How to delete a string based on spaces - c++

Question

I have a std::string
I want to keep the string after two spaces like in Newa and Newb
std::string a = "Command send SET Command comes here";
std::string b = "Command GET Command comes here";
std::string Newa = "SET Command comes here";
std::string Newb = "Command comes here";
what comes to my mind is that i can do std::string::find(' ') two times and use std::string::substr to get the desired result.
Can we do it in more refined manner.

Answer 1

The following is a more generalized approach. find_n returns an iterator past the element that matches the n-th element that needs to be searched for. It can be used to split after two spaces, three spaces, etc. You can in fact use it as a building block for other algorithms. The split function will return the input string in case the string contains less than two spaces.
#include <iostream>
template<class InputIt, class T>
InputIt find_n(InputIt first, InputIt last, const T& value, size_t n)
{
size_t count{0};
while (first != last && count < n) {
if (*first++ == value) ++count;
}
return first;
}
std::string split_after_two_spaces(const std::string& s)
{
const auto it{find_n(s.begin(), s.end(), ' ', 2)};
if (it != s.end()) {
return std::string(it, s.end());
}
return s;
}
int main()
{
const std::string a = "Command send SET Command comes here";
const std::string b = "Command GET Command comes here";
const std::string c = "Command GET";
std::cout << split_after_two_spaces(a) << '\n';
std::cout << split_after_two_spaces(b) << '\n';
std::cout << split_after_two_spaces(c) << '\n';
return 0;
}
I wanted to verify how much worse this approach would be in terms of performance compared to the straightforward double find() and substr() approach, and it turns out to be a bit faster for small strings, and it is slower for longer input strings. std::string::find() is going to be faster than std::find since it is likely to be optimized to deal with strings specifically.
Short string benchmark: http://quick-bench.com/sgKeT333zoXYBQS_1EXFzprCNrY
Longer string benchmark: http://quick-bench.com/FEhPTU4YfDPvemWMqgg4oTHZlLU
Update: The following implementation of find_n is more efficient, but also a bit more complex unfortunately. It has nicer semantics in the sense that it returns an iterator to the n-th matching element, instead of an iterator one past the n-th matching element.
template<class InputIt, class T>
InputIt find_n(InputIt first, InputIt last, const T& value, size_t n)
{
if (first != last && n > 0)
{
size_t count{0};
do
{
first = std::find(first, last, value);
}
while (first != last && ++count < n);
}
return first;
}

Answer 2

There's no need to construct sstream, you can just use find() function twice.
The function below can remove n first words from your string (based on spaces, but it can also be parametrized). All you need is to find the first space occurence using find, replace input string with a substring (starting from a next character after that space) and repeat the procedure depending on number of words you want to remove.
#include <iostream>
#include <string>
std::string removeWords(std::string s, int n) {
for(int i=0; i<n; ++i) {
const auto spaceIdx = s.find(' ');
s = s.substr(spaceIdx+1, s.length());
}
return s;
}
int main() {
std::cout << removeWords("Command send SET Command comes here", 2) << '\n';
std::cout << removeWords("Command GET Command comes here", 2) << '\n';
return 0;
}

Answer 3

You can try this, with the help of sstream and getline:
#include <iostream>
#include <sstream>
#include <string>
using namespace std;
int main() {
string a = "Command send SET Command comes here";
stringstream ss(a);
string w1, w2, w3, Newa;
ss >> w1 >> w2 >> w3;
getline(ss, Newa); //get rest of the string!
cout << w3 + Newa << endl;
return 0;
}
"SET Command comes here"

Answer 4

Hm, maybe I have a gross misunderstanding. I see many lines of descriptions, many lines of code, votings and an accepted answer.
But normally such text replacements can always be done with a one-liner. There is a dedicated function available for such tasks: std::regex_replace.
There are no requirements regarding performance, so I would recommend to use the dedicated function. Please see the following one-liner:
#include <iostream>
#include <iterator>
#include <string>
#include <regex>
int main()
{
std::string a{"Command send SET Command comes here"};
std::regex_replace(std::ostreambuf_iterator<char>(std::cout), a.begin(), a.end(), std::regex(R"(.*? .*? (.*$))"), "$1");
return 0;
}
I guess that because if its simplicity, further explanaions are not needed.