Converting knots to mph using map() function

Forum 2005-2010 (read only) Software Syntax & Programs
1

I have a question, I'm parsing NMEA data and would like to convert the SOG in knots to mph. I want the data to be in integer format (don't need anything past the decimal point) and thought about using the map function map(X,0,200,0,230). Am I correct in assuming this is a very simple way to do the conversion or am I missing something? Is there an easier way?

Wayne

2

Multiplying by 2.3 would be faster, but map will work.

3

It should work well so long as you're not taveling in a fast jet. At about 1141 MPH your calculated value will start to diverge from the actual value.

You should read through this...
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1257716581

4

I think you mean 1.151, but since you mention it, if I multiple an integer is the resultant value still an integer or is reclassified as a float?

5

It is an integer of the largest type in the expression after promoting to int...

Multiply two bytes - result is a byte.
Multiply two bytes - result is an int.

Multiply a byte and a short - result is an int.

Multiply a byte and a long - result is a long.

[Corrected byte times byte. The result is an int. Sorry about the mistake.]

6 
#include <iostream>
#define k  6076
#define m  5280

using namespace std;

int main()
{
int kts = 25;

int mph;

mph = int( ( (long) kts * (long) k)/ (long) m); 
      cout << mph <<"\n";
      return 0;
}

prints 28

i compiled and ran it with g++ rather than on Arduino

edit correct result with floats is 28.76----

7

And here's how you would do rounding to the nearest mph:

#include <iostream>
#define k 6076L
#define m 5280L
#define half_m 2640L

using namespace std;

int main()
{
int kts = 25;
int mph;

mph = (int)((((long)kts * k) + half_m) / m);
        cout << mph << "\n";
        return 0;
}

Regards,

-Mike

8

Mike i cut and pasted that, compiled and ran it and it prints 28.

(6078*25 +2640)/5280 = 29.27840909091

changed it to this.

#include <iostream>
#define k 6076L
#define m 5280L
#define half_m 2640L

using namespace std;

int main()
{
      int kts = 25;
      int mph;
      long tmp;

      tmp = (long)kts * k;
      cout << tmp <<"\n";

      tmp = tmp + half_m;
      cout << tmp <<"\n";

      mph = (int)(tmp / m);
      cout << mph << "\n";
      return 0;
}

prints:
151900
154540
29

this---

#include <iostream>
#define k 6076L
#define m 5280L
#define half_m 2640L

using namespace std;

int main()
{
int kts = 25;
int mph;

mph = (int)(  ( (long)kts * k + half_m ) / m);
        cout << mph << "\n";
        return 0;
}

prints 29

I sure couldn't see what was wrong until i did the intermediate prints. Too many (((( for me to sight read.