向量搜索

Question

基本上，我的字符串向量 vector<string> words 中有很多单词。

我需要创建一个函数，在整个向量中搜索所有带有“ly”的单词并返回它们，例如 (golly, helpfully, mostly, nearly)。

我如何使用std::find_if 函数来执行此操作，或者有其他方法可以执行此操作吗？

我还需要在我的向量中找到比7 字母长的单词，我是否仍将std::find_if 函数与>=7 一起使用？

Answer 1

首先，标准库中有一个更合适的算法，称为std::copy_if，而不是std::find_if（根据您的要求）。

其次，您需要根据不同的情况获得不同的单词列表。这听起来像是有一个 template function 包装了 std::copy_if 并且还提供了一种提供自定义比较（例如 lambda function）功能的方法。

因此我建议如下：

#include <algorithm> // std::copy_if
#include <iterator>  // std::cbegin, std::cend


template<typename Container, typename Predicate>
auto getElelmentsOf(const Container& container, const Predicate condition) /* noexcept */
{
    Container result;
    std::copy_if(std::cbegin(container), std::cend(container), std::back_inserter(result),
        condition);
    return result;
}

现在你可以写类似的东西

// all the words with "ly"
const auto words_with_ly = [](const auto& ele) {
    return ele.find(std::string{ "ly" }) != std::string::npos;
};
const auto elemtsOfLy = getElelmentsOf(words, words_with_ly);  // function call


// find words that are longer than 7 letters
const auto words_with_size_7_more = [](const auto& ele) { return ele.size() > 7; };
const auto elemtsOfsize7More = getElelmentsOf(words, words_with_size_7_more);  // function call

(See a Live Demo Online)

Answer 2

您可以使用std::copy_if 来获取满足某些条件的所有元素。

#include <iostream>
#include <vector>
#include <string>
#include <algorithm> // for std::copy_if
#include <iterator> // for std::back_inserter

using std::vector;
using std::string;

int main(void) {
    vector<string>words={
        "golly", "hoge", "lyric", "helpfully",
        "mostly", "abcdefg", "nearly", "terrible"
    };
    vector<string> res_ly, res_7;

    // get all words that contains "ly"
    std::copy_if(words.begin(), words.end(), std::back_inserter(res_ly),
        [](const string& x){ return x.find("ly") != string::npos; });

    // get all words that are longer than 7 letters
    std::copy_if(words.begin(), words.end(), std::back_inserter(res_7),
        [](const string& x){ return x.length() > 7; });

    // print what we got
    std::cout << "words with \"ly\":\n";
    for (const string& s : res_ly) std::cout << "  " << s << '\n';
    std::cout << "\nwords longer than 7 letters:\n";
    for (const string& s : res_7) std::cout << "  " << s << '\n';

    return 0;
}

输出：

words with "ly":
  golly
  lyric
  helpfully
  mostly
  nearly

words longer than 7 letters:
  helpfully
  terrible

如果你想使用std::find_if，你可以像这样重复搜索：

#include <iostream>
#include <vector>
#include <string>
#include <algorithm> // for std::find_if
#include <iterator> // for std::next

using std::vector;
using std::string;

int main(void) {
    vector<string>words={
        "golly", "hoge", "lyric", "helpfully",
        "mostly", "abcdefg", "nearly", "terrible"
    };
    vector<string> res_ly;

    // get all words that contains "ly"
    for (vector<string>::iterator start = words.begin(); ;) {
        vector<string>::iterator next = std::find_if(start, words.end(),
            [](const string& x){ return x.find("ly") != string::npos; });
        if (next == words.end()) {
            break;
        } else {
            res_ly.push_back(*next);
            start = std::next(next, 1);
        }
    }

    // print what we got
    std::cout << "words with \"ly\":\n";
    for (const string& s : res_ly) std::cout << "  " << s << '\n';

    return 0;
}

Answer 3

我可以建议以下解决方案。

#include <iostream>
#include <string>
#include <vector>
#include <iterator>
#include <algorithm>

std::vector<std::string> copy_strings( const std::vector<std::string> &v, const std::string &s )
{
    auto present = [&s]( const auto &item ) 
    {
        return item.find( s ) != std::string::npos; 
    };

    auto n = std::count_if( std::begin( v ), std::end( v ), present );
                            
    std::vector<std::string> result;
    result.reserve( n );
    
    std::copy_if( std::begin( v ), std::end( v ), 
                  std::back_inserter( result ),
                  present );
                  
    return result;                
}

int main() 
{
    std::vector<std::string> v =
    {
        "golly", "helpfully", "mostly", "nearly"
    };
    
    auto result = copy_strings( v, "ly" );
    
    for (const auto &item : result )
    {
        std::cout << item << ' ';
    }
    std::cout << '\n';
    
    return 0;
}

程序输出是

golly helpfully mostly nearly