strange button problem...

Forum 2005-2010 (read only) Software Syntax & Programs
1

hey guys, so i have 2 buttons, one does program changes up and the other is program changes down, one interval at a time.

the problem is this:
lets say i am on program 10
i press program down, it goes to 9, press again it goes to 8.
now i want to go back up to program 9
i press program up and it will go down to 7, press again 8, press again 9.
then if i press program down it will go up one before going down and so on.
hopefully that makes sense.

i am using AlphaBeta's button library for the unique press function.

this is the section of code in question.

 Button progUp = Button(11,PULLUP); //program up pin 11
Button progDn = Button(12,PULLUP); //program up pin 12
int prog = 114;



    if(progUp.uniquePress()){
    MIDI.send(PC,prog++,0,1);
}
   if(progDn.uniquePress()){
    MIDI.send(PC,prog--,0,1);
}

*note that is not the entire sketch, just the button assignment and loop section.

2

Try ++prog and --prog; I think you are running into a pre vs. post decrement/increment issue...

:slight_smile:

3

that was exactly the problem!

thanks so much, i could not figure out what was going wrong, and it was so simple!

4

The real question is - do you understand the difference as to where and how things work based on where the increment/decrement operators are placed?

5

actually...i guess i don't really understand why it makes a difference.
i just know that i will be using the -- or ++ int he front now.

would you mind explaining whats going on? it would be cool to truly know what was going on.

thanks cr0sh!

6

pre Xcrement pun slightly intended makes the value change before it's passed, and post changes the value after it's passed.
:slight_smile:

7

assume prog = 5

preincrement:
x = ++prog;
y = prog;
results in x = 6 and y = 6 (increment the value of prog then use the incremented value)

but
post increment:
x = prog++;
y = prog;

results in x = 5 and y = 6 (use the value of prog then increment it)

8

wow, thanks guys not that makes sense!
great to know why i should do something rather than just knowing what to do.

9

Note that this same logic applies everywhere; for instance:

if (i++ < 10) {
  // do stuff
}

...will function differently than:

if (++i < 10) {
  // do stuff
}

As you have seen, if you aren't taking it into account (perhaps from a night of drinkin' - ;D ), it can trip you up!

:slight_smile: