在字符串 C++ 中的字符之间添加空格

Question

I am looking for the most efficient way to add a space " " between every character in the string in C++.

So let's say I pass a string "123" and I want to get "1 2 3 " or "1 2 3" (it doesn't really matter).

Does anyone know the fastest way of doing such a thing?

My method so far looks like this...

    string output;
    for(int i = 0 ; i < s.length(); ++i) {
        output += s.substr(i, 1) + " ";
    }
    return output;

...but I'm just wondering if there's a better way to do it

Thanks

Answer 1

You can write this conveniently with the range-v3 library. So given:

namespace rs = ranges;
namespace rv = ranges::views;

std::string input = "123";

If you just want to print the string with interspersed spaces, you can do:

rs::copy(input | rv::intersperse(' '), 
         rs::ostream_iterator<char>(std::cout));

And if you want to store the result in a new string, you can do:

auto result = input | rv::intersperse(' ') | rs::to<std::string>;

Here's a demo .

I think this is about as efficient as it can reasonably get, but more importantly, it's very readable.

Answer 2

The inefficiency of your code is repeated re-allocation of the string.

You know upfront the size of the output , so you should allocated that, filled with spaces. Then you simply place all character in their new position:

std::string foo(const std::string& s) {
  std::string output(s.length()*2-1, ' ');
  for (int i = 0, j = 0; i < s.length(); ++i, j+=2) {
    output[j] = s[i];
  }
  return output;
}