Overview. Common tasks. Observation. Chapter 20 The STL (containers, iterators, and algorithms) 8/13/18. Bjarne Stroustrup

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1 Overview Chapter 20 The STL (cotaiers, iterators, ad algorithms) Bjare Stroustrup Commo tasks ad ideals Geeric programmig Cotaiers, algorithms, ad iterators The simplest algorithm: fid() Parameterizatio of algorithms fid_if() ad fuctio objects Sequece cotaiers vector ad list Associative cotaiers map, set Stadard algorithms copy, sort, Iput iterators ad output iterators List of useful facilities Headers, algorithms, cotaiers, fuctio objects Stroustrup/Programmig - Nov'13 3 Commo tasks Observatio Collect data ito cotaiers Orgaize data For pritig For fast access Retrieve data items By idex (e.g., get the th elemet) By value (e.g., get the first elemet with the value "Chocolate") By properties (e.g., get the first elemets where age<64 ) Add data Remove data Sortig ad searchig Simple umeric operatios We ca (already) write programs that are very similar idepedet of the data type used Usig a it is t that differet from usig a double Usig a vector<it> is t that differet from usig a vector<strig> Stroustrup/Programmig - Nov'13 4 Stroustrup/Programmig - Nov'13 5 1

2 Ideals Ideals (cotiued) We d like to write commo programmig tasks so that we do t have to re-do the work each time we fid a ew way of storig the data or a slightly differet way of iterpretig the data Fidig a value i a vector is t all that differet from fidig a value i a list or a array Lookig for a strig igorig case is t all that differet from lookig at a strig ot igorig case Graphig experimetal data with exact values is t all that differet from graphig data with rouded values Copyig a file is t all that differet from copyig a vector Code that s Easy to read Easy to modify Regular Short Fast Uiform access to data Idepedetly of how it is stored Idepedetly of its type Stroustrup/Programmig - Nov'13 6 Stroustrup/Programmig - Nov'13 Ideals (cotiued) Examples Type-safe access to data Easy traversal of data Compact storage of data Fast Retrieval of data Additio of data Deletio of data Stadard versios of the most commo algorithms Copy, fid, search, sort, sum, Stroustrup/Programmig - Nov'13 8 Sort a vector of strigs Fid a umber i a phoe book, give a ame Fid the highest temperature Fid all values larger tha 800 Fid the first occurrece of the value 1 Sort the telemetry records by uit umber Sort the telemetry records by time stamp Fid the first value larger tha Peterse? What is the largest amout see? Fid the first differece betwee two sequeces Compute the pairwise product of the elemets of two sequeces What are the highest temperatures for each day i a moth? What are the top 10 best-sellers? What s the etry for C++ (say, i Google)? What s the sum of the elemets? Stroustrup/Programmig - Nov'13 9 2

3 Geeric programmig Liftig example (cocrete algorithms) Geeralize algorithms Sometimes called liftig a algorithm The aim (for the ed user) is Icreased correctess Through better specificatio Greater rage of uses Possibilities for re-use Better performace Through wider use of tued libraries Uecessarily slow code will evetually be throw away Go from the cocrete to the more abstract The other way most ofte leads to bloat double sum(double array[], it ) // oe cocrete algorithm (doubles i array) double s = 0; for (it i = 0; i < ; ++i ) s += array[i]; retur s; struct Node Node* ext; it data; ; it sum(node* first) it s = 0; while (first) s += first->data; first = first->ext; retur s; // aother cocrete algorithm (its i list) // termiates whe expressio is false or zero Stroustrup/Programmig - Nov'13 10 Stroustrup/Programmig - Nov'13 11 Liftig example (abstract the data structure) Liftig example (STL versio) // pseudo-code for a more geeral versio of both algorithms it sum(data) // somehow parameterize with the data structure it s = 0; // iitialize while (ot at ed) // loop through all elemets s += get value; // compute sum get ext data elemet; retur s; // retur result We eed three operatios (o the data structure): ot at ed get value // Cocrete STL-style code for a more geeral versio of both algorithms template<class Iter, class T> // Iter should be a Iput_iterator // T should be somethig we ca + ad = T sum(iter first, Iter last, T s) // T is the accumulator type while (first!=last) s += *first; ++first; retur s; Let the user iitialize the accumulator float a[] = 1,2,3,4,5,6,,8 ; double d = 0; d = sum(a,a+sizeof(a)/sizeof(*a),d); get ext data elemet Stroustrup/Programmig - Nov'13 12 Stroustrup/Programmig - Nov'

4 Liftig example The STL Almost the stadard library accumulate I simplified a bit for terseess (see 21.5 for more geerality ad more details) Works for arrays vectors lists istreams Rus as fast as had-crafted code Give decet iliig The code s requiremets o its data has become explicit We uderstad the code better Part of the ISO C++ Stadard Library Mostly o-umerical Oly 4 stadard algorithms specifically do computatio accumulate, ier_product, partial_sum, adjacet_differece Hadles textual data as well as umeric data E.g. strig Deals with orgaizatio of code ad data Built-i types, user-defied types, ad data structures Optimizig disk access was amog its origial uses Performace was always a key cocer Stroustrup/Programmig - Nov'13 14 Stroustrup/Programmig - Nov'13 15 The STL Desiged by Alex Stepaov Geeral aim: The most geeral, most efficiet, most flexible represetatio of cocepts (ideas, algorithms) Represet separate cocepts separately i code Combie cocepts freely wherever meaigful Geeral aim to make programmig like math or eve Good programmig is math works for itegers, for floatig-poit umbers, for polyomials, for Algorithms sort, fid, search, copy, Basic model iterators Cotaiers vector, list, map, uordered_map, Separatio of cocers Algorithms maipulate data, but do t kow about cotaiers Cotaiers store data, but do t kow about algorithms Algorithms ad cotaiers iteract through iterators Each cotaier has its ow iterator types Stroustrup/Programmig - Nov'13 16 Stroustrup/Programmig - Nov'13 1 4

5 The STL A ISO C++ stadard framework of about 10 cotaiers ad about 60 algorithms coected by iterators Other orgaizatios provide more cotaiers ad algorithms i the style of the STL Boost.org, Microsoft, SGI, Probably the curretly best kow ad most widely used example of geeric programmig The STL If you kow the basic cocepts ad a few examples you ca use the rest Full documetatio: Google C++ STL More accessible ad less complete documetatio Appedix B Stroustrup/Programmig - Nov'13 18 Stroustrup/Programmig - Nov'13 19 Basic model A pair of iterators defies a sequece The begiig (poits to the first elemet if ay) The ed (poits to the oe-beyod-the-last elemet) vector Cotaiers (hold sequeces i differece ways) begi: ed: list (doubly liked) A iterator is a type that supports the iterator operatios ++ Go to ext elemet * Get value == Does this iterator poit to the same elemet as that iterator? Some iterators support more operatios (e.g. --, +, ad [ ]) Stroustrup/Programmig - Nov'13 20 set (a kid of tree) Stroustrup/Programmig - Nov'

6 begi: The simplest algorithm: fid() // Fid the first elemet that equals a value template<class I, class T> I fid(i first, I last, cost T& val) while (first!=last && *first!= val) ++first; retur first; ed: void f(vector<it>& v, it x) // fid a it i a vector vector<it>::iterator p = fid(v.begi(),v.ed(),x); if (p!=v.ed()) /* we foud x */ We ca igore ( abstract away ) the differeces betwee cotaiers Stroustrup/Programmig - Nov'13 22 fid() geeric for both elemet type ad cotaier type void f(vector<it>& v, it x) // works for vector of its vector<it>::iterator p = fid(v.begi(),v.ed(),x); if (p!=v.ed()) /* we foud x */ void f(list<strig>& v, strig x) // works for list of strigs list<strig>::iterator p = fid(v.begi(),v.ed(),x); if (p!=v.ed()) /* we foud x */ void f(set<double>& v, double x) // works for set of doubles set<double>::iterator p = fid(v.begi(),v.ed(),x); if (p!=v.ed()) /* we foud x */ Stroustrup/Programmig - Nov'13 23 Algorithms ad iterators A iterator poits to (refers to, deotes) a elemet of a sequece The ed of the sequece is oe past the last elemet ot the last elemet That s ecessary to elegatly represet a empty sequece Oe-past-the-last-elemet is t a elemet You ca compare a iterator poitig to it You ca t dereferece it (read its value) Returig the ed of the sequece is the stadard idiom for ot foud or usuccessful A empty sequece: some iterator: the ed: begi: ed: Stroustrup/Programmig - Nov'13 24 Simple algorithm: fid_if() Fid the first elemet that matches a criterio (predicate) Here, a predicate takes oe argumet ad returs a bool template<class I, class Pred> I fid_if(i first, I last, Pred pred) while (first!=last &&!pred(*first)) ++first; retur first; void f(vector<it>& v) vector<it>::iterator p = fid_if(v.begi(),v.ed,odd()); if (p!=v.ed()) /* we foud a odd umber */ Stroustrup/Programmig - Nov'13 A predicate 25 6

7 Predicates A predicate (of oe argumet) is a fuctio or a fuctio object that takes a argumet ad returs a bool For example A fuctio bool odd(it i) retur i%2; // % is the remaider (modulo) operator odd(); // call odd: is odd? A fuctio object struct Odd bool operator()(it i) cost retur i%2; ; Odd odd; // make a object odd of type Odd odd(); // call odd: is odd? Fuctio objects A cocrete example usig state template<class T> struct Less_tha T val; // value to compare with Less_tha(T& x) :val(x) bool operator()(cost T& x) cost retur x < val; ; // fid x<43 i vector<it> : p=fid_if(v.begi(), v.ed(), Less_tha(43)); Stroustrup/Programmig - Nov'13 26 // fid x<"perfectio" i list<strig>: q=fid_if(ls.begi(), ls.ed(), Less_tha("perfectio")); Stroustrup/Programmig - Nov'13 2 Fuctio objects A very efficiet techique iliig very easy ad effective with curret compilers Faster tha equivalet fuctio Ad sometimes you ca t write a equivalet fuctio The mai method of policy parameterizatio i the STL Key to emulatig fuctioal programmig techiques i C++ Policy parameterizatio Wheever you have a useful algorithm, you evetually wat to parameterize it by a policy. For example, we eed to parameterize sort by the compariso criteria struct Record strig ame; char addr[24]; ; // stadard strig for ease of use // old C-style strig to match database layout vector<record> vr; sort(vr.begi(), vr.ed(), Cmp_by_ame()); sort(vr.begi(), vr.ed(), Cmp_by_addr()); // sort by ame // sort by addr Stroustrup/Programmig - Nov'13 28 Stroustrup/Programmig - Nov'13 29

8 Comparisos // Differet comparisos for Rec objects: struct Cmp_by_ame bool operator()(cost Rec& a, cost Rec& b) cost retur a.ame < b.ame; // look at the ame field of Rec ; struct Cmp_by_addr bool operator()(cost Rec& a, cost Rec& b) cost retur 0 < strcmp(a.addr, b.addr, 24); // correct? ; // ote how the compariso fuctio objects are used to hide ugly // ad error-proe code Policy parameterizatio Wheever you have a useful algorithm, you evetually wat to parameterize it by a policy. For example, we eed to parameterize sort by the compariso criteria vector<record> vr; sort(vr.begi(), vr.ed(), [] (cost Rec& a, cost Rec& b) retur a.ame < b.ame; // sort by ame ); sort(vr.begi(), vr.ed(), [] (cost Rec& a, cost Rec& b) retur 0 < strcmp(a.addr, b.addr, 24); // sort by addr ); Stroustrup/Programmig - Nov'13 30 Stroustrup/Programmig - Nov'13 31 Policy parameterizatio Use a amed object as argumet If you wat to do somethig complicated If you feel the eed for a commet If you wat to do the same i several places Use a lambda expressio as argumet If what you wat is short ad obvious Choose based o clarity of code There are o performace differeces betwee fuctio objects ad lambdas Fuctio objects (ad lambdas) ted to be faster tha fuctio argumets vector template<class T> class vector T* elemets; usig value_type = T; usig iterator =???; // the type of a iterator is implemetatio defied // ad it (usefully) varies (e.g. rage checked iterators) // a vector iterator could be a poiter to a elemet usig cost_iterator =???; iterator begi(); cost_iterator begi() cost; iterator ed(); cost_iterator ed() cost; // poits to first elemet // poits to oe beyod the last elemet ; iterator erase(iterator p); // remove elemet poited to by p iterator isert(iterator p, cost T& v); // isert a ew elemet v before p Stroustrup/Programmig - Nov'13 32 Stroustrup/Programmig - Nov'

9 isert() ito vector vector<it>::iterator p = v.begi(); ++p; ++p; ++p; vector<it>::iterator q = p; ++q; p = v.erase(p); erase() from vector // leaves p poitig at the elemet after the erased oe 5 p=v.isert(p,99); // leaves p poitig at the iserted elemet Note: q is ivalid after the isert() Note: Some elemets moved; all elemets could have moved Stroustrup/Programmig - Nov' vector elemets move whe you isert() or erase() Iterators ito a vector are ivalidated by isert() ad erase() Stroustrup/Programmig - Nov'13 35 template<class T> class list Lik* elemets; list usig value_type = T; usig iterator =???; // the type of a iterator is implemetatio defied // ad it (usefully) varies (e.g. rage checked iterators) // a list iterator could be a poiter to a lik ode usig cost_iterator =???; iterator begi(); cost_iterator begi() cost; iterator ed(); cost_iterator ed() cost; // poits to first elemet Lik: // poits oe beyod the last elemet T value Lik* pre Lik* post iterator erase(iterator p); // remove elemet poited to by p iterator isert(iterator p, cost T& v); // isert a ew elemet v before p ; Stroustrup/Programmig - Nov'13 36 isert() ito list list<it>::iterator p = v.begi(); ++p; ++p; ++p; list<it>::iterator q = p; ++q; 6 v = v.isert(p,99); // leaves p poitig at the iserted elemet Note: q is uaffected 99 Note: No elemets moved aroud Stroustrup/Programmig - Nov'13 3 9

10 p = v.erase(p); 6 erase() from list // leaves p poitig at the elemet after the erased oe Ways of traversig a vector for(it i = 0; i<v.size(); ++i) // do somethig with v[i] for(vector<t>::size_type i = 0; i<v.size(); ++i) // do somethig with v[i] for(vector<t>::iterator p = v.begi(); p!=v.ed(); ++p) // do somethig with *p Less paiful: for (auto = v.begi(); p!=v.ed(); ++p) // do somethig with *p // why it? // loger but always correct Note: list elemets do ot move whe you isert() or erase() Stroustrup/Programmig - Nov'13 38 Stroustrup/Programmig - Nov'13 39 Ways of traversig a vector Kow all three ways (iterator, subscript, auto) The subscript style is used i essetially every laguage The iterator style is used i C (poiters oly) ad C++ The iterator style is used for stadard library algorithms The subscript style does t work for lists (i C++ ad i most laguages) Use either way for vectors There are o fudametal advatages of oe style over the other But the iterator style works for all sequeces Prefer size_type over plai it pedatic, but quiets compiler ad prevets rare errors Ways of traversig a vector for(vector<t>::iterator p = v.begi(); p!=v.ed(); ++p) // do somethig with *p for(vector<t>::value_type x : v) // do somethig with x for(auto& x : v) // do somethig with x Rage for Use for the simplest loops Every elemet from begi() to ed() Over oe sequece Whe you do t eed to look at more tha oe elemet at a time Whe you do t eed to kow the positio of a elemet Stroustrup/Programmig - Nov'13 40 Stroustrup/Programmig - Nov'

11 Vector vs. List Some useful stadard headers By default, use a vector You eed a reaso ot to You ca grow a vector (e.g., usig push_back()) You ca isert() ad erase() i a vector Vector elemets are compactly stored ad cotiguous For small vectors of small elemets all operatios are fast compared to lists If you do t wat elemets to move, use a list You ca grow a list (e.g., usig push_back() ad push_frot()) You ca isert() ad erase() i a list List elemets are separately allocated Note that there are more cotaiers, e.g., map uordered_map Stroustrup/Programmig - Nov'13 42 <iostream> I/O streams, cout, ci, <fstream> file streams <algorithm> sort, copy, <umeric> accumulate, ier_product, <fuctioal> fuctio objects <strig> <vector> <map> <uordered_map> hash table <list> <set> Stroustrup/Programmig - Nov'13 43 Next lecture Map, set, ad algorithms Stroustrup/Programmig - Nov'