Global variable arduino - arduino

Question

I'm using I2C to communicate a Master Arduino to 4 Slaves Arduinos, and an Shield (OULIMEX Shield LCD 16x2) on every Arduino slave.
I send Data from the master to slaves using I2C. So I use this code in the master :
#include <Wire.h>
#include <math.h>
#include <floatToString.h>
double incomingData;
void setup()
{
Wire.begin();
Serial.begin(9600);
incomingData = Serial.parseFloat(); //read incoming data
}
void loop()
{
delay (1000);
if (Serial.available())
{
incomingData = Serial.parseFloat(); //read incoming data
Wire.beginTransmission(8); // transmit to device #8
if ((M==0) || (M==1) || (M==2))
Wire.beginTransmission(8); // transmit to device #8 *****************************************************************
else
Wire.beginTransmission(7); // transmit to device #7 *****************************************************************
M++;
if (M==5)
M=0;
String a = "";
a = floatToString(test,incomingData,3,5,true);
for(i=0; a[i]!='\0'; ++i); // length of the string
Wire.write(i);
Wire.write(floatToString(test,incomingData,3,5,true)); // sends one byte
Wire.endTransmission(); // stop transmitting
}
}
I wanted the Data to be printed on the Shield, but I'm connecting all slaves with the same way with the master. For that I have two problems :
1- The global data I'm using to print value is always printed as 0, and not giving the real value;
2- All Shields print the same thing : For exemple, I print "hello" in the first Shield, and I print "hi" in the second Shield, but bouth are printing the same thing (hello or hi).
The code using for the first slave is :
#include <LCD16x2.h>
#include <Wire.h>
LCD16x2 lcd;
int buttons;
int sensorPin = A0; // select the input pin for the potentiometer
int sensorValue = 0; // variable to store the value coming from the sensor
float numOut;
int comp=1 ;
String wordd = "";
int M =0;
void setup()
{
Wire.begin(8); // join i2c bus with address #8
Wire.onReceive(receiveEvent); // register event
Serial.begin(9600); // start serial for output
}
void loop()
{
delay(500);
}
// function that executes whenever data is received from master
// this function is registered as an event, see setup()
void receiveEvent(int howMany) {
wordd = "";
int x = Wire.read();
for (int i=0; i<=x; i++)
{
char c = Wire.read();
wordd += c;
}
numOut = wordd.toFloat();
Serial.println(numOut,3); // print the integer
}
Thank you in advance !!

Answer 1

I think it's due to a poor program structure in master shield.
This block selects the slave but on the first line you already select #8
I think this is confusing for the slaves.
Wire.beginTransmission(8); // transmit to device #8
if ((M==0) || (M==1) || (M==2))
Wire.beginTransmission(8); // transmit to device #8
else
Wire.beginTransmission(7); // transmit to device #7
This block should be at the end of the function
M++;
if (M==5)
M=0;
Then you parse the value in a string.
But leave out the first char because you write ++i instead of i++
Moreover you close the loop with ; so it does nothing
String a = "";
a = floatToString(test,incomingData,3,5,true);
for(i=0; a[i]!='\0'; ++i); // length of the string
Finally you write the ordinal number of the byte
And then again the Whole string
So you should get "0" (or "1" because of ++i)
followed by your number if Wire.write() supports it
Wire.write(i);
Wire.write(floatToString(test,incomingData,3,5,true)); // sends one byte
Wire.endTransmission(); // stop transmitting
}
Your sketch should be:
if (Serial.available())
{
incomingData = Serial.parseFloat(); //read incoming data
String a = "";
a = floatToString(test,incomingData,3,5,true);
if ((M==0) || (M==1) || (M==2))
Wire.beginTransmission(8); // transmit to device #8
else
Wire.beginTransmission(7); // transmit to device #7
for(i=0; a[i]!='\0'; ++i) // length of the string
Wire.write(a[i]); // write one byte
Wire.endTransmission(); // stop transmitting
M++;
if (M==5) M=0;
}
Let me know if this works.

Answer 2

I already ask this question but I think I have the answer of it. A global variable have to be diclared befor the void setup, and the void loop too, like that :
type YourGlobalVariable;
void setup()
{
}
void loop()
{
}
So, it is exactly how I did already. The reason it didn't work for me, it was cause of I used this function :
void receiveEvent(int howMany) {}
I don't really know what are the properties of it that let it not work for a global variables, but It works like I sayd already.
Thank you all