convert time into millisecond in C

Question

I have a list of data formatted as

08:01:00.064

Notation: hour,minute,second and millisecond

I need convert them into the notation of: second.millisecond.

Please find my code below,

float calculateTime(char *line) {

    int hour;
    int minute;
    int second;
    int millisecond;
    //transfer from char to int
    hour = ((line[0] - '0')*10 + (line[1] - '0'))*3600;
    minute = ((line[3] - '0')*10 + (line[4] - '0'))*60;
    second = (line[6] - '0')*10 + (line[7] - '0');
    millisecond = (line[9] - '0') *100 + (line[10] - '0') *10 + (line[11] - '0');

    float time;

    time = hour + minute + second + millisecond *0.001;

    return (time);

}

There is a problem with my code that the milliseconds will be inaccurate. I think it's due to the float format when calculating.

Result below,

37831.945312 source millisecond = 944

The output I expected was

37831.945312

Answer 1

A typical float (IEEE754 32 bit) will only be accurate to about 7 significant digits. You exceed that.

A solution? Use a double instead. That will give you about 15 significant figures. But note that the result may well still not be exact . That's the nature of floating point arithmetic.

A decent solution? Use a stuct instead containing the time values. There's one called tm in <time.h> .

Answer 2

Your problem lies with the lack of precision in the 32 bit float type. Use type double instead for the time variable. It has the precision you require.