Question

I’m not so sure about correct casts, here especially from unsigned int and #defines (whatever type that is) to double.

In this example

#define SPEEDSENSOR_EDGES_NUM 10

int speed_counter_left;

double result = speed_counter_left / SPEEDSENSOR_EDGES_NUM;

the result always is 0 when speed_counter_left is < 10.

So I put in some casts:

double result = (double) ((double) speed_counter_left / (double) SPEEDSENSOR_EDGES_NUM);

This is correct, but I think there are too many casts. How can I figure out the correct way with least casts?

Answer 1

Having one double operand should be enough:

double result = (double)speed_counter_left / SPEEDSENSOR_EDGES_NUM;

The cast “sticks” closest to speed_counter_left. So it’s essentially equivalent to:

double result = ((double)speed_counter_left) / SPEEDSENSOR_EDGES_NUM;

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and #defines (whatever type that is) to double.

The standard says:

6.4.4.1 – 5

The type of an integer constant is the first of the corresponding list
in which its value can be represented.

And (for decimal constants without suffixes) the list goes: int, long int, long long int.