2016-07-17

Useful functions of string in C++

String append

string::append

append经常被+代替,他们作用相似。

示例代码：

#include <iostream>
#include <string>

int main ()
{
  std::string str;
  std::string str2="Writing ";
  std::string str3="print 10 and then 5 more";

  // used in the same order as described above:
  str.append(str2);                       // "Writing "
  str.append(str3,6,3);                   // "10 "
  str.append("dots are cool",5);          // "dots "
  str.append("here: ");                   // "here: "
  str.append(10u,'.');                    // ".........."
  str.append(str3.begin()+8,str3.end());  // " and then 5 more"
  str.append<int>(5,0x2E);                // "....."

  std::cout << str << '\n';
  return 0;
}
// Output: Writing 10 dots here: .......... and then 5 more.....

Upcase and lowercase

std::tolower

把大写字母转化为小写字母，每次只能对一个char进行转换，如果要对string进行转换，可以按照以下代码，来自这里：

#include <algorithm>
#include <string> 

std::string data = "Abc"; 
std::transform(data.begin(), data.end(), data.begin(), ::tolower);

// For converting to lower case, you need to replace ::tolower with ::toupper .

std::toupper

示例代码：

#include <iostream>       // std::cout
#include <string>         // std::string
#include <locale>         // std::locale, std::toupper

int main ()
{
  std::locale loc;
  std::string str="Test String.\n";

  for (std::string::size_type i=0; i<str.length(); ++i)
      std::cout << std::toupper(str[i], loc);
  
  return 0;
}

transform

也可以参考这篇博客

Code sample:

transform(rs[i].begin(), rs[i].end(), rs[i].begin(), ::tolower);

String split

cstring::strtok

/* strtok example */ 
#include <stdio.h> 
#include <string.h> 

int main () 
{ 
    char str[] ="a,b,c,d*e"; 
    const char * split = ","; 
    char * p; 
    p = strtok (str,split); 
    while(p!=NULL) { 
        printf ("%s\n",p); 
        p = strtok(NULL,split); 
    } 

    getchar(); 
    return 0; 
}

本例中，实现对字符串a,b,c,d\*e用逗号(,)来作界定符对字符串进行分割。输出结果将如下所示:

a
b
c
d*e

因为delimiters支持多个分割符, 我们将本示例中的语句行const char * split = ",";改成 const char * split = ",*"; //用逗号(,)和星号(*)对字符串进行分割。这样输出结果将如下所示:

a
b
c
d
e

split

c++中没有对string直接split的函数，string::strtok只能对char *进行split，如果要对string进行split可以使用下面的代码，同样支持多个分隔符：

#include <string>
#include <sstream>
#include <vector>

using namespace std;

vector<string> &split(const string &s, char delim, vector<string> &elems) {
    stringstream ss(s);
    string item;
    while (getline(ss, item, delim)) {
        elems.push_back(item);
    }
    return elems;
}


vector<string> split(const string &s, char delim) {
    vector<string> elems;
    split(s, delim, elems);
    return elems;
}

规定自己就算死记硬背也要把上面的代码记住，做到快速手写出来。

除了上面的方法，还可以使用boost库（需要安装boost库，或者直接把include的cpp直接复制过来）:

#include <boost/algorithm/string.hpp>
std::vector<std::string> strs;
boost::split(strs, "string to split", boost::is_any_of("\t "));

String search

string::find

Code sample:

#include <iostream>       // std::cout
#include <string>         // std::string

int main ()
{
  std::string str ("There are two needles in this haystack with needles.");
  std::string str2 ("needle");

  // different member versions of find in the same order as above:
  std::size_t found = str.find(str2);
  if (found!=std::string::npos)
    std::cout << "first 'needle' found at: " << found << '\n';

  found=str.find("needles are small",found+1,6);
  if (found!=std::string::npos)
    std::cout << "second 'needle' found at: " << found << '\n';

  found=str.find("haystack");
  if (found!=std::string::npos)
    std::cout << "'haystack' also found at: " << found << '\n';

  found=str.find('.');
  if (found!=std::string::npos)
    std::cout << "Period found at: " << found << '\n';

  // let's replace the first needle:
  str.replace(str.find(str2),str2.length(),"preposition");
  std::cout << str << '\n';

  return 0;
}
//Notice how parameter pos is used to search for a second instance of the same search string.

Output:
first 'needle' found at: 14
second 'needle' found at: 44
'haystack' also found at: 30
Period found at: 51
There are two prepositions in this haystack with needles.

transformation between string and other data types

string and int

可以查看这篇博客

Other functions

string::erase

erase函数有三种用法：

• erase(pos,n); 删除从pos开始的n个字符，比如erase(0,1)就是删除第一个字符
• erase(position);删除position处的一个字符(position是个string类型的迭代器)
• erase(first,last);删除从first到last之间的字符（first和last都是迭代器）

Code sample:

// string::erase
#include <iostream>
#include <string>

int main ()
{
  std::string str ("This is an example sentence.");
  std::cout << str << '\n';
                                           // "This is an example sentence."
  str.erase (10,8);                        //            ^^^^^^^^
  std::cout << str << '\n';
                                           // "This is an sentence."
  str.erase (str.begin()+9);               //           ^
  std::cout << str << '\n';
                                           // "This is a sentence."
  str.erase (str.begin()+5, str.end()-9);  //       ^^^^^
  std::cout << str << '\n';
                                           // "This sentence."
  return 0;
}

trim

1. 自己定义

   Code sample:

   #include <iostream>
   #include <string>
   
   std::string& trim(std::string &);
   
   int main() 
   {
       std::string s = " Hello World!! ";
       std::cout << s << " size:" << s.size() << std::endl;
       std::cout << trim(s) << " size:" << trim(s).size() << std::endl;
   
       return 0;
   }
   
   std::string& trim(std::string &s) 
   {
       if (s.empty()) 
       {
           return s;
       }
   
       s.erase(0,s.find_first_not_of(" "));
       s.erase(s.find_last_not_of(" ") + 1);
       return s;
   }

2. 使用boost

   可以参考这篇博客，很强大的boost……。

Technique Summaries

Algorithm, String, C++