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float to int question

 
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evsource



Joined: 21 Nov 2006
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float to int question
PostPosted: Tue Sep 01, 2009 6:23 pm     Reply with quote
Hi,

I know I can cast a float to an int, e.g:

Code:

int the_int;
float the_float;

the_float = 8.5;

the_int = (int) the_float;




... and the result will be "8".

However, if I have a float variable that is percentages, and want to do a fast conversion on a 18F chip to an int, how can I do that?

If I cast the float to an int, anything below 100% will be 0. Sure, I can multiply by 100, then cast to an int, but on a 18F chip at 40mHz, that's going to cost me about 35us ... too long.

Any suggestions?
vinniewryan



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PostPosted: Tue Sep 01, 2009 6:40 pm     Reply with quote
You could do
Code:
the_int=(100 - the_float)
which would return a value you can work with, if indeed 100% is the greatest value the_float will hold.
evsource



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PostPosted: Tue Sep 01, 2009 9:39 pm     Reply with quote
vinniewryan wrote:
You could do
Code:
the_int=(100 - the_float)
which would return a value you can work with, if indeed 100% is the greatest value the_float will hold.


Hmm... I'm not sure that's what I'm after.

e.g.
Code:

the_float = 0.08; // 8%

the_int = (int)(100 - the_float);


the_int will be "99" if I understand things correctly. The desired result is "8".

Just some background, the value that is a float needs to be transmitted over a serial communications line. I know I could do fixed-point arithmetic to avoid using the float altogether, but it seems like the right thing to do in code, using the float. But to transmit it, I only want to send one byte, since all I want in the transmitted data is 0-100%, in integer increments.

I was just curious if there was a way to get the float into an integer in less than 35us on a 40mHz 18F chip.
vinniewryan_guest
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PostPosted: Wed Sep 02, 2009 12:03 am     Reply with quote
Oh I see, I didn't realize you would be working with numbers less than 1. Now that I think about it, it was just an overall thoughtless answer.

I can't think of any other methods of rounding a percent to a whole number without multiplying. Sorry.
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