Rounding with significant digits

Question

In Xcode /Objective-C for the iPhone.

I have a float with the value 0.00004876544. How would I get it to display to two decimal places after the first significant number?

For example, 0.00004876544 would read 0.000049.

Would this work: http://stackoverflow.com/questions/202302/rounding-to-an-arbitrary-number-of-significant-digits — TreyA, Jul 19 '11 at 17:16
@Tom Are you sure? The algorithm in the accepted answer looks good, you'd just have to convert it to C and it should work fine. — Nate Thorn, Jul 19 '11 at 17:27
@Nate really, I just looked at the examples and it didn't seem to apply. I'm not very mathematical. and wouldn't know how to convert this to objective-c. any help? — Tom Kelly, Jul 19 '11 at 17:40
possible duplicate of [How do I "round" a number to end in certain digits?](http://stackoverflow.com/questions/2012266/how-do-i-round-a-number-to-end-in-certain-digits) — jscs, Jul 19 '11 at 18:18
There's also this question from a few hours ago: http://stackoverflow.com/questions/6749649/trim-a-floating-point-value — jscs, Jul 19 '11 at 18:25
@Josh Neither of those are talking about *significant* digits. — Nate Thorn, Jul 19 '11 at 18:48

score 4 · Accepted Answer · edited May 23 '17 at 12:34

I didn't run this through a compiler to double-check it, but here's the basic jist of the algorithm (converted from the answer to this question):

-(float) round:(float)num toSignificantFigures:(int)n {
    if(num == 0) {
        return 0;
    }

    double d = ceil(log10(num < 0 ? -num: num));
    int power = n - (int) d;

    double magnitude = pow(10, power);
    long shifted = round(num*magnitude);
    return shifted/magnitude;
}

The important thing to remember is that Objective-C is a superset of C, so anything that is valid in C is also valid in Objective-C. This method uses C functions defined in math.h.

Rounding with significant digits

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