Using the map() function with limits?

[richierich1]

Hi

I'm wondering if it's possible to put limits into the map fanuction.

If I use this;

val = map(average, 0, 1023, 15, 255);

The output will look like this,

However what I really need is this

I have modified the map like to look like this,

val = map(average, 0-400 , 1023, 15, 255);

and to my surprise it verifys but I can't actually test it yet.
Will it work in a Nano or do I need to set the lower limit another way?

Many thanks, Rich.

[UKHeliBob]

The code will evaluate the parameter values before using them in the map() so what you have written is the same as

val = map(average, -400 , 1023, 15, 255);

Is that what you actually want ? I suspect not

Take a look at the constrain() function if you want to limit the range of the output of map()

[jfjlaros]

Hi @richierich1, welcome to the forum.

Perhaps you mean something like this?

val = max(15, map(average, 400, 1023, 15, 255));

[richierich1]

Two good suggestions striaght away, and from reading the documentation both look like they'd work just fine.
Many thanks for swifting putting me back on the right track.

[robtillaart]

@richierich1

Have a look at my fastMap library. It is developed for floats and the performance for integers is worse on some platforms.

It implements exactly the functions you need: lowerConstrainedMap().
Give it a try

GitHub - RobTillaart/FastMap: Arduino library for Fast mapping and constraining

[alto777]

Indeed lowerConstrainedMap from @robtillaart's library is perfect.

Saves me the time of saying this can be easily programmed. Here's what the function looks like

float FastMap::lowerConstrainedMap(float value)
{
    if (value <= _in_min) return _out_min;
    return this->map(value);
}

@richierich1 can do the same thing "by hand". No objection to libraries, but there are things that should just be taken in stride sometimes; eveyone should know how sling a few lines of code

    int value = analogRead(A0);   // for example

    if (value <= knee) return zeroValue;
    else return map(value - knee, 1024, zeroValue, maxValue);

map() often finds itself nearby to calls to constrain(). map will happily extrapolate, so constrain() can be necessary.

I don't like map at all and only use it when I am too lazy to do without. For "real" mapping, there are better libraries, like @robtillaart's, available.

a7

[richierich1]

Okay, I'll bite.

Over the weekend I'll set up a test nano and play around with all the options you've all shown me as it'll be a good learning exercise.

The actual project this is used on is just a dimmer on some accessory lights on my motorcycle, so speed or accuracy isn't that important. I just noticed yesterday evening that it would better if the auto dimming ramped down sooner to the minimum brightness.

Changing the code in the nano on this motorcycle is a PITA as it's buried in the bike, and I have already broken off (and re-soldered) the micro USB connector once. I'm not sure how many more chances I'll get to plug back into it before it breaks again and needs replacing.

[richierich1]

I've just been testing out the suggestion made by @UKHeliBob and @jfjlaros and both work great, but I'll probably use @jfjlaros suggested code as it is the simplest.

[richierich1]

@robtillaart Many thanks for linking to your library. It's a bit of sledgehammer to crack a nut for this simple project but I will try to use it on future projects..
Just have to work out how to use GitHub first.

@alto777 Many thanks for your input here, I'll try your suggestions when I come back to looking at the FastMap..

So.. thanks again everyone.
Rich.