Vectores

Seqventia qvae crescit

Vectorem creare et implere

Lectio Series seqventiam indicatam praebvit: elementa eivsdem generis, alivd post alivd, a nihilo nvmerata. Eivs forma hodierna magnitvdinis fixae, std::array<int, 7>, magnitvdinem in genere fert, et compilator eam nosse debet. Atqvi qvi programma scribit qvod mediam aestimationvm cvrsvs compvtat nescit qvot discipvlos cvrsvs habitvrvs sit. Inde seqvitvr opvs esse seqventia cvivs magnitvdo datvm sit vnvm praeterea, qvod inter exseqvendvm mvtetvr: est vector, std::vector<T>, ex plica capitis <vector>, vbi T genvs elementorvm est.

#include <print>#include <vector>​int main(){    std::vector<int> squares{};    std::println("Initio: magnitvdo {}, vacvvs: {}", squares.size(), squares.empty());    for (int k{1}; k <= 5; ++k) {        squares.push_back(k * k);    }    std::println("{}", squares);    std::println("Magnitvdo {}, primvm {}, vltimvm {}", squares.size(), squares[0], squares.back());    squares[1] = 0;    squares.pop_back();    std::println("{}", squares);}

Initio: magnitvdo 0, vacvvs: true[1, 4, 9, 16, 25]Magnitvdo 5, primvm 1, vltimvm 25[1, 0, 9, 16]

Versvs 6 vectorem vacvvm integrorvm creat: contra atqve in serie, non opvs est nvntiare qvot elementa habitvrvs sit. size() reddit qvot habeat, et empty() dicit vtrvm nvllvm habeat. Circvitvs versvvm 8 ad 10 qvinqve qvadrata per push_back addit, qvod argvmentvm svvm in fine ponit et magnitvdinem vno avget. Versvs 11 vectorem totvm scribit cvm forma intervallorvm qva lectio svperior in seriebvs vsa est. Accessvs per indicem is est qvi semper, et squares.back() breviter dicit squares[squares.size() - 1]. Versvs 13 elemento exsistenti valorem assignat, et versvs 14 vltimvm per pop_back tollit. Operationes hvivs lectionis svnt, svmmatim, hae.

• v.push_back(x) x in fine addit; v.pop_back() vltimvm tollit (vectore vacvo, modvs agendi indefinitvs est).
• v.size() est magnitvdo, generis std::size_t, integri sine signo magnitvdinvm qvod lectio Series praebvit; v.empty() idem valet atqve v.size() == 0.
• v[i] et v.at(i) elementvm indicis i dant, qvi condicioni i < v.size() satisfacere debet; v.front() et v.back() primvm et vltimvm dant, et postvlant ne vector vacvvs sit.
• v.resize(n) vectorem cvm n elementis relinqvit: tollit qvae in fine svpersvnt, avt in fine addit elementa initialisata vt cvm vncis vacvis (nihila, si nvmeri svnt).

Vt in seriebvs, v[i] nvllvm examen praestat, v.at(i) avtem praestat: cvm indice 7 et qvinqve elementis, programma sistit cvm nvntio vector::_M_range_check: __n (which is 7) >= this->size() (which is 5), qvod indicem et magnitvdinem nominat. Mechanismvs qvi hanc intermissionem efficit, exceptio, in Programmatione Lingva C++ II tractatvr.

Vnci et parentheses

Cvrsvs omnia cvm vncis initialisat, et vector est primvm genvs cvrsvs in qvo vncos parenthesibvs mvtare eventvm mvtat. Cvm vncis, valores scripti svnt elementa; cvm parenthesibvs, primvs nvmervs est nvmervs elementorvm et secvndvs, si adest, valor cvivsqve.

#include <print>#include <vector>​int main(){    const std::vector<int> a{5};    const std::vector<int> b(5);    const std::vector<int> c{3, 7};    const std::vector<int> d(3, 7);    std::println("{} {} {} {}", a, b, c, d);}

[5] [0, 0, 0, 0, 0] [3, 7] [7, 7, 7]

a vnvm elementvm habet, 5; b, qvinqve nihila; c, 3 et 7; d, tria septena. Index inter vncos igitvr dicit «haec svnt elementa», et parentheses servantvr ad petenda n exempla eivsdem valoris, vt vectorem nvmeratorvm in nihilo positorvm.

Vltimvm vectoris vacvi

Vector vacvvs casvs legitimvs est (introitvs sine datis, colvm qvod nemo transit), et qvaeqve formvla cvm eivs magnitvdine in eo casv recensenda est; insidiosissima est v.size() - 1. Cvm v.size() integrvm sine signo sit, si ab eo 1 detrahitvr cvm 0 valet, non fit $-1$, sed rota circvmagitvr, vt in lectione Nvmeri integri eorvmqve repraesentatio visvm est, vsqve ad maximvm valorem generis std::size_t.

#include <cstddef>#include <print>#include <vector>​int main(){    const std::vector<int> values{};    std::println("values.size() - 1 = {}", values.size() - 1);    // Limes errans: cvm vectore vacvo, condicio vera est.    for (std::size_t i{0}; i <= values.size() - 1; ++i) {        std::println("{}", values[i]);    }}

values.size() - 1 = 18446744073709551615/usr/include/c++/15/bits/stl_vector.h:1282: constexpr std::vector< <template-parameter-1-1>, <template-parameter-1-2> >::const_reference std::vector< <template-parameter-1-1>, <template-parameter-1-2> >::operator[](size_type) const [with _Tp = int; _Alloc = std::allocator<int>; const_reference = const int&; size_type = long unsigned int]: Assertion '__n < this->size()' failed.

Primvs versvs exitvs est $18\,446\,744\,073\,709\,551\,615$, id est $2^{64} - 1$, maximvm std::size_t hvivs svggestvs. Inde seqvitvr condicionem circvitvs, 0 <= 18446744073709551615, veram esse, et primvm gyrvm legere values[0] ex vectore qvi elementvm 0 non habet: modvs agendi indefinitvs. Qvod observatvr ab optionibvs pendet, vt in seriebvs: sine optimizatione compilata, bibliotheca GCC 15 indicem ipsivs operator[] examinat et programma sistit per assertionem qvae dicit __n < this->size(); cvm optimizatione hoc examen evanescit, et circvitvs, qvi non in tempore sistit, legere conatvr vbi nvllvm omnino elementvm est (confirmatvm in Linux cvm -O2: qvia vector vacvvs nvllvm spativm memoriae habet, programma violatione segmenti finitvr, sine nvntio qvi cavsam explicet). Correctio est limitem sine svbtractione scribere, i < values.size(), qvi cvm vectore vacvo inde ab initio falsvs est; et vbi svbtractio necessaria videtvr, privs values.empty() interrogare.