Vector searching - c++

Question

Basically I have a load of words in my string vector vector<string> words.
I need to make a function that searches for all the words with "ly" throughout my vector and return them, for example (golly, helpfully, mostly, nearly).
How do I use the std::find_if function to do this or is there any other way that I can do this?
I also need to find words that are longer than 7 letters in my vector, do I still use the std::find_if function with >=7 or something else?

Answer 1

First of all, there is a more appropriate algorithm in the standard library called std::copy_if than the std::find_if (for what you have asked).
Secondly, you need to get a different list of words asper different cases. This sounds like having a template function which wraps the std::copy_if and also provide a way to give the custom compare (e.g. a lambda function) functionalities.
Therefore I would suggest something like as follows:
#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;
}
Now you could write something like
// 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

You can use std::copy_if to get all elements that satisfy some conditions.
#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;
}
Output:
words with "ly":
golly
lyric
helpfully
mostly
nearly
words longer than 7 letters:
helpfully
terrible
If you want to use std::find_if, you can repeat searching like this:
#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

I could suggest the following solution.
#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;
}
The program output is
golly helpfully mostly nearly