Lecture 7-3 : STL Algorithms

STL Algorithms The Standard Template Library not only contains container classes, but also algorithms that operate on sequence containers. To use them, we must write #include <algorithm> in our program. In this lesson we will see a few different algorithms contained in the STL (for others see the textbook): sort() find(), find_if() Etc binary_search, count, min/max, swap, partition, rotate set_difference, set_union, set_intersection

Example: STL Algorithm –sort() Let vector<T> A; for some class T. Let vector<T>::iterator p, q sort(p, q) sorts A between p and q. Common case is sort(A.begin(), A.end()); sorts all of A.

example Output composer: Mozart Bach Chopin Beethoven // sort without comparators #include<iostream> #include<vector> #include<string> #include<algorithm> using namespace std; template<class Iterator> void Display(Iterator start, Iterator end) { for( Iterator p = start; p != end; ++p ) cout<< *p << “ “; } int main() vector<string> composer; composer.push_back(“Mozart”); composer.push_back(“Bach”); composer.push_back(“Chopin”); composer.push_back(“Beethoven”); cout<< “composer: “; Display(composer.begin(), composer.end()); cout<< endl; sort(composer.begin(), composer.end()); vector<int> v; for (int i = 1; i < 13; i++) { v.pushback(i*i % 13); cout<< “v: “; Display(v.begin(), v.end()); cout<< endl; sort(v.begin(), v.end()); Output composer: Mozart Bach Chopin Beethoven composer: Bach Beethoven Chopin Mozart v: 1 4 9 3 12 10 1012 3 9 4 1 v: 1 1 3 3 4 4 9 9 10 1012 12

Example: STL Algorithm –sort() sort() also works with deque objects but not with list objects. In general, sort() works with any random access sequence container. Guaranteed O(n log n) running time.

See also sort() for user defined types example : stl_sort.cpp

Another Example: STL Algorithm –find() #include <algorithm> #include <string> #include <list> int main() { list<string> composer; composer.push_back(“Mozart”); composer.push_back(“Bach”); composer.push_back(“Chopin”); composer.push_back(“Beethoven”); list<string>::iterator p; p = find(composer.begin(), composer.end(), “Bach”); if (p == composer.end()) { cout << “Not found.” << endl; } else if (++p != composer.end()) { cout<< “Found before: “ << *p << endl; } else { cout<< “Found at the end.” << endl; }

Algorithms, Iterators, and Sub-sequences Sequences/Sub-sequences are specified using iteratorsthat indicate the beginning and the end for an algorithm to work on. Here we find the 2nd occurrence of the value, 341, in a sequence. // File “init.cpp” int f(int x) { return -x*x + 40*x + 22; } // 22 61 98 133 166 197 226 253 278 301 322 341 358 373 386 397 // 406 413 418 421 422 421 418 413 406 397 386 373 358 341 322 301 template<typename T> void my_initialization(T& x) { const int N = 39; for (int j = 0; j < N; ++j) { x.push_back( f(j) ); }

Example #include <vector> #include <algorithm> #include ''init.cpp'' int main() { const int search_value= 341; vector<int> x; my_initialization(x); vector<int>::iteratorp; p = find(x.begin(), x.end(), search_value); if (p != x.end()) {// Value found! p = find(++p, x.end(), search_value); // Find again if (p != x.end()) {// Value found again! cout<< “Found after: “ << *--p << endl; }

STL find()–‘Implementation’ equivalent to : find() searches linearly through a sequence, and stops when an item matches the 3rd argument. A big limitation of find()is that it requires an exact match by value. template <class IteratorT, class T> IteratorT Find( IteratorT begin, IteratorT end, const T& value ) { for ( ; begin != end ; begin++) if (*begin == value) break; return begin; }

find() for user defind type operator== is necessary (no necessary for built-in type) #include <algorithm> #include <vector> #include <iostream> using namespace std; class mytype { public : int key; int height; int weight; mytype(int k, int h, int w) : key(k) , height(h) , weight(w){} bool operator==(const int a) if (key==a) return true; return false; } }; int main() { vector<mytype> vec; mytype x1(1,172,70); mytype x2(2,163,50); mytype x3(3,178,74); mytype x4(4,185,80); vec.push_back(x1); vec.push_back(x2); vec.push_back(x3); vec.push_back(x4); for (int i=0;i<vec.size();i++) { cout << vec[i].key << " “ << vec[i].height << " “ << vec[i].weight << endl; } vector<mytype>::iterator p; p=find(vec.begin(),vec.end(),3); cout << "found: " << p->key << " " << p->height << " " << p->weight << endl; return 1;

find_if Returns an iterator to the first element in the range   [first,last) for which applying f to it, is true. // find_if example #include <iostream> #include <algorithm> #include <vector> using namespace std; bool IsOdd (int i) { return ((i%2)==1); } int main () { vector<int> myvector; vector<int>::iterator it; myvector.push_back(10); myvector.push_back(25); myvector.push_back(40); myvector.push_back(55); it = find_if (myvector.begin(), myvector.end(), IsOdd); cout << "The first odd value is " << *it << endl; return 0; Output : The first odd value is 25

Many other useful <algorithm> functions…