The C++ Algorithm Libraries

1. The C++ Algorithm Libraries • A standard collection of generic algorithms • Applicable to various types and containers • E.g., sorting integers (int) vs. intervals (pair<int, int>) • E.g., sorting elements in a vector vs. in a C-style array • Polymorphic even without inheritance relationships • Types substituted need not have a common base class • Must only provide the operators the algorithm needs • Significantly used with the sequence containers • To reorder elements within a container’s sequence • To store/fetch values into/from a container • To calculate various values and properties from it

2. Motivating Example: Searching a String • From Austern: “Generic Programming and the STL” • Sequential (linear) search: find char c in string s char * strchr (char* s, char c) { while (*s != 0 && *s != c){ ++s; } return *s == c ? s : (char *) 0; } • Problem: not very general • “Range” of iteration is always defined up to ‘\0’ character • Only works for a “zero terminated” string in C/C++

3. Improving Linear Search with Ranges • First generalization (Austern, pp. 11): use a range (something that sequential containers can give us!) char * find1 (char* first, char* last, char c){ while (first != last && *first != c) ++first; return first; } • Gives an explicit range (calculate its length – how?) • Assumes first is before last (can check – how?) • Note how caller checks for success changed: why?

4. Linear Search over Parameterized Types • Second generalization: use templates to parameterize the function argument types template <typename T> T* find2(T* first,T* last, const T &value){ while (first != last && *first != value) ++first; return first; } • How much did the find1 code need to change? • One last problem • What if we want to apply this to a container (e.g., list) whose range can’t be traversed via simple pointers?

5. Linear Search with Generic Iterators • Third generalization: separate iterator type parameter • We arrive at the find algorithm (Austern pp. 13): template <typename Iterator,typename T> Iterator find (Iterator first,Iterator last, const T & value) { while (first != last && *first != value) ++first; return first; } • Notice how algorithm depends on the iterators • Notice how refinements made algorithm more abstract • … but still essentially does the same thing • i.e., algorithm structure (and time complexity) is the same

6. Organization of C++ Algorithm Libraries • The <algorithm> header file contains • Non-modifying sequence operations • Do some calculation but don’t change sequence itself • Examples include count, count_if • Mutating sequence operations • Modify the order or values of the sequence elements • Examples include copy, random_shuffle • Sorting and related operations • Modify the order in which elements appear in a sequence • Examples include sort, next_permutation • The <numeric> header file contains • General numeric operations • Scalar and matrix algebra, especially used with vector<T> • Examples include accumulate, inner_product

7. count algorithm Moves through iterator range Checks each position for equality Increases count if equal #include <iostream> #include <vector> #include <algorithm> using namespace std; int main (int, char * []) { vector<int> v; v.push_back(1); v.push_back(2); v.push_back(3); v.push_back(2); int i = 7; cout << i << " appears " << count(v.begin(), v.end(), i) << " times in v" << endl; i = 2; cout << i << " appears " << count(v.begin(), v.end(), i) << " times in v" << endl; return 0; } Example of Using Non-Modifying Algorithms /* output is 7 appears 0 times in v 2 appears 2 times in v */

8. copy algorithm Copies from an input iterator range into an output iterator Note use of default constructor to get an “off-the-end” (here, “end-of-file”) input iterator Note use of noskipws (need to make sure container behavior matches what you want to do) ifstream input_file (input_file_name.c_str()); ofstream output_file (output_file_name.c_str()); input_file >> noskipws; istream_iterator<char> i (input_file); ostream_iterator<char> o (output_file); copy (i, istream_iterator<char>(), o); cout << "copied input file: " << input_file_name << endl << " to output file: " << output_file_name << endl; return 0; } /* output: cdgill@hive> ./copytest Makefile Makefile2 copied input file: Makefile to output file: Makefile2 cdgill@hive> diff Makefile Makefile2 cdgill@hive> */ Example of Using Mutating Algorithms #include <iostream> #include <string> #include <fstream> #include <iterator> #include <algorithm> using namespace std; int main (int argc, char * argv[]) { if (argc != 3) {return 1;} string input_file_name (argv[1]); string output_file_name (argv[2]);

9. sort algorithm Reorders a given range Can also plug in a functor to change the ordering function next_permutation algorithm Generates a specific kind of reordering, called a “permutation” Can use to generate all possible orders of a given sequence #include <iostream> #include <string> #include <algorithm> using namespace std; int main (int, char * []) { string s = "asdf"; cout << "original: " << s << endl; sort (s.begin(), s.end()); cout << "sorted: " << s << endl; string t (s); cout << "permutations:" << endl; do { next_permutation (s.begin(), s.end()); cout << s << " "; } while (s != t); cout << endl; return 0; } Example of Using Sorting Algorithms /* output is original: asdf sorted: adfs permutations: adsf afds afsd asdf asfd dafs dasf dfas dfsa dsaf dsfa fads fasd fdas fdsa fsad fsda sadf safd sdaf sdfa sfad sfda adfs */

10. accumulate algorithm Sums up elements in a range (based on a starting sum value) inner_product algorithm Computes the inner (also known as “dot”) product of two matrixes: sum of the products of their respective elements #include <iostream> #include <vector> #include <numeric> using namespace std; int main (int, char * []) { vector<int> v; v.push_back(1); v.push_back(2); v.push_back(3); v.push_back(2); cout << "v contains "; for (size_t s = 0; s < v.size(); ++s) { cout << v[s] << " "; } cout << endl; cout << "the sum of the elements in v is " << accumulate (v.begin(), v.end(), 0) << endl; cout << "the inner product of v and itself is " << inner_product (v.begin(), v.end(), v.begin(), 0) << endl; return 0; } Example of Using Numeric Algorithms /* output is: v contains 1 2 3 2 the sum of the elements in v is 8 the inner product of v and itself is 18 */

11. Concluding Remarks • C++ libraries give you useful, generic algorithms • Combine easily with a variety of containers/iterators • Support many common data structure manipulations • Finding and modifying values, re-ordering, numeric operations • Reusing them saves you from writing code • Many STL algorithms can be extended further • Especially by plugging functors into them • Next time we’ll look at how functors work, and how to use them, as well as at algorithms and iterators in more detail