How to copy a char array / String a variable number of times C++

Question

Question:

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?

possible solutions

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, e.g. 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.

SO clarifications

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, e.g. the String Constructor and insert methods, which did not answer the original question and were unhelpful when I then looked for an efficient method.

How would a loop be inefficient? How else could it possibly be implemented? — Carl Norum, Jun 28 '13 at 17:21
Don't be angry when everyone gives you the actual answer and it isn't what you hoped for. — Captain Skyhawk, Jun 28 '13 at 17:33
Regarding your recent edit - do you really think that `'string'*n` is not a loop in its implementation? — Carl Norum, Jun 28 '13 at 17:35
both of you used +=. tell me when that has ever been efficient and I will upvote your answers. @CaptainSkyhawk — , Jun 28 '13 at 17:35
yes because queues should recopy their values every time you append, that's efficient. — , Jun 28 '13 at 17:41
I don't see why this question was at -2 points; I +1'ed it as this is exactly a question I had a moment before, and the accepted answer seems to be very valuable. — akavel, Aug 18 '14 at 11:05

KyleL · Accepted Answer · 2013-06-28T17:42:06.110

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;
}

used for copying into it rather than copying from it. but yes this is more efficient as it reduces allocations +1. — , Jun 28 '13 at 17:29

juanchopanza · Answer 2 · 2013-06-28T17:35:17.047

1

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;
}

edited Jun 28 '13 at 17:35

answered Jun 28 '13 at 17:20

juanchopanza

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@EiyrioüvonKauyf what is the backend array? State your requirements clearly. – juanchopanza Jun 28 '13 at 17:22

How to copy a char array / String a variable number of times C++

Question:

possible solutions

SO clarifications

2 Answers2