in C++ : Given the std::string s = "copyme" how can I make a string b which is "copymecopymecopyme" or any generalized s + s + ... n times?
A possible solution would be a method/macro that does a for loop, but that would be horribly inefficient.
What is an efficient method of doing this string copy operation, eg as the previous author says in the style of 'string'*n in python or ruby.
a relatively efficient loop would be allocating strlen*n and then copying the chars over multiple times rather than for(...) {str+=copy_this_string;} which does multiple copies and allocates.
another solution is using a stringbuffer.
key: without using a macro. clarification: the 'char array' in the question is because in the original question, due to a single char in the string, all answers ignored the copying of a general stream and focused on using '.' since it had easily accessible methods such as the stream constructor.
"Duplicate" : How to repeat a string a variable number of times in C++? Except most the answers all used methods that only had a char repeated, eg the String Constructor and insert methods, which did not answer the original question and were unhelpful when I then looked for an efficient method.
If you're worried about the string '+=' operation being inefficient, one optimization you can make is to reserve memory in the target string container. This may avoid inefficient re-allocations of memory.
For example:
std::string s = "copyme";
std::string b;
target.reserve(s.length()*5);
for (int index = 0; index < 5; ++index)
{
b += s;
}
I checked the std::string implementation on my linux system and it appears this method will not cause re-allocation of the array as long as the initial reservation is large enough.
template<typename _CharT, typename _Traits, typename _Alloc>
basic_string<_CharT, _Traits, _Alloc>&
basic_string<_CharT, _Traits, _Alloc>::
append(const basic_string& __str)
{
const size_type __size = __str.size();
if (__size)
{
const size_type __len = __size + this->size();
if (__len > this->capacity() || _M_rep()->_M_is_shared())
this->reserve(__len);
_M_copy(_M_data() + this->size(), __str._M_data(), __size);
_M_rep()->_M_set_length_and_sharable(__len);
}
return *this;
}
A simple loop would do. If you are concerned about re-allocations, you can call std::string::reserve
on the copy string:
#include <string>
#include <iostream>
int main()
{
const int num + 5;
std::string s = "copyme";
std::string b;
b.reserve(s.size() * num); // reserve space for num* "copyme"
for (int i = 0; i < num; ++i) b += s;
std::cout << b << std::endl;
}
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.