19 size capacity data 0 1 2 3 4 5 6 7 size() and capacity() From this, it should be obvious that the vector function member size() simply returns the value of its size attribute: A vector also has a function member named capacity() that returns the value of its capacity attribute.

  5. 0 0 f size capacity data 8 8 8 8 1 0 0 1 1 0 1 0 0 1 1 0 0 1 0 1 size size capacity capacity data data 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 Declarations: 3 Forms vector<double> aVector; // empty const int N = 8; // N default vector<double> aVector(N); // values const int N = 8; // N specified vector<double> aVector(N, 1); // values

  6. 8 8 8 8 0 Hi 0 Hi 0 Hi 0 Hi 0 Hi Hi 0 0 Hi Hi 0 size size capacity capacity data data 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 Vector Declarations You can pass any type to a vector when it is declared: const int N = 8; vector<bool> boolVec(N, false); const int N = 8; vector<string> stringVec(N, “Hi”);

  7. 0 0 f size capacity data 1 8 13 size capacity data 0 1 2 3 4 5 6 7 Vector push_back() Values can be appended to a vector using the push_back() function member, which causes the vector to “grow” if necessary. vector<double> aVector; // empty aVec.push_back(13);

  8. 3 8 13 6 21 size capacity data 0 1 2 3 4 5 6 7 Appending (Ct’d) aVec.push_back(6); aVec.push_back(21); Subsequent calls to push_back() continue to append: When the size equals the capacity, a call to push_back() causes the vector to grow again. The only limit on a vector’s growth is the amount of memory available.

  9. 2 3 8 8 13 13 6 6 21 size size capacity capacity data data 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 Vector pop_back() To remove the most recently appended value, the vector function member pop_back() can be used: aVec.pop_back(); Note that push_back() and pop_back() alter size.

  10. 2 8 13 6 size capacity data 0 1 2 3 4 5 6 7 Vector front() and back() The first and last values in a vector can be retrieved with the front() and back() function members: double alpha = aVec.front(); // alpha == 13 double omega = aVec.back(); // omega == 6 The front() and back() function members provide a convenient way to access the ends of a vector without finding using subscripts.

  11. 2 8 13 6 size capacity data 0 1 2 3 4 5 6 7 Vector Subscript Like a string, the individual values in a vector can be accessed using the subscript ([]) operator. double first = aVec[0]; // first == 13 double second = aVec[1]; // second == 6 The subscript operation aVec[i] provides a convenient way to access the value in aVec whose index is i, where i is a for loop control variable.

  12. 2 8 13 6 size capacity data 0 1 2 3 4 5 6 7 Safe Vector Element Access The subscript operator does not check that the index being accessed is valid. The at() function member does check the index for validity, making it safer, but slower. double first = aVec.at(0); // first == 13 double second = aVec.at(1); // second == 6

  13. aVec.begin() aVec.end() aVec 3 8 13 6 21 size capacity data 0 1 2 3 4 5 6 7 Vector Iterators Each STL container provides 2 function members: • begin(), that points to the first data value; and • end(), that points beyond the last data value. STL uses such “pointers” to iterate through the values of a container, and so they are called iterators.

  14. STL Algorithms In addition to containers like class vector, the STL defines a rich set of algorithms that can be applied to any STL container, just by using the directive: #include <algorithm> To make these algorithms independent of any particular container, they take as arguments a container’s iterators, rather than the container itself.

  15. aVec.begin() aVec.end() aVec 3 8 13 6 21 size capacity data 0 1 2 3 4 5 6 7 Algorithm accumulate() The accumulate() algorithm returns the result of applying + to each of an STL container’s values: double sum = accumulate(aVec.begin(), // sum aVec.end(), 0); // == // 40 The third argument to accumulate() lets you sum starting with a base value other than zero.

  16. aVec.begin() aVec.begin() aVec.end() aVec.end() aVec aVec 3 3 8 8 13 6 13 6 21 21 size size capacity capacity data data 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 Algorithm sort() The sort() algorithm arranges the values of an STL container in ascending order: sort(aVec.begin(), aVec.end());

  17. aVec.begin() aVec.end() aVec 3 8 13 6 21 size capacity data 0 1 2 3 4 5 6 7 pos aVec.begin() aVec.end() aVec 3 8 13 6 21 size capacity data 0 1 2 3 4 5 6 7 Algorithm find() The find() algorithm searches an STL container for a particular value, returning an iterator to it: vector<double>::iterator pos = find(aVec.begin(), aVec.end(), 6);

  18. pos aVec.begin() aVec.begin() aVec.end() aVec.end() aVec aVec 3 2 8 8 13 13 6 21 21 size size capacity capacity data data 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 Vector erase() The erase() function member lets you remove a value at a position pointed to by an iterator: vector<double>::iterator pos = find(aVec.begin(), aVec.end(), 6); aVec.erase(pos);

  19. aVec.begin() aVec.end() aVec 3 8 13 6 21 size capacity data 0 1 2 3 4 5 6 7 aVec.begin() aVec.end() aVec 2 8 13 21 size capacity data 0 1 2 3 4 5 6 7 Vector erase() Alternatively: aVec.erase(find(aVec.begin(), aVec.end(), 6));

  20. aVec.begin() aVec.end() aVec.begin() aVec.end() aVec 3 8 9 13 21 aVec 2 8 13 21 size capacity data 0 1 2 3 4 5 6 7 size capacity data 0 1 2 3 4 5 6 7 Vector insert() The insert() function member lets you insert a value within a vector, before an iterator: aVec.insert(aVec.begin(), 9);

  21. erase() and insert() The erase() and insert() function members are both expensive in terms of time, because of the extensive copying each performs. • insert() must copy all values to the right of the insertion point, to “make room” for the new value. • erase() must copy all values to the right of the erased element, to “fill in” the empty spot. As a result, these should be used with caution.

  22. Summary The vector class provides many function members for operating on vectors. The C++ standard template library (STL) provides containers, algorithms, and iterators to connect algorithms and containers. A vector is an STL container, so that many STL algorithms can be used to operate on vectors.

  23. Summary (Ct’d) There are many more STL algorithms beyond the ones presented here (we barely scratched the surface). For a complete list, see the most recent edition of “The C++ Programming Language”, by Bjarne Stroustrup, Addison-Wesley.

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