The function map() -- the math behind it; does it work?

[MaxG]

This seems to be the math behind the map() function:

long map(long x, long in_min, long in_max, long out_min, long out_max)
{
  return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min;
}

I have renamed the variables to calculate the dimmer level of a light in relation to a lux value.

dimmer_level = (lux - lux_min) * (dim_max - dim_min) / (lux_max - lux_min) + dim_max

Or as code:

int main (void)
{
    int lux_min	=    0;
    int lux_max	= 2000;
    int dim_min	=    4;
    int dim_max	=   35;

    int lux = 2000;

    int dimmer_level = (lux - lux_min) * (dim_max - dim_min) / (lux_max - lux_min) + dim_max;

    printf("dimmer value: %d\n", dimmer_level);

    return 0;
}

However, my result does not stay within out limits.

What did I get wrong?
Any hints appreciated.

[MaxG]

Huh, found it!

The last value in my formula is dim_max; which should be dim_min.

[groundFungus]

The int data type will hold values from -32768 to 32767. Are any of the intermediate results of your calculations outside of that range?

See this thread >> Basic Calculation Results Wrong Value

[TomGeorge]

Hi, @MaxG

@MaxG

However, my result does not stay within out limits.

The map function uses y = mx + c.
It does not just work within your input and output limits, if you enter a value outside your expected input values, the function will output a value outside your expected output values.

You may need to look at this function to use in conjunction with your values.
https://www.arduino.cc/reference/en/language/functions/math/constrain/

Tom..

[MaxG]

Yes, I wanted to keep it simple and focus on the map function only.

I actually constrain the in put between min and max.

[jremington]

The point of having a function is that you don't have to rename variables or use the wrong data types, thus avoiding the sorts of mistakes that you made in doing so.