I know the map() function has been discussed before and I have read the previous posts but no one has stated the obvious problem. The equation in the function stated at http://arduino.cc/en/Reference/Map

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;

}

is wrong. The example given:

val = map(val, 0, 1023, 0, 255);

In this example 0-1023 is mapped to 0-255. The equation gets it wrong by mapping 1023 numbers to 255 numbers, when it should be mapping 1024 numbers to 256 numbers.

0 through 1023 is 1024 numbers not 1023 : in_max - in_min : 1023-0=1023

0 through 255 is 256 numbers not 255 : out_max - out_min : 255-0=255

The corrected equation in the function should be:

long map(long x, long in_min, long in_max, long out_min, long out_max)

{

return (x - in_min) * (out_max - out_min + 1) / (in_max - in_min + 1) + out_min;

}

In the following simpler example it will be easier to see:

val = map(val, 0, 3, 0, 1);

In this example 0,1,2 and 3 is mapped to 0 and 1. Mapped correctly it should be:

0 maps to 0

1 maps to 0

2 maps to 1

3 maps to 1

Using the equation in the the function as it is now, it is mapped as such:

0 maps to 0

1 maps to 0

2 maps to 0

3 maps to 1

This is a common problem programmers have dealt with before. When dealing with arrays you can not determine array size by simply subtracting first position from last position you will always be off by 1. That is why you add 1 to get proper size. I know this is for the most part a minor problem, but if your project requires accurate results this could be a serious issue.