Serial Print negative numbers in Binary form

Projects Programming
1

I was curious why printing negative numbers in binary results in output that far exceeds the size of an int data type. With this code:

void setup()
{
 Serial.begin(115200);
 int a = -1;
 Serial.print("A = ");
 Serial.println(a);
 Serial.print("A = ");
 Serial.print(a,BIN);
 Serial.println(" in Binary");

 
 
}

void loop()
{}

I get this output:

A = -1
A = 11111111111111111111111111111111 in Binary

What is happening here?

2

Negative number in Binary are in 2's complement form

So -1 =

Binary of 1 say in 16bits =

00000000000000001

  • 1
    to form 2 comp[lement = 1111111111111111.

Remember the 2'complement of a numbe = Binary of positive number then add 1

so -1 = 11111111111111111

  • 2 = 11111111111111110
3

I would have expected to see 16 1's in a row to represent a -1 in an int data type. What printed was 32 1's in a row. This is what gets printed with -2.

a = 11111111111111111111111111111110

This is on an Arduino mega2560

4

jerseyguy1996:
I would have expected to see 16 1's in a row to represent a -1 in an int data type. What printed was 32 1's in a row. This is what gets printed with -2.

a = 11111111111111111111111111111110

This is on an Arduino mega2560

It does not matter what size the data is the 2's complement is always the positve + 1

in 8,16 ,24 , 32 wtc., its just the format - 1 is alway a string if 1's

Useually the hex format ids used

  • 1 in 8 buts = FF
    -1 in 16 bits = FFFF
    -1 in 24 bits = FFFFFF
    -1 in 32 biss = FFFFFFFF

Any other size will be exntened to pacxk FF into the upper order.

5

What printed was 32 1's in a row

What this means is that Serial.print is treating "-1" as an unsigned long (32 bit) integer.

6

You have the source, so checking how the function overloads work is easy.

7

.print(int) just calls .print(long). This causes a sign extension to 32 bits which is a problem in BIN and HEX formats. If you cast the value to 'unsigned' it will call .print(unsigned) which will call .print(unsigned long) and therefore no sign extension.

8

Thank you everyone for the great explanations!

9

Unwashed solution, wrap word() around the negative number. eg. Serial.println(word(-1000), HEX);