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Скачать Chapter 33 Bitwise Operations

unsigned char d = a & b;

printf("Hex: %x & %x = %x\n", a, b, d); printf("Decimal: %d & %d = %d\n", a, b, d);

return 0;

}

When you run it, you will see the two bytes bitwise-ANDed together:

Hex: 3c & a9 = 28

Decimal: 60 & 169 = 40

In Objective-C, we use bitwise-AND to see if a certain bit, or flag, is on. For example, if you were handed an instance of NSDataDetector, you could check if it was set to look for phone numbers like this:

if ([currentDetector checkingTypes] & NSTextCheckingTypePhoneNumber) { NSLog(@"This one is looking for phone numbers");

}

The checkingTypes method returns an integer that is the bitwise-OR result of all the flags this instance of NSDataDetector has on. You bitwise-AND this integer with a particular NSTextCheckingType constant and check the result. If the bit that is on in NSTextCheckingTypePhoneNumber is not on in the data detector’s setting, then the result of bitwise-ANDing them will be all zeroes. Otherwise, you’ll get a non-zero result, and you’ll know that this data detector does look for phone numbers.

Note that when we use bits this way, we don’t care what the integers in these cases equate to numerically. We use the bit placement within the integer to represent something other than a certain power of 2.

Other bitwise operators

For completeness, here are the other bitwise operators. These are less commonly used in Objective-C but good to know.

Exclusive OR

You can exclusive-or (XOR) two bytes together to create a third. A bit in the third byte is 1 if exactly one of the corresponding bits in the input bytes is 1.

Figure 33.5 Two bytes bitwise-XORed together

242 Complement

This is done with the ^ operator. Add to main.c:

unsigned char e = a ^ b;

printf("Hex: %x ^ %x = %x\n", a, b, e); printf("Decimal: %d ^ %d = %d\n", a, b, e);

return 0;

}

When you run it you will see:

Hex: 3c ^ a9 = 95

Decimal: 60 ^ 169 = 149

This operator sometimes causes beginners some confusion. In most spreadsheet programs, the ^ operator is exponentiation: 2^3 means 23. In C, we use the pow() function for exponentiation:

double r = pow(2.0, 3.0); // Calculate 2 raised to the third power

Complement

If you have a byte, the complement is the byte that is the exact opposite: each 0 becomes a 1 and each 0 becomes a 1.

Figure 33.6 The complement

This is done with the ~ operator. Add a few lines to main.c:

unsigned char f = ~b;

printf("Hex: The complement of %x is %x\n", b, f); printf("Decimal: The complement of %d is %d\n", b, f);

return 0;

}

You should see:

Hex: The complement of a9 is 56

Decimal: The complement of 169 is 86

Left-shift

If you left-shift the bits, you take each bit and move it toward the most significant bit. The ones that are on the left side of the number are forgotten, and the holes created on the right are filled with zeros.

243 Chapter 33 Bitwise Operations

Figure 33.7 Left-shifting by 2

Left-shift is done with the << operator. Add a shift of two places to main.c:

unsigned char g = a << 2;

printf("Hex: %x shifted left two places is %x\n", a, g); printf("Decimal: %d shifted left two places is %d\n", a, g);

return 0;

}

When this code runs, you will see:

Hex: 3c shifted left two places is f0

Decimal: 60 shifted left two places is 240

Every time you left-shift a number one place, you double its value.

Right-shift

The right-shift operator should not be much of a surprise. Add code to main.m:

unsigned char h = a >> 1;

printf("Hex: %x shifted right one place is %x\n", a, h); printf("Decimal: %d shifted right one place is %d\n", a, h);

return 0;

}

When run:

Hex: 3c shifted right one places is 1e

Decimal: 60 shifted right two places is 30

Figure 33.8 Right-shifting by 1

244

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