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I am programming effects for an LED cube. The cube and the independent functions are working correctly. The problem occurs when I call turnon_layer function in a loop. The loop counter is not incremented. 

for(i=0; i<4; i++)
{
  Serial.println(i);
  turnon_layer(1000,i);
}

The Serial monitor shows that i has a constant value of 0 throughout.

When I call the functions with constants, then the output is as expected.

turnon_layer(1000,0);
turnon_layer(1000,1);
turnon_layer(1000,2);
turnon_layer(1000,3);

My turnon_layer function is :

void turnon_layer(long time, int layer)
{
  boolean state = HIGH;
  long wait, start = millis();
  for(i=0; i<4; i++)
  {
      if(i!=layer) digitalWrite(levelPin[i], !LOW);
      else digitalWrite(levelPin[i], !HIGH);
  }

  if(time/2<10)  wait = time/2;
  else  wait = 10;

  while(millis()-start<time)
  {
    state = !state;
    //Serial.println(state);
    //First 8 LEDs
    for(i=0;i<2;i++)
    for(j=0;j<4;j++)
    digitalWrite(ledPin[i][j],state);

    //Next 8 LEDs
    for(i=2;i<4;i++)
    for(j=0;j<4;j++)
    digitalWrite(ledPin[i][j],!state);
    delay(wait);
  }
}

I am using the Arduino IDE.

What is going wrong ? Why ?

share|improve this question
Are you able to post the entire code including the setup() and void() methods as well as any variable declarations at the top? – Marko Feb 14 at 4:25
@Marko: Did you mean setup() and loop()? – Dave Tweed Feb 14 at 4:32
1  
@AnindoGhosh Now I definitely look mentally challenged laughing at my laptop. Thanks for that. – Marko Feb 14 at 4:39
1  
@Swanand do you have anything concrete to back that statement? A quick scan at the Wikipedia For Loop Page reveals that just about every example uses i. – Marko Feb 14 at 6:48
1  
But i means a lot to me ;) No pun intended. – Marko Feb 14 at 6:51
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1 Answer

up vote 4 down vote accepted

Is the variable i declared as a global variable? i.e. at the top of the file outside all methods?

It may be getting overwritten by the loop inside your turnon_layer() method.

To test, just change the variable to letter k inside just one of your loops.

share|improve this answer
1  
The experiment you're proposing will fail, because j is also used in the called function. In general, loop index variables should pretty much always be declared local to each function in which they're used. – Dave Tweed Feb 14 at 4:33
Agreed, I have seen Arduino examples where i is declared with other variables at the top of the file. Generally bad practice. – Marko Feb 14 at 4:36
@DaveTweed I saw your update and have updated it to letter k. – Marko Feb 14 at 4:37
Yes, that was the issue. Making it a local solved the problem. – AshRj Feb 14 at 5:42
@DaveTweed I just had another look and I believe that unless j was declared at the top it wouldn't interfere because when specified in the loop declaration it becomes a local variable to that loop body. – Marko Feb 14 at 20:33

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