如何插入大量空格以使其更具可读性？

Question

我想出了这个，因为 stackoverflow 上提供的其他示例是用 C# 编写的

string number_fmt(ulong n)
{
    // cout << "(" << n << ")" << endl;
    char s[128];
    sprintf(s, "%lu", n);
    string r(s);
    reverse(r.begin(), r.end());
    int space_inserted = 0;
    size_t how_many_spaces = r.length() / 3;

    if(r.length() % 3 != 0)
        how_many_spaces += 1;

    for(int i = 1; i < how_many_spaces; ++i)
    {
        r.insert(3 * i + space_inserted, " ");
        space_inserted += 1;
    }
    reverse(r.begin(), r.end());

    return r;
}

你知道更好的解决方案吗？

Answer 1

这个是不同的，但更好的是主观的。我认为它非常简洁明了，但它在做什么：

string number_fmt(unsigned long long n, char sep = ',') {
    stringstream fmt;
    fmt << n;
    string s = fmt.str();
    s.reserve(s.length() + s.length() / 3);

    // loop until the end of the string and use j to keep track of every
    // third loop starting taking into account the leading x digits (this probably
    // can be rewritten in terms of just i, but it seems more clear when you use
    // a seperate variable)
    for (int i = 0, j = 3 - s.length() % 3; i < s.length(); ++i, ++j)
        if (i != 0 && j % 3 == 0)
            s.insert(i++, 1, sep);

    return s;
}

像这样使用它

cout << number_fmt(43615091387465) << endl;

打印

43,615,091,387,465

Answer 2

如果“更好”意味着更高效，您应该：

1. 在输出字符串上使用reserve（你知道它的大小...）

2. 避免在字符串中间使用insert，因为每次这样做时都必须复制大部分字符串。

我会这样说（未经测试）：

std::string number_fmt (ulong n)
{
  std::ostringstream buff;
  buff << n;
  std::string without_spaces = buff.str ();
  std::string with_spaces;
  with_spaces.reserve ((without_spaces.size () * 4) / 3);
  std::size_t nb_inserted = 0;

  for (auto it = without_spaces.rbegin (); it != without_spaces.rend (); ++it)
  {
     if (nb_inserted % 3  ==  0  &&  nb_inserted != 0)
     {
         with_spaces.push_back (' ');
     }
     ++ nb_inserted;

     with_spaces.push_back (*it);   
  }
  std::reverse (with_spaces.begin (), with_spaces.end ());
  return with_spaces;
}

Answer 3

我不知道“更好”，但是这个版本使用std::locale等。

#include <iostream>
#include <locale>
#include <sstream>

template<class Char>
class MyFacet : public std::numpunct<Char> {
public:
  std::string do_grouping() const { return "\3"; }
  Char do_thousands_sep() const { return ' '; }
};

std::string number_fmt(unsigned long n)
{
  std::ostringstream oss;
  oss.imbue(std::locale(oss.getloc(), new MyFacet<char>));
  oss << n;
  return oss.str();
}

int main() {
  std::cout << number_fmt(123456789) << "\n";
}

---

编辑：当然，如果您的最终目标是在ostream 上打印值，您可以完全跳过将它们存储在string 中。

#include <iostream>
#include <locale>
#include <sstream>
#include <cwchar>

template <class Char>
class MyFacet : public std::numpunct<Char> {
public:
  std::string do_grouping() const { return "\3"; }
  Char do_thousands_sep() const { return ' '; }
};

int main(int ac, char **av) {
  using std::locale;
  using std::cout;

  // Show how it works to start with
  cout << 123456789 << "\n";

  // Switch it to spacey mode
  locale oldLoc =
    cout.imbue(locale(cout.getloc(), new MyFacet<char>));

  // How does it work now?
  cout << 456789123 << "\n";

  // You probably want to clean up after yourself
  cout.imbue(oldLoc);

  // Does it still work?
  cout << 789123456 << "\n";
}

Answer 4

诚然，如果想要获得最高效的版本并且不介意针对手头的情况对其进行专门化，那么使用本地 char 缓冲区会大有帮助。

#include <iostream>
#include <string>

std::string format(unsigned long long i) {
  char buffer[128]; // can be adapted more tightly with std::numeric_limits

  char* p = buffer + 128;
  *(--p) = '\0';

  unsigned char count = 0;
  while (i != 0) {
    *(--p) = '0' + (i % 10);
    i /= 10;

    if (++count == 3) { count = 0; *(--p) = ' '; }
  }

  return p;
}

int main() {
  std::cout << format(1234567890) << '\n';
}

在ideone行动中：

1 234 567 890

（关键点：数字打印，往后走）

Answer 5

这已由语言环境完成。

默认本地是“C”，不进行格式化。但您可以将其设置为当前特定语言的本地（由您的计算机设置定义，将当前本地设置为main 的第一行）。

int main()
{
    std::locale::global(std::locale("")); // Set the default local of the machine
                                          // Will be used by all streams.
                                          // The "" will find the machine specific local
                                          // and use that instead of the "C" locale
                                          // Note: The C local should only be used for programmers.

    // Alternatively you can imbue particular stream with the local
    // To achieve a localized effect
    // std::cout.imbue(std::locale(""));

    // Now all you do is print the number.
    std::cout << "123456789\n";  // This will print the number according to your local
}                                // For me US-en this is   123,456,789
                                 // Your may very.

如果你想明确地做某事，那么你可以在本地设置一个方面来打印数字。

#include <iostream>
#include <locale>
#include <string>


template<typename CharT>
struct Sep : public std::numpunct<CharT>
{
        virtual std::string do_grouping()      const   {return "\003";}
        virtual CharT       do_thousands_sep() const   {return ':';}
};

int main()
{
        std::cout.imbue(std::locale(std::cout.getloc(), new Sep <char>()));

        std::cout << 123456789 << "\n";   // this prints 123:456:789
}

Answer 6

不是很理想，但很小

QString str = QString::number(d);
for (int i = 3; str.size() > i; i += 4)
   str.insert(str.size() - i, ' ');