C++ 相当于 python 的 expandtabs() function？

Question

I need to implement the C++ equivalent of the expandtabs() function. Can someone help me with converting this code to C++?

def expandtabs(string, n):
    result = ""
    pos = 0
    for char in string:
        if char == "\t":
            # instead of the tab character, append the
            # number of spaces to the next tab stop
            char = " " * (n - pos % n)
            pos = 0
        elif char == "\n":
            pos = 0
        else:
            pos += 1
        result += char
    return result

This is what I have:

std::string ExpandTabs(const std::string &str, int tabsize =4){

  std::string ReturnString = str;
  std::string result = " ";
  int pos = 0;

  for(std::string::size_type i = 0; i < ReturnString.size(); ++i) {
    if (ReturnString[i] == '\t'){
      int spaces = tabsize - pos % tabsize ;
      ReturnString.append(" ", spaces);
      pos = 0;
    }
    else{
      pos+=1;
    }

}
  return ReturnString;

Answer 1

You need to build up the string character by character. Currently you assign str to ReturnString at the start of the function and then append whatever spaces you decide are necessary to the end of the string, instead of in place of the tabs.

There are no doubt more idiomatic ways to achieve the same result, but a like for like conversion of the python might look like.

#include <iostream>
#include <string>

std::string expand_tabs(const std::string &str, int tabsize=4)
{
    std::string result = "";
    int pos = 0;

    for(char c: str)
    {
        if(c == '\t')
        {
            // append the spaces here.
            result.append(tabsize - pos % tabsize, ' ');
            pos = 0;
        } else
        {
            result += c;
            pos = (c == '\n') ? 0: pos + 1;
        }         
    }

    return result;
}

int main()
{
    std::cout << expand_tabs("i\tam\ta\tstring\twith\ttabs") << '\n';
    std::cout << expand_tabs("1\t2\t3\t4", 2) << '\n';
}

It basically steps through the input forwarding on any non tab characters to the result string, otherwise it adds the correct number of spaces to the result.

Output:

i   am  a   string  with    tabs
1 2 3 4

Answer 2

A straight translation of the python code is problematic, since char cannot be both a string and a single character, but apart from that it's straightforward:

std::string expandtabs(std::string const&str, std::string::size_type tabsize=8)
{
    std::string result;
    std::string::size_type pos = 0
    result.reserve(str.size());  // avoid re-allocations in case there are no tabs
    for(c : str)
        switch(c) {
        default:
            result += c;
            ++pos;
            break;
        case '\n':
            result += c;
            pos = 0;
            break;
        case '\t':
            result.append(tabsize - pos % tabsize,' ');
            pos = 0;
        }
    return result
}