Simple math doing my head in - when is an int treated like an int?

Using Arduino Programming Questions
1

Hi there

I'm trying to do something simple - convert analogue pin input (0 - 1023) to byte equivalent (0 - 255).

All variables declared as int.

I do this (i.e. input value x 255/1023) and all starts going strange at about value 130.

// read potentiometer pin
potValue = analogRead(potPin);
Serial.print("Pin Value: ");
Serial.println(potValue);
// pot pin value will be from 0 - 1023
// convert this to 0 - 255
result = potValue * 255/1023;
Serial.print("Result: ");
Serial.println(result);

Pin Value: 128
Result: 31

Pin Value: 131
Result: -31

If I replace the "255 / 1023" with 0.25, then all works fine

result = potValue * 0.25; //255 / 1023;

Pin Value: 215
Result: 53

Now I understand that intermidiate int calcs are treated as ints, which may cause a problem with 255/1023, but 0.25 is no more an int than 255/1023?

Can anyone tell me what's up?

2

An int can represent any integer value between -32768 and +32767

Anything outside this range wraps around.

130*255/1023. Think about how that sum works.

130 * 255 = 33150

That's outside the amount that an int will store.

You should either use an "unsigned int" that can store between 0 and 65535, or a "long" which can store considerably more (we're talking billions here).

3

Is there a reason you are not dividing by 4 (in other words why 255 and not 256, 1023 and not 1024?)

result = potValue / 4 ;

or with shifting right:

result = potValue >> 2 ;

Or if the slight difference matters, there is a built-in function specifically for this task:

result := map (potValue, 0, 1023, 0, 255) ;
4

I'm trying to do something simple - convert analogue pin input (0 - 1023) to byte equivalent (0 - 255).

So why are you going about it in such a complicated way?
0..1023 represents a span of ten bits.
0..255 represents a span of eight bits.

1023 / 22 = ?

5 
result := map (potValue, 0, 1023, 0, 255) ;

Have we got our Pascal head on ? :wink:

6

Sorry, = not := (and its Spin, not Pascal, in fact!)

7

majenko:
130*255/1023. Think about how that sum works.

130 * 255 = 33150

That's outside the amount that an int will store.

Ah. Ok. :blush: Thanks.

Thanks for the input guys - as for the why not divide by 4, I saw the 1023 and decided it would not quite be 4. doh.

8

Is this behaviour (intermediate calcs being influenced by data type of variable) just Arduino, or is it C? Damn sure this has never happened to me in years of Java..?

9

Mostly C, I'd say, with a smidgin of AVR - it is all to do with the platform and the default size of an "int".

10

Mostly AVR. it's one of the few platforms where an int is only 16 bits.

11

davetapson:
Is this behaviour (intermediate calcs being influenced by data type of variable) just Arduino, or is it C? Damn sure this has never happened to me in years of Java..?

C/C++ have that issue everywhere... Just try this anywhere =)

int foo = 20;
int bar = 35;
float baz = 0.0;

baz = foo / bar;