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arrayshexbyte

Confused with a statement

I want to check if the 4th byte of my array is equal to any of these : 0xe0 0xe1 0xe2 0xe3 0xe4 0xe5 0xe6 0xe7 0xe8 0xe9 0xeA 0xeB 0xeC 0xeD 0xeE 0xeF. My friend told me to use the statement (array[3] & 0xf0) == 0xe0. This works but i can't actually understand why.

Solution

  • Remember, with bitwise AND (&) operation, the Nth bit of result is only equal to 1 if both corresponding bits of the operands are equal to 1.

    So &-ing 0xf0 to the value of 4th byte (array[3]) essentially replaces the lowest half of that byte with 0, leaving the highest one intact. 

     1010  1010
 &
 1111  0000
 ====  ====
 1010  0000
 ^^^^  ^^^^ 
  ||    ||
as is  zeroed

    Now all that remains is check that value against 0xe0.