Break an int into 2 char's - Electronic Circuits Projects Diagrams Free

gualala · 30th April 2008, 01:56 PM

Hi,
I'm writing some codes in C18. I have an unsigned int that I wish to break into two chars. I saw something on union struct for a byte, but I don't know how to change it into int. Could anyone help? Thanks.

I wish I can have a variable, say "Value",
Value = 0x8FFF;
Value.HighByte // gives 0x8F
Value.LowByte // gives 0xFF
or any other way that works?

The code I saw for char, so that we can use something like Flags = 0; to set all to zeros or Flags.Timeout = 1; to set a specific value.

Code:

union
{
  struct
  {
    unsigned Timeout:1;
    unsigned None:7;
  } Bit;
  unsigned char Byte;
} Flags;

eng1 · 30th April 2008, 02:58 PM

Quote:

Originally Posted by gualala

Hi,
I'm writing some codes in C18. I have an unsigned int that I wish to break into two chars. I saw something on union struct for a byte, but I don't know how to change it into int. Could anyone help? Thanks.

I wish I can have a variable, say "Value",
Value = 0x8FFF;
Value.HighByte // gives 0x8F
Value.LowByte // gives 0xFF
or any other way that works?

I would do a right-shift and use casts.

Code:

unsigned char low_byte = Value;    // cast
unsigned int tmp = Value>>8;       // right-shift
unsigned char high_byte = tmp;     // cast

kchriste · 1st May 2008, 03:49 AM

You can do it eng1's way with shifts or this way with unions:

Code:

union {
	unsigned int Integer;
	struct{
		unsigned char LowByte;
		unsigned char HighByte;
		};
	} Value;

	Value.Integer = 0x1234;
	// Value.HighByte == 0x12;
	// Value.LowByte == 0x34;

The advantage of the union is you can operate directly on the Bytes of the Integer.

Torben · 1st May 2008, 05:17 AM

Or you could do it with a little pointer arithmetic if you like:

Code:

#include <stdio.h>

int main(void)
{
    unsigned int value = 0x8FFF;
    unsigned char *low_byte = (unsigned char *) &value,
        *high_byte = low_byte + 1;

    printf("0x%04x => Low: 0x%02x; High: 0x%02x\n", value, *low_byte, *high_byte);

    return 0;
}

This also allows you access to the individual bytes, but just remember this: http://xkcd.com/371/

Torben

Pommie · 1st May 2008, 06:33 AM

Quote:

Originally Posted by Torben

Or you could do it with a little pointer arithmetic if you like:

Torben

Are we sure that C18 stores ints in little endian format.

Mike.

Torben · 1st May 2008, 06:42 AM

Quote:

Originally Posted by Pommie

Are we sure that C18 stores ints in little endian format.

Mike.

Nope!

Just going with what the OP posted, figuring s/he'd just swap the labels if not. Checking the C18 User's Guide confirms it though (page 12).

Torben

ikalogic · 2nd May 2008, 10:05 AM

Quote:

Originally Posted by Torben

This also allows you access to the individual bytes, but just remember this: http://xkcd.com/371/
Torben

That's orriginal! loooooooool

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30th April 2008, 01:56 PM	(permalink)
gualala	Break an int into 2 char's Hi, I'm writing some codes in C18. I have an unsigned int that I wish to break into two chars. I saw something on union struct for a byte, but I don't know how to change it into int. Could anyone help? Thanks. I wish I can have a variable, say "Value", Value = 0x8FFF; Value.HighByte // gives 0x8F Value.LowByte // gives 0xFF or any other way that works? The code I saw for char, so that we can use something like Flags = 0; to set all to zeros or Flags.Timeout = 1; to set a specific value. Code: union { struct { unsigned Timeout:1; unsigned None:7; } Bit; unsigned char Byte; } Flags;

1st May 2008, 03:49 AM	(permalink)
kchriste	You can do it eng1's way with shifts or this way with unions: Code: union { unsigned int Integer; struct{ unsigned char LowByte; unsigned char HighByte; }; } Value; Value.Integer = 0x1234; // Value.HighByte == 0x12; // Value.LowByte == 0x34; The advantage of the union is you can operate directly on the Bytes of the Integer. __________________ --- The days of the digital watch are numbered. ---