Combining bytes into multiple integers

Projects Programming
1

I'm fairly new to type union for combining bytes into a float/integer. This issue is anyways well covered on the internet. Now, my question is is there a way of combining bytes into multiple integers without creating a for loop?

My sensor data looks like so:
D0 D1 D2 D3 D4 D5 D6 D7
16 128 63 126 110 120

Every pair D0-D1 consists of 1 byte. I want to assign each byte to an int.

I have coded this:

void loop(){

int combined;
int Fx,Fy,Fz;

for(int i=0;i<5;){

combined = message.data[i+1];
combined = combined<<8;
combined |= message.data*;*

  • if (message.id == 970){ //filtering message data*
  • if (i==0){*
    Fx = (((combined - 32768)*scale_factor)/32768)+0;
  • }*
  • if (i==2){*
    Fy = (((combined - 32768)*scale_factor)/32768)+21;
  • }*
  • if (i==4){*
    Fz = (((combined - 32768)*scale_factor)/32768)+46;
  • }*
  • }*
  • i=i+2;*
    }
    Serial.print(Fx)
    Serial.print(", ")
    Serial.print(Fy)
    Serial.print(", ")
    Serial.print(Fz)
    but it seems what I read is wrong.
    I wanted to use a union like this:
    union{
  • byte dataBytes[3];*
  • int value;*
  • } SensorData;*
    void loop(){
    SensorData.dataBytes[0]=message.data[0]
    SensorData.dataBytes[1]=message.data[1]
    SensorData.dataBytes[2]=message.data[2]
    Serial.println(SensorData.value)
    But I think this will just combine the 3 bytes into a single int. Do I need to use 3 unions to solve this?
2

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3

Three bytes won't fit into an int type.

4

aarg:
Three bytes won't fit into an int type.

They will if the first one is 0.

5

PaulS:
They will if the first one is 0.

Yes but then there is no point in sending or receiving it.

6

I worked out a possible solution:

union convertor
{
  int value;
  byte bytes[1];
};

void loop()
{
int Fx,Fy,Fz;

convertor sensorDataX;
sensorDataX.bytes[0] = message.data[0];
sensorDataX.bytes[1] = message.data[1];
Fx = sensorDataX.value;

convertor sensorDataY;
sensorDataY.bytes[0] = message.data[2];
sensorDataY.bytes[1] = message.data[3];
Fy = (sensorDataY.value;

convertor sensorDataZ;
sensorDataZ.bytes[0] = message.data[4];
sensorDataZ.bytes[1] = message.data[5];
Fz = sensorDataZ.value;

}
7

Should be:

union convertor
{
  int value;
  byte bytes[2];
};

You're lucky the two byte int was covering you. Otherwise, you'd be writing into memory that you don't own.

8

Something worth noting from K&R 2nd Edition regarding unions:

“the results are implementation-dependent if something is stored as one type and extracted as another”

It says “implementation-dependent”, not “undefined”. So, I assume this means that if what you did works for a given processor and given version of the compiler, then it will always work for that combination. But, if the compiler changes over time ……

What you showed in your original code is a more portable solution:

intVariable = byteVariable1 << 8 | byteVariable0;
9

You can just type cast it and use the data directly.

void loop()
{
  int Fx,Fy,Fz;

  Fx = ((int *)message.data)[0];
  Fy = ((int *)message.data)[1];
  Fx = ((int *)message.data)[2];
}
10

If the values never exceed the size of a byte, why assign them to int's at all? It just wastes memory.