map function issue ?

Projects Programming
1

Hello everybody,

I have a strange issue with the map function.

I have a value from 4000 to 12000 and I need to map it to 1 to 8, so 4000 is 1 and 12000 is 8.

here is my code:

int result = map(value, 4000, 12000, 1, 8);

and here is the result:

value: 2803 result : 0
value: 2803 result : 0
value: 2803 result : 0
value: 2857 result : 0
value: 2857 result : 0
value: 2857 result : 0
value : 3296 result : 1
value : 3296 result : 1
value : 5084 result : 1
value : 5084 result : 1
value : 5357 result : 2
value : 6000 result : 2
value : 6122 result : 2
value : 6382 result : 3
value : 6382 result : 3
value : 7317 result : 3
value : 7142 result : 3
value : 7500 result : 4
value : 7500 result : 4
value : 8333 result : 4
value : 8333 result : 4
value : 8823 result : 5
value : 8571 result : 4
value : 8571 result : 4
value : 8823 result : 5
value : 8823 result : 5
value : 9677 result : 5
value : 9677 result : 5
value : 10000 result : 6
value : 9677 result : 5
value : 10344 result : 6
value : 10000 result : 6
value : 10000 result : 6
value : 10714 result : 6
value : 11111 result : 7
value : 11538 result : 7
value : 11111 result : 7
value : 12000 result : 8
value : 12000 result : 8
value : 12000 result : 8

As you can see the upper limit is good, when value reach 12000 it is map to 8.
But the lower limit is not good, value is map to 1 around 3200 instead of 4000.

Do you have any explain for this result ? Did I miss something ?

3

Thank you :slight_smile:

I was expected a reply like this and the two tests is my actual workaround, I'll let it like this.

4

There is also the constrain function.

5

Delta_G:
It's a consequence of how the map function is written using integer math.

Not really. It's a consequence of the OP stating that the input range would be 4000 to 12000, then testing it with a value of 2803. The results are entirely as expected, it's just a shame that a lot of the tutorials fail to mention that the result will not be constrained.

6

just some thought

Sometimes one must look through the map function

output = map(value, 4000, 12000, 1, 8);

is almost

output = value/1000 - 3;

Check for a FastMap Class - http://forum.arduino.cc/index.php?topic=276194.0

update: added code tags

7

Sometimes one must look through the map function

output = map(value, 4000, 12000, 1, 8);

is almost

output = value/1000 - 3;

If the upper limit is a smiley face...

8

Oops:)

9

Yes, the map fuction is not well described in the tutorial or on the arduino reference sheet.

But with constraint or some tests it works well :slight_smile:

10

M4vrick:
I have a strange issue with the map function.

I have a value from 4000 to 12000 and I need to map it to 1 to 8, so 4000 is 1 and 12000 is 8.

here is my code:

int result = map(value, 4000, 12000, 1, 8);

The 'map()' function is one of the worst functions in the Arduino core library.
Undocumented function.
Non-intuitive function.
Example programs show how to use this function the wrong way.

So nearly everybody seems to use the function the wrong way.

Correct would be:

  • Include the lower bound into the range
  • but DO NOT include the upper bound into the range

So the correct usage in your case (map 4000 to value 1, map 12000 to value 8 ) would be indeed:

   int result = map(value, 4000, 12001, 1, 9);

lower bounds (4000 and 1): included in range
upper bounds (12001 and 9): NOT included in intended mapping range by one

11

We know it's bad, thats why there was a whole discussion about improving it.

http://forum.arduino.cc/index.php?topic=49982.0#bot