Bdtjtk

I'm doing a simple calculation with integers (on Arduino with ESP8266 12E), but I can't get the expected result and can't find the error. Can someone guide me?

#define A      200

#define B      A * 62

#define C      500



void setup() {

   Serial.begin(9600);

   Serial.println("");



   unsigned long aux = 0;



   aux = (B * 500) / C;    // (12400 * 500) / 500 = 12400

   Serial.printf("aux = %dn", aux);

   aux = aux * C;          // 12400 * 500 = 6200000

   Serial.printf("aux = %dn", aux);



   // ERROR: Should result in "500", but is resulting in "1922000"

   aux = aux / B;          // 6200000 / 12400 = 500



   Serial.printf("aux = %dn", aux); // It's printing "1922000"

}

In your #define of B you missed parenthesis (). Change your definition to:

#define B      (A * 62)

Without parenthesis you first divide 6200000 by 200 and then multiply result by 62, which is not what you intend.

Fully-parenthesizing macros (as noted in answer by dmz) solves one class of problem.

Another thing you should do is, in any arithmetic expression which involves literal constants, use the L suffix on at least one of the constants involved if there's any chance the result will exceed 32767 (the maximum guaranteed-representable value for int). The type of an arithmetic operation in C is based on the types of the operands of that operation only; the type of the variable to which the result is assigned is irrelevant.

(edit)
For example:

long q, r;

unsigned char v = 231;

unsigned char w = 197;



q = v * w * 5;

r = v * w * 5L;

q might contain 30927, since after the Usual Arithmetic Conversions (6.3.1.8) [in this case, the Integer Promotions (6.3.1.1-2)] all operands are of type int and it's possible for INT_MAX to be 32767 in which case each operation would be performed modulo 32768.

r will contain 227535, since the constant 5L is of type long and thus all operations in this expression will be performed on values of type long.

As explained in previous answers, fully parenthesizing the macros is the
standard solution to this problem in C. However, on Arduino you are
programming in C++, and in C++ it is considered good practice to replace
this usage of #define by explicit constants:

const int A = 200;

const int B = A * 62;

const int C = 500;

Not only this makes the initial problem go away, it also provides some
type safety: you can choose to give these constants other types (e.g.
long) if appropriate.