Java有一个方便的分割方法:
String str = "The quick brown fox";
String[] results = str.split(" ");
在c++中有简单的方法来做到这一点吗?
Java有一个方便的分割方法:
String str = "The quick brown fox";
String[] results = str.split(" ");
在c++中有简单的方法来做到这一点吗?
当前回答
使用strtok。在我看来,没有必要围绕标记化构建类,除非strtok不能提供您所需要的东西。可能不会,但在用C和c++编写各种解析代码的15年多时间里,我一直在使用strtok。这里有一个例子
char myString[] = "The quick brown fox";
char *p = strtok(myString, " ");
while (p) {
printf ("Token: %s\n", p);
p = strtok(NULL, " ");
}
一些注意事项(可能不适合您的需要)。该字符串在该过程中被“销毁”,这意味着EOS字符内联放置在分隔符点中。正确的用法可能需要创建字符串的非const版本。还可以在解析过程中更改分隔符列表。
在我看来,上面的代码比为它单独编写一个类要简单得多,也更容易使用。对我来说,这是语言提供的功能之一,而且它做得很好,很干净。这只是一个“基于C”的解决方案。它很合适,很简单,而且你不需要写很多额外的代码:-)
其他回答
您可以使用流、迭代器和复制算法来相当直接地做到这一点。
#include <string>
#include <vector>
#include <iostream>
#include <istream>
#include <ostream>
#include <iterator>
#include <sstream>
#include <algorithm>
int main()
{
std::string str = "The quick brown fox";
// construct a stream from the string
std::stringstream strstr(str);
// use stream iterators to copy the stream to the vector as whitespace separated strings
std::istream_iterator<std::string> it(strstr);
std::istream_iterator<std::string> end;
std::vector<std::string> results(it, end);
// send the vector to stdout.
std::ostream_iterator<std::string> oit(std::cout);
std::copy(results.begin(), results.end(), oit);
}
在我看来很奇怪的是,SO网站上有这么多注重速度的书呆子,却没有人给出一个使用编译时生成的分隔符查找表的版本(下面是示例实现)。使用查找表和迭代器应该在效率上击败std::regex,如果你不需要击败regex,就使用它,它是c++ 11的标准,超级灵活。
有些人已经建议使用正则表达式,但对于新手来说,这里有一个打包的示例,应该完全符合OP的期望:
std::vector<std::string> split(std::string::const_iterator it, std::string::const_iterator end, std::regex e = std::regex{"\\w+"}){
std::smatch m{};
std::vector<std::string> ret{};
while (std::regex_search (it,end,m,e)) {
ret.emplace_back(m.str());
std::advance(it, m.position() + m.length()); //next start position = match position + match length
}
return ret;
}
std::vector<std::string> split(const std::string &s, std::regex e = std::regex{"\\w+"}){ //comfort version calls flexible version
return split(s.cbegin(), s.cend(), std::move(e));
}
int main ()
{
std::string str {"Some people, excluding those present, have been compile time constants - since puberty."};
auto v = split(str);
for(const auto&s:v){
std::cout << s << std::endl;
}
std::cout << "crazy version:" << std::endl;
v = split(str, std::regex{"[^e]+"}); //using e as delim shows flexibility
for(const auto&s:v){
std::cout << s << std::endl;
}
return 0;
}
如果我们需要更快并接受所有字符必须为8位的约束,我们可以在编译时使用元编程创建一个查找表:
template<bool...> struct BoolSequence{}; //just here to hold bools
template<char...> struct CharSequence{}; //just here to hold chars
template<typename T, char C> struct Contains; //generic
template<char First, char... Cs, char Match> //not first specialization
struct Contains<CharSequence<First, Cs...>,Match> :
Contains<CharSequence<Cs...>, Match>{}; //strip first and increase index
template<char First, char... Cs> //is first specialization
struct Contains<CharSequence<First, Cs...>,First>: std::true_type {};
template<char Match> //not found specialization
struct Contains<CharSequence<>,Match>: std::false_type{};
template<int I, typename T, typename U>
struct MakeSequence; //generic
template<int I, bool... Bs, typename U>
struct MakeSequence<I,BoolSequence<Bs...>, U>: //not last
MakeSequence<I-1, BoolSequence<Contains<U,I-1>::value,Bs...>, U>{};
template<bool... Bs, typename U>
struct MakeSequence<0,BoolSequence<Bs...>,U>{ //last
using Type = BoolSequence<Bs...>;
};
template<typename T> struct BoolASCIITable;
template<bool... Bs> struct BoolASCIITable<BoolSequence<Bs...>>{
/* could be made constexpr but not yet supported by MSVC */
static bool isDelim(const char c){
static const bool table[256] = {Bs...};
return table[static_cast<int>(c)];
}
};
using Delims = CharSequence<'.',',',' ',':','\n'>; //list your custom delimiters here
using Table = BoolASCIITable<typename MakeSequence<256,BoolSequence<>,Delims>::Type>;
有了这些,创建getNextToken函数就很容易了:
template<typename T_It>
std::pair<T_It,T_It> getNextToken(T_It begin,T_It end){
begin = std::find_if(begin,end,std::not1(Table{})); //find first non delim or end
auto second = std::find_if(begin,end,Table{}); //find first delim or end
return std::make_pair(begin,second);
}
使用它也很简单:
int main() {
std::string s{"Some people, excluding those present, have been compile time constants - since puberty."};
auto it = std::begin(s);
auto end = std::end(s);
while(it != std::end(s)){
auto token = getNextToken(it,end);
std::cout << std::string(token.first,token.second) << std::endl;
it = token.second;
}
return 0;
}
这里有一个生动的例子:http://ideone.com/GKtkLQ
对于简单的事情,我只使用以下方法:
unsigned TokenizeString(const std::string& i_source,
const std::string& i_seperators,
bool i_discard_empty_tokens,
std::vector<std::string>& o_tokens)
{
unsigned prev_pos = 0;
unsigned pos = 0;
unsigned number_of_tokens = 0;
o_tokens.clear();
pos = i_source.find_first_of(i_seperators, pos);
while (pos != std::string::npos)
{
std::string token = i_source.substr(prev_pos, pos - prev_pos);
if (!i_discard_empty_tokens || token != "")
{
o_tokens.push_back(i_source.substr(prev_pos, pos - prev_pos));
number_of_tokens++;
}
pos++;
prev_pos = pos;
pos = i_source.find_first_of(i_seperators, pos);
}
if (prev_pos < i_source.length())
{
o_tokens.push_back(i_source.substr(prev_pos));
number_of_tokens++;
}
return number_of_tokens;
}
懦弱的免责声明:我编写实时数据处理软件,其中数据通过二进制文件、套接字或一些API调用(I/O卡、摄像头)传入。除了在启动时读取外部配置文件以外,我从未将这个函数用于更复杂或时间要求更严格的事情。
pystring是一个小型库,实现了Python的一系列字符串函数,包括split方法:
#include <string>
#include <vector>
#include "pystring.h"
std::vector<std::string> chunks;
pystring::split("this string", chunks);
// also can specify a separator
pystring::split("this-string", chunks, "-");
我以前只用标准库做了一个lexer/tokenizer。代码如下:
#include <iostream>
#include <string>
#include <vector>
#include <sstream>
using namespace std;
string seps(string& s) {
if (!s.size()) return "";
stringstream ss;
ss << s[0];
for (int i = 1; i < s.size(); i++) {
ss << '|' << s[i];
}
return ss.str();
}
void Tokenize(string& str, vector<string>& tokens, const string& delimiters = " ")
{
seps(str);
// Skip delimiters at beginning.
string::size_type lastPos = str.find_first_not_of(delimiters, 0);
// Find first "non-delimiter".
string::size_type pos = str.find_first_of(delimiters, lastPos);
while (string::npos != pos || string::npos != lastPos)
{
// Found a token, add it to the vector.
tokens.push_back(str.substr(lastPos, pos - lastPos));
// Skip delimiters. Note the "not_of"
lastPos = str.find_first_not_of(delimiters, pos);
// Find next "non-delimiter"
pos = str.find_first_of(delimiters, lastPos);
}
}
int main(int argc, char *argv[])
{
vector<string> t;
string s = "Tokens for everyone!";
Tokenize(s, t, "|");
for (auto c : t)
cout << c << endl;
system("pause");
return 0;
}