What is the most efficient way to format strings with rounding and thousand separator in C?

Question

So I have an inner loop situation with a buffer of floating point and integer values that are to be copied over to a another buffer in string format.

What are my alternatives to round and insert a thousand separator when formatting strings? Whatever the approach I end up using, it has to be flexible enough in permitting different formats. Also, because this is a inner loop scenario, I want to optimize any solution as far as possible.

It would seem locale.h is one way to do it. But in that case, how can I setup customized locales, and how do I actually use them? Or is there a better alternative altogether? If this is a noob question please just point me in the right direction.

EDIT:

Here are a few examples to clarify:

1000 gives 1,000 (If I want to use , as thousand separator)

1000 gives 1 000 (If I want to use space as thousand separator)

1000.123 gives 1,000.1 (If I want to round to one digit and use , as thousand separator)

0 gives `` (If I want to show zero as a blank string)

I am on a POSIX system btw...

Is C++ okay? Or does it need to be plain-old C? – csd May 08 '13 at 14:13 — csd, May 08 '13 at 14:13
@csd: Sorry, C only... – c00kiemonster May 08 '13 at 14:16 — c00kiemonster, May 08 '13 at 14:16

score 3 · Accepted Answer · answered May 08 '13 at 14:32

3

You can try to set a locale using setlocale and use printf with the ' flag and a precision value for rounding. Whether this will work, depends on your C library.

See the following program:

#include <locale.h>
#include <stdio.h>

int main()
{
    double value = 1234567890.123;

    if (!setlocale(LC_ALL, "en_US.utf8")) {
        fprintf(stderr, "Locale not found.\n");
        return 1;
    }

    printf("%'.0f\n", value);
    printf("%'.1f\n", value);

    return 0;
}

On my Ubuntu system, the output is:

1,234,567,890
1,234,567,890.1

answered May 08 '13 at 14:32

nwellnhof

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It simply enables the thousands' separator of the current locale. – nwellnhof May 08 '13 at 14:35
is it possible to change the separetor? using space instead – MOHAMED May 08 '13 at 14:40
1

Not really. You could try to find a locale the uses space as thousands' separator. The French locale usually uses a space character, but it uses comma as decimal separator. If you want a portable solution, you have to write your own formatting functions. – nwellnhof May 08 '13 at 14:56

What is the most efficient way to format strings with rounding and thousand separator in C?

1 Answers1