Converting to ASCII in C

Question

Using a microcontroller (PIC18F4580), I need to collect data and send it to an SD card for later analysis. The data it collects will have values between 0 and 1023, or 0x0 a

Answer 1

If the values are correctly in range (0..1023), then your last conversion is unnecessarily wasteful on the divisions; the last line could be replaced with:

temp[3] = 1023 / 1000;

or even:

temp[3] = 1023 >= 1000;

Since division is repeated subtraction, but you have a very special case (not a general case) division to deal with, I'd be tempted to compare the timings for the following code with the division version. I note that you put the digits into the string in 'reverse order' - the least significant digit goes in temp[0] and the most in temp[4]. Also, there is no chance of null-terminating the string given the storage. This code uses a table of 8 bytes of static data - considerably less than many of the other solutions.

void convert_to_ascii(int value, char *temp)
{
    static const short subtractors[] = { 1000, 100, 10, 1 };
    int i;
    for (i = 0; i < 4; i++)
    {
        int n = 0;
        while (value >= subtractors[i])
        {
            n++;
            value -= subtractors[i];
        }
        temp[3-i] = n + '0';
    }
}

Performance testing - Intel x86_64 Core 2 Duo 3.06 GHz (MacOS X 10.6.4)

This platform is probably not representative of your microcontroller, but the test shows that on this platform, the subtraction is considerably slower than the division.

void convert_by_division(int value, char *temp)
{
    temp[0] = (value %    10)        + '0';
    temp[1] = (value %   100) /   10 + '0';
    temp[2] = (value %  1000) /  100 + '0';
    temp[3] = (value % 10000) / 1000 + '0';
}

void convert_by_subtraction(int value, char *temp)
{
    static const short subtractors[] = { 1000, 100, 10, 1 };
    int i;
    for (i = 0; i < 4; i++)
    {
        int n = 0;
        while (value >= subtractors[i])
        {
            n++;
            value -= subtractors[i];
        }
        temp[3-i] = n + '0';
    }
}

#include 
#include 
#include 

static void time_convertor(const char *tag, void (*function)(void))
{
    int r;
    Clock ck;
    char buffer[32];

    clk_init(&ck);
    clk_start(&ck);
    for (r = 0; r < 10000; r++)
        (*function)();
    clk_stop(&ck);
    printf("%s: %12s\n", tag, clk_elapsed_us(&ck, buffer, sizeof(buffer)));
}

static void using_subtraction(void)
{
    int i;
    for (i = 0; i < 1024; i++)
    {
        char temp1[4];
        convert_by_subtraction(i, temp1);
    }
}

static void using_division(void)
{
    int i;
    for (i = 0; i < 1024; i++)
    {
        char temp1[4];
        convert_by_division(i, temp1);
    }
}

int main()
{
    int i;

    for (i = 0; i < 1024; i++)
    {
        char temp1[4];
        char temp2[4];
        convert_by_subtraction(i, temp1);
        convert_by_division(i, temp2);
        if (memcmp(temp1, temp2, 4) != 0)
            printf("!!DIFFERENCE!! ");
        printf("%4d: %.4s %.4s\n", i, temp1, temp2);
    }

    time_convertor("Using division   ", using_division);
    time_convertor("Using subtraction", using_subtraction);

    time_convertor("Using division   ", using_division);
    time_convertor("Using subtraction", using_subtraction);

    time_convertor("Using division   ", using_division);
    time_convertor("Using subtraction", using_subtraction);

    time_convertor("Using division   ", using_division);
    time_convertor("Using subtraction", using_subtraction);

    return 0;
}

Compiling with GCC 4.5.1, and working in 32-bit, the average timings were (optimization '-O'):

• 0.13 seconds using division
• 0.65 seconds using subtraction

Compiling and working in 64-bit, the average timings were:

• 0.13 seconds using division
• 0.48 seconds using subtraction

Clearly, on this machine, using subtraction is not a winning proposition. You would have to measure on your machine to make a decision. And removing the modulo 10000 operation will only skew results in favour of the division (it knocks about 0.02 seconds off the time with division when replaced with the comparison; that's a 15% saving and worth having).