How can I improve formatting number with commas performance?

Question

I'm using the following method to format a number with commas:

template<class T>
static std::string FormatNumberWithCommas(T value, int numberOfDecimalPlaces = 0)
{
    std::stringstream ss;
    ss.imbue(std::locale(""));
    ss.precision(numberOfDecimalPlaces);
    ss << std::fixed << value;
    return ss.str();
}

Profiling has show this method to take a significant amount of time relative to other code. Specifically the profiler has identified the line:

ss.imbue(std::locale(""));

And within that I believe it is the std::locale("") that is taking long. How can I improve the performance of this method? If it requires using something other than stringstream or doing something semi-hacky in this particular method I'm open to it.

Beals

Cache the result? (Assuming you don't expect the locale to change while your program is running. (If that can even happen.)) – Rexferd

Hooray for profiling! :) – Cleasta

Answer 1

You could start by making the string stream a static variable:

{
  static std::stringstream ss;
  static bool ss_init = false;
  static std::string emtpy_string;
  if (!ss_init) { ss.imbue(std::locale("")); ss_init = true; }

  ss.str(empty_string);
  // ...
}

If that's still a bottleneck, you could look at an alternative formatting library like fastformat.

Gillie

ah nice.. since this method is in a class I was thinking of using a static class member, didn't even think of static local variable thanks – Beals

In a class you can just make the stream an ordinary member variable (not static) and have the constructor set the locale. (If you made it static, you'd have to figure out a way to initialize it, probably something similar to what I did with the static bool.) – Gillie

@okorz001 It is true for c++03, for c++11 the static locals are guaranteed by runtime in a thread-safe manner. – Oskar

Answer 2

1 using namespace std;

2 template <typename T>
3 string AddCommas(T data);

4 template <>
5 string AddCommas<int>(int data)
6 { 
7   stringstream ss; ss << data; string s = ss.str();
8   if (s.length() > 3)
9       for (int i = s.length()-3; i > 0; i -= 3)
10          s.insert(i,",");
11  return s;
12 }

You can use the same code for long. Double is a little trickier.

13 template <>
14 string AddCommas<double>(double data)
15 {
16  stringstream ss; ss << fixed << data; string s = ss.str();
17  string intPart, decPart = "";
18  int pos = s.find('.');
19  if (pos != string::npos) {
20      intPart = s.substr(0,pos);
21      decPart = s.substr(pos,pos-s.length());
22      //remove trailing zeros
23      for (int i = decPart.length()-1; i > 0; i--) {
24          if (decPart[i] == '0') decPart.erase(i);
25          else break;
26      }
27      // remove decimal point if no decimals
28      if (decPart.length() == 1) decPart = "";
29  }
30  else intPart = s;
31  //insert commas
32  if (intPart.length() > 3) {
33      for (int i = intPart.length()-3; i > 0; i -= 3) intPart.insert(i,",");
34  }
35  s = intPart + decPart;
36  return s;
37 }

You can make one for float. The only problem arises if you want to set precision. Then you have to just make a stand alone function and add a parameter for precision. Also you would need to change some of the code:

ss << fixed << setprecision(precision) << data; //...

As well as remove lines 22 through 28.

Roland