Fastest C++ way to convert float to string

Question

I have encountered problem of converting float to string where to_string is too slow for me as my data might involves few millions floats.

I already have solution on how to write those data out fast.

However, after solving that problem, I soon realized that the conversion of float to string is leaving a big impact.

So, is there any ideas or solution for this other than using other non standard library?

Answer 1

Here are some of the fastest algorithms for converting floating point numbers into decimal string representation:

• Grisu by Florian Loitsch: Printing Floating-Point Numbers Quickly and Accurately with Integers
• Ryū by Ulf Adams: Ryū: fast float-to-string conversion
• Schubfach by Raffaello Giulietti: The Schubfach way to render doubles
• Dragonbox by Junekey Jeon: Dragonbox: A New Floating-Point Binary-to-Decimal Conversion Algorithm

At the time of writing Dragonbox is the fastest of these methods, followed by Schubfach, then a variation of Grisu called Grisu-Exact (not to be confused with Grisu2 and Grisu3) and then Ryū:

An implementation of Dragonbox is available here. It is also included in the {fmt} library integrated into a high-level formatting API. For maximum performance you can use format_to with a stack-allocated buffer, for example:

fmt::memory_buffer buf;
fmt::format_to(buf, "{}", 4.2);
// buf.data() returns a pointer to the formatted data & buf.size() gives the size

Answer 2

An optimization that comes in mind is to not directly use to_string, which creates a new string every time you call it. You probably end up copying that string too, which is not so efficient.

What you could do is to allocate a char buffer big enough to store all the string representations that you need, then use printf

http://www.cplusplus.com/reference/cstdio/printf/

reusing the same buffer all the time. If you limit the precision of your floats to a fixed amount of decimals, you can compute the offset to which your float is represented in the array.

for example if we only had an array of values:

index = 1;
float f = value[index];
//corrresponding 6 chars float
const char* s = char_array[index*1];
//the representation will start at position 6, and it will be null terminated so you can use it as a string

for clarification your char_array will look like:

1.2000\02.4324\0...