Converting byte array to long and long to byte array in specific position

Projects Programming
1

Hi

I'm trying to work with byte arrays and pointers, as I need to extract from a byte array, which I had received and stored from an I2C connection, however from this array I would need to extract the last 4 bytes and converted to a long.

Also the opposite is required, where for instance I have another long and I need to stored in a different byte array (for my testing) so far I have this code

byte I2C_rev [7] ={0x07,0x043,0x40,0x00,0x23,0xAB,0x2F};
byte I2C_rev2 [7] ={0,0,0,0,0,0,0};
// Integer to use 2337583, 0x0023AB2F in hex, which is equal to 0x00, 0x23, 0xAB, 0x2F
// 0x07, size of the byte array
// 0x43 the I2C address
// 0x10 the Register (function to use)

byte msg_size, I2C_Address, Register;
long data;
long data2 = 214050;

void setup() {
//--- receive string, and put data from a pointer to a long variable ------
Serial.begin(9600);
Serial.println ("ready");
delay (1000);
}

void loop() {

msg_size =I2C_rev[1];
I2C_Address=I2C_rev[2];
Register=I2C_rev[3];
Serial.println (" to extract from Byte array last 4 bytes and converted to long");
long intpos = (long)&I2C_rev+3;
data=*intpos;
Serial.println (*intpos);
Serial.println (data);
delay (1000);

//--------------- Store a string in to the byte array

Serial.println ("to store a long in the last 4 byte of the array");
I2C_rev[1]=0x07; //message size 7 bytes
I2C_rev[2]=0x042; //I2C address
I2C_rev[3]=0x25; //Register value

// to store the long number in to the last 4 bytes of the array

long intpos2 = (long)&I2C_rev2+3;
*intpos2 = data2;
long test;
Serial.println (*intpos2);
Serial.println (data2);

}

So far I have apparently been successful in the last part - storing the long in to the array, but not the extraction

Can anyone please help me

Thanks

2

Looks like this is a problem:

long *intpos =(long*)&I2C_rev+3;

I think the compiler is interpreting the +3 as +3 longs, or 12 bytes.

This seems to work better:

long *intpos = (long*)&I2C_rev[3];

Also, it looks like you need to reverse the order of the bytes that make up the long.

3

Please keep the comments and code unambiguous as in the end the code is allways right :wink:

// 0x43 the I2C address

I2C_rev[2]=0x042; //I2C address

4

Couple of comments on pointers and arrays:

  • The first element in an array has an index of 0, not 1. So you probably want:
  msg_size =I2C_rev[0];
  I2C_Address=I2C_rev[1];
  Register=I2C_rev[2];

  • A leading zero in hex values like 0x043 is slightly confusing - only the last two digits will end up in a byte anyway.

  • Is the correct array being used here? Or should it be I2C_rev2? Also mind the indexes

  I2C_rev[1]=0x07; //message size 7 bytes