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C++ sprintf float Decimal Degrees to Degrees Minutes Seconds


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#1 CyberMike   Banned   -  Reputation: 120

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Posted 17 January 2013 - 05:28 AM

Hi

 how to format a float alike 50.3502 to show degrees minutes:

 

    sprintf(bufflon,"Longitude: %2.4f\n", lon);

 

Same as:

http://transition.fcc.gov/mb/audio/bickel/DDDMMSS-decimal.html

 

basically multiply the .3502 with 60

Many thanks

 



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#2 Brother Bob   Moderators   -  Reputation: 9467

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Posted 17 January 2013 - 05:37 AM

Round the value to an integer to get the integer part of the angle. Then subtract that from the real angle to get the decimal part, and multiply by 60 to convert to minutes.

float degrees = floor(lon);
float minutes = (lon - degrees) * 60;
 
sprintf(bufflon,"Longitude: %.0f degrees, %f minutes\n", degrees, minutes);



#3 Bacterius   Crossbones+   -  Reputation: 11533

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Posted 17 January 2013 - 05:43 AM

Create yourself a function for convenience:

float frac(float x) { return x - floor(x); }

 

Template it if you want, etc..


“If I understand the standard right it is legal and safe to do this but the resulting value could be anything.”


#4 Milcho   Crossbones+   -  Reputation: 1177

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Posted 17 January 2013 - 05:45 AM

This should work, in c++, for degree, minutes and second (just ignore the last part if you don't need seconds)

float  angle = 50.3502; // or whatever value
int degree = (int)angle;
int minutes = (int) ((angle - (float)degree) * 60.f);
int seconds = (int)(angle - (float)degree - (float)minutes/60.f) * 60.f * 60.f); // ignore this line if you don't need seconds

You don't strictly need all the explicit conversions, but I like to make sure that values are treated as they should be.

Note this will give you only whole values of degree/minutes/seconds. Your specific value (50.3502) actually ends up having a fraction of a second.



#5 CyberMike   Banned   -  Reputation: 120

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Posted 18 January 2013 - 03:29 AM

Thanks Milcho and Brother Bob. The seconds from Milcho don't work and I cannot figure the error. Also I would like to write E or W:

int seconds=?

sprintf (bufflon, "Longitude: "(lon<0?"W ":"E ")""%d*%d.%d", degree, minutes,seconds);

 

Many thanks again



#6 rip-off   Moderators   -  Reputation: 9787

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Posted 18 January 2013 - 04:07 AM

The seconds from Milcho don't work and I cannot figure the error.

We aren't psychic, so please tell us exactly what you mean by a statement like "don't work":

  • Does it compile? Post the compiler error message in full.

  • Does it crash at runtime? Post any error message you get.

  • Does it behave unexpectedly? Post what behaviour you expected, and the observed behaviour.

  • Do demons escape your computer? Post the spells you used to banish them.

Regardless of how it isn't working, it is always good to post the code you are using. Sometimes there are subtle differences between the code someone suggested and the code you ended up writing. Better still, prepare a minimal program demonstrating the erroneous behaviour. Smaller programs are easier to post, to understand and to debug than large programs.

 

Also I would like to write E or W:

You can only concatenate string literals like that if they are known at compile time. Why not make the direction an argument too:

char direction = (lon < 0 ? 'W' : 'E');
sprintf (bufflon, "Longitude: %c %d*%d.%d", direction, degree, minutes,seconds);



#7 CyberMike   Banned   -  Reputation: 120

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Posted 18 January 2013 - 04:50 AM

Thanks Rip-off, now for seconds it's probably a bracket problem?

 

    int degree = (int)lon;
    int minutes = (int) ((lon - (float)degree) * 60.f);
    int seconds = (int)(lon - (float)degree - (float)minutes * 60.f * 60.f); // tried all kind of variants but all were different to the output as in the above site.

 

Thanks again for help with seconds.



#8 Milcho   Crossbones+   -  Reputation: 1177

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Posted 18 January 2013 - 05:18 AM

My bad, my original post was missing a parenthasis in the seconds column. The way you corrected that however won't give you the right results. Try this: (this time I tested this by compiling with gcc):

int degree = (int)lon;
int minutes = (int) ( (lon - (float)degree) * 60.f);
int seconds = (int) ( (lon - (float)degree - (float)minutes / 60.f) * 60.f * 60.f );

I tested the above to compare with the site - and it works fine, but due to the fact that it's using floats and not doubles there's some accuracy lost that sometimes gave me a result that was a second off. In order to fix this, you would have to make sure you store your 'lon' variable as a double, not a float, and use this version:

int degree = (int)lon;
int minutes = (int) ( (lon - (double)degree) * 60.0);
int seconds = (int) ( (lon - (double)degree - (double)minutes / 60.0) * 60.0 * 60.0 );

- this will also work with your lon variable being stored as a 'float' - but it won't give any improvements unless you store the original variable as a double (and no, converting from float to double right before calculation won't help either).






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