Bitwise and "&" question

Projects Programming
1

Hello!

Why do the following if comparisons show different results?

unsigned int i = 0;
unsigned int test = 0;
void setup() {
  Serial.begin(115200);
  for ( i=0; i<4; i++ ) {
    Serial.println( i&3 );
    test = i&3;
    if ( i&3==0 ) {
      Serial.println( "zero" );         // not show
    }
    if ( test==0 ) {
      Serial.println( "test is zero" ); // show
    }
  }
}
void loop() {
  delay(10);
}
/* Serial print results:
0
test is zero
1
2
3
*/
2

Take a look at the C++ operator precedence table and think about what it means for the above logical construction.

Hint: if in doubt, use parentheses to enforce your intent.

3

( (i&3)==0 )
That solved it!
Thank you very much!

4

Do you not have your Compiler Warnings (in Preferences...) turned up to "All"? If you did so you would have gotten the helpful message:

sketch_oct09a.ino: In function 'void setup()':
sketch_oct09a.ino:10:16: warning: suggest parentheses around comparison in operand of '&' [-Wparentheses]
     if ( i & 3 == 0 )
              ~~^~~~

That means "The '3 == 0' comparison is one side of your 'i & something' expression and that's often a mistake. You should put parens around the '3 == 0' part, to let people know you really mean it."

5 
if(i & 3 == 0)

The referred Table says that the "equality operator" has the higher precedence. If so, will the Compiler attempt to evaluate "3 == 0" or will just ignore with an error message?

6

There is nothing wrong with expression if(i & 3 == 0). 3==0 is evaluated first, leading to the observed and sometimes unexpected result.

1 Like
7

The warning just tells you it is unusual and may not be what you meant. The compiler will continue on and do what you told it to.

That is why I always turn on all warnings and use parentheses when I am not sure (it saves looking up the rules).

8

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