Modifying an array in void function

system · March 2, 2012, 4:59pm

Probably a stupid question, but is there a way to modify an array in a void function to be used outside of that function?

consider the structure

byte array[6];
byte *ptr=&array[0];

void function()
{
while (blabla)
{
*(ptr+i)=something;
i++;
}
}

void loop()
{
void function()
//uses array
}

With that code, my array is always empty. I have to do this instead

byte function()
{
while (blabla)
{
*(ptr+i)=something;
i++;
}
return 0;
}

void loop()
{
byte uselessvariable=function()
//uses array
}

The code works, but I'd like to get rid of that useless variable. Is that possible with a void type?

system · March 2, 2012, 5:02pm

Why don't you post your actual code?

The return type of a function has no bearing on the function's ability to modify a global array.

system · March 2, 2012, 5:18pm

my actual code is kinda long, messy and commented in french, but here it is anyway.

#include <Wire.h>
#include <string.h>
#include <avr/interrupt.h> 
#include <avr/io.h>
#undef int
#include <stdio.h>
#include <Servo.h> 

Servo patind;
Servo pating;
byte outbuf[6];
byte *bufpoint=&outbuf[0];// stockage des données du Nunchuck
byte zero=0x00;
int deg=0;
int degf=0;
int count=0;
boolean cd=1;  // vérifie le contact des patins avec le sol
boolean cg=1;  // cd:droit cg:gauche
long mil[2];
byte pente=2;

void setup ()
{
  mil[1] = 0;
  pinMode(52, OUTPUT);
  digitalWrite(52, HIGH);
// TCCR1B=13;
// TIFR1=0x00;
// TIMSK1=2;
// OCR1A=174; 
// TCCR3B=13;
// TIFR3=0x00;
// TIMSK3=2;
// OCR3A=1024; 
  patind.attach(50);
  pating.attach(51); // Liaision de la pin 2 avec le contrôleur du moteur 
  Wire.begin ();      // Initialisation du I2C pour le nunchuck
  nunchuck_init ();   // Initialisation du Nunchuck
  Serial.begin (19200);  
  Serial.print ("Finished setup\n");
  TCNT2=0x00; 
   
}

//ISR(TIMER1_COMPA_vect)          // timer compare interrupt service routine
//{
//  while (0b11 & outbuf[5]==0b11)
//  {
//    if (outbuf[1]<100 && pente>0)
//      pente+=1;
//    else if (outbuf[1]>160 && pente<60)
//      pente-=1;
//    
//  }
//}



void loop()
{
   byte caliss=nunchuck();                  // Appel de la fonction pour recevoir la valeur du joystick
  deg= map(outbuf[1], 131, 230, 0, 180);    // Étalonnage des données pour avoir une valeur entre 0 et 180
mil[0]=millis();
  if (deg>degf && mil[0]-mil[1]>=pente)
    {
    degf+=1;
    mil[1]=mil[0];
    
    }
  else if (deg<degf && deg>0)
    degf=deg; 
  else if (deg<0)
  degf=0;
  if (cd==1)
    patind.write(degf);  // Écris la valeur étalonnée dans le PPM de sortie pour le moteur
  if (cg==1)
    pating.write(degf);
    Serial.println(degf); 

//  if (millis()>1000 && millis()<2000)
//      count+=1;
//else
//      Serial.println (count);  
}

code used for acquiring data from a nunchuck

void nunchuck_init ()
{
  
  Wire.beginTransmission (0x52);	// transmet 0x52 au Nunchuck
  Wire.write (0x40);		        // envoie l'adresse de mémoire
  Wire.write (zero);		        // envoie un zéro
  Wire.endTransmission ();	        // arrête la transmission
}

void send_zero ()
{
  Wire.beginTransmission (0x52);	// transmet 0x52 au Nunchuck
  Wire.write (zero);		        // envoie un zéro
  Wire.endTransmission ();	        // arrête la transmission
}

byte nunchuck ()
{
  int cnt = 0;
  Wire.requestFrom (0x52, 6);	        // Demande les 6 bytes de données du Nunchuck
  while (Wire.available ())
    {
      *(bufpoint+cnt) = nunchuk_decode_byte (Wire.read ());	// Reçoit un byte de données du Nunchuck
      cnt++;
    }
//  Serial.println(outbuf[1]); 
  send_zero ();
return 0;  // Envoie la demande pour recevoir les prochains bits
//  byte joy=outbuf[1];  // Stock la variable du Joystick y dans joy et retourne la valeur
////  Serial.println(joy);
//  return joy;
}

char nunchuk_decode_byte (char x)       // Décodage des valeurs du Nunchuck
{
  x = (x ^ 0x17) + 0x17;
  return x;
}

The function called is nunchuck()

system · March 2, 2012, 5:18pm

void loop()
{
void function()
//uses array
}

That comment is WRONG. That is not how to call a function.

system · March 2, 2012, 5:20pm

PaulS:

void loop()

{
void function()
//uses array
}



That comment is WRONG. That is not how to call a function.

I meant that I use the array in loop

wildbill · March 2, 2012, 6:45pm

It would help if you could post the code that didn't work, but extrapolating from what you have above, I assume it's this:

void nunchuck();                  // Appel de la fonction pour recevoir la valeur du joystick

What PaulS is pointing out is that that is a function prototype and has no effect, when what you really want is:

nunchuck();                  // Appel de la fonction pour recevoir la valeur du joystick

system · March 2, 2012, 6:46pm

I meant that I use the array in loop

If you don't actually call the function that modifies the array, how can you expect the array to be modified?

marcello.romani · March 2, 2012, 9:38pm

(Did the OP modify the code after posting ?)

You should always use { } even when you only have one statement after "if" or "else", and learn to properly indent your code. Hit CTRL-T in the Arduino IDE. Subtle bugs might lie in "misindented" code.

system · March 2, 2012, 9:47pm

(Did the OP modify the code after posting ?)

Yes. I hate when people do that.

marcello.romani · March 2, 2012, 9:49pm

PaulS:

(Did the OP modify the code after posting ?)

Yes. I hate when people do that.

Definitely. At least one should append an edit: did this and that.

system · March 13, 2012, 3:21pm

Thanks, but I feel my question was not clear enough: If I call a void function that change an array declared elsewhere with a pointer, the array resets when the function ends. If I call that same function, but declare it an int or byte or whatever and I let it return a useless variable, the array is correctly modified. Why does that happens? A void function can't change a value that lasts after it ends?

system · March 13, 2012, 3:26pm

Please, stop calling it a "void function".
It's just a function that happens to not return anything.

If I call a void function that change an array declared elsewhere with a pointer, the array resets when the function ends.

You can't declare an array with a pointer; you have to declare a pointer and point it to some memory.

You question still isn't clear (to me) - can you post some code that displays the sort of thing you think you're seeing?

system · March 13, 2012, 3:46pm

AWOL:
Please, stop calling it a "void function".
It's just a function that happens to not return anything.

If I call a void function that change an array declared elsewhere with a pointer, the array resets when the function ends.

You can't declare an array with a pointer; you have to declare a pointer and point it to some memory.

You question still isn't clear (to me) - can you post some code that displays the sort of thing you think you're seeing?

the array is declared elsewhere and I use a pointer to modify it in the function.

system · March 13, 2012, 3:56pm

OK, so "post some code" means something different to you.
Like this?

char array [100];

void setup ()
{

}

void loop ()
{
  char* p = array;
  p [0] = 'A';
}

// or this

void foo (char* p)
{
  p [1] = 'B';
}

marcello.romani · March 13, 2012, 7:15pm

mxmcharbonneau:
Thanks, but I feel my question was not clear enough: If I call a void function that change an array declared elsewhere with a pointer, the array resets when the function ends. If I call that same function, but declare it an int or byte or whatever and I let it return a useless variable, the array is correctly modified. Why does that happens? A void function can't change a value that lasts after it ends?

There is no void function, nor int function, nor anything like that. There are functions that return data, and one must declare what type that data is, and there are functions that don't return anything, and one tells that to the compiler by using the special type void.
The data type returned by a function, be it void or anything else, has no influence whatsoever on what happens to the variables used inside that function. That depends only on the function code. What you are observing is definitely not a direct effect of what type of data that function returns.
"Post the code" in this context means two things:

copy-n-paste into a reply the entire code where you use the "void function"
copy-n-paste into a reply the entire code where you use the "int function"
Highlight the differences between the two codes and how they differ in their behaviour.

IMHO this is the only way you can get some help.

nickgammon · March 13, 2012, 10:12pm

void loop()
  {
  byte uselessvariable=function();
  //uses array
  }

Can be replaced by:

void loop()
  {
  function();
  //uses array
  }

In other words you can ignore the return code. And then if you are planning to ignore it, declare "function" as (returning) void - that is, nothing.

Topic		Replies	Views
Void functions without return Programming Questions	10	4773	May 6, 2021
How to return the value from void function and use it inside in another function or in the void loop? Programming Questions	6	3307	August 16, 2021
Function Clarification Programming Questions	10	1425	May 5, 2021
use a byte array in a function Programming Questions	4	1084	May 5, 2021
Void or not void for global variables Programming Questions	4	1517	May 5, 2021

Modifying an array in void function

Related Topics