## float precision problem

Dec 18, 2011 at 10:59am

# float precision problem

I want to calculate distance with gps. I moved code from processing (java) to max patches.
Problem is that in processing i have those values for example:
dLat: -1.3315805E-7
dLon: 1.7476995E-7

which is like 0.00000017476995 if i’m correct.

In max the value is zero, and so after a few calculations i get nan (not a number).
What can i do?

:(

– Pasted Max Patch, click to expand. –

In case someone know’s processing, here is the proceesing code.

/*
//pc
testP: gps 5.910325 30. 51.988632 57. 28.372467

//keuken
testP: gps 5.91006 10. 51.988777 19. 29.984245
*/

void setup() {
// println(getDistance(51.988632, 5.910325, 51.988777, 5.91006)); // pc > keuken
// println(getDistance(51.988777, 5.910006, 51.988739, 5.910056));
/*

String[] tokens = split(lines[0], “t”);
float lon1 = float(tokens[2]);
float lat1 = float(tokens[4]);

for (int i = 1; i < 2; i++) { //lines.length-1

tokens = split(lines[i+1], “t”);
float lon2 = float(tokens[2]);
float lat2 = float(tokens[4]);

//println(lat2+”t”+lon2);
float d = getDistance(lon1, lat1, lon2, lat2);
println(d*1000);
}
*/
float d = getDistance(5.910161, 51.988705, 5.910171, 51.988697);
println(d*1000);

}

float getDistance(float lon1, float lat1, float lon2, float lat2) {
int R = 6371; // Radius of the earth in km

println(“dLat: “+dLat+”ndLon: “+dLon);

float a = sin(dLat/2) * sin(dLat/2) + cos(radians(lat1)) * cos(radians(lat2)) * sin(dLon/2) * sin(dLon/2);
float c = 2 * atan2(sqrt(a), sqrt(1-a));
float d = R * c; // Distance in km

return d;
}

#60728
Dec 18, 2011 at 11:35am

Hi
I can’t find the corresponding expression in your Processing code (maybe the last 5 lines?); but inside your getDistance subpatch, you are trying to find the sqrt of a negative number (on the right, above [atan2]); this generates a complex number, unrepresentable in Max. That’s where the “nan” is generated. Sorry if you already knew this.

http://stackoverflow.com/questions/347920/what-do-1-inf00-1-ind00-and-1-ind-mean

Brendan

#218588
Dec 18, 2011 at 11:50am

…..to get it to “work”, I inverted the result AFTER the sqrt calculation; probably messes up the result you’re expecting but here it is anyway:

– Pasted Max Patch, click to expand. –

Brendan

#218589
Dec 18, 2011 at 5:23pm

Max works with single-precision floating point values, which gives you approximately 6-digit precision in the mantissa and can handle exponents (base 10) of about +/-38.

This is all considerably less powerful than the double-precision floats used in Java, but should not be a limitation with the values you say you are using.

Square-roots of negative numbers will NaN no matter what representation you have. From a quick look, it seems that what you have implemented in your max patch is float c = 2 * atan2(sqrt(a), sqrt(0-a)); rather than float c = 2 * atan2(sqrt(a), sqrt(1-a));.

The problem is further exacerbated by your using send/receive pairs and, apparently, relying on left-to-right order. When you s/r, all bets on execution order are off.

The following is only the [p GetDistance] part of your patch, you’ll have to manually copy/paste it into your top-level patch.

– Pasted Max Patch, click to expand. –
#218590
Dec 20, 2011 at 12:32pm

Thanks peter,

i allready had it fixed with javascript which is much easier to read for such things (didn’t know before that javascript was possible).

But good to know it is possible.

#218591

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