From 074785cea106179cb3305637055ab0a009ca74f2 Mon Sep 17 00:00:00 2001 From: Peter Bengtsson Date: Tue, 8 Dec 2020 14:42:52 -0500 Subject: initial commit --- .../operators/bitwise_operators/index.html | 558 +++++++++++++++++++++ 1 file changed, 558 insertions(+) create mode 100644 files/pt-br/web/javascript/reference/operators/bitwise_operators/index.html (limited to 'files/pt-br/web/javascript/reference/operators/bitwise_operators') diff --git a/files/pt-br/web/javascript/reference/operators/bitwise_operators/index.html b/files/pt-br/web/javascript/reference/operators/bitwise_operators/index.html new file mode 100644 index 0000000000..7e9d2de505 --- /dev/null +++ b/files/pt-br/web/javascript/reference/operators/bitwise_operators/index.html @@ -0,0 +1,558 @@ +--- +title: Bitwise operators +slug: Web/JavaScript/Reference/Operators/Bitwise_Operators +translation_of: Web/JavaScript/Reference/Operators +--- +
{{jsSidebar("Operators")}}
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Operadores bit-a-bit são são operadores tratados como sequência de 32 bits ( zeros e uns ), preferencialmente como decimal, hexadecimal, ou números octais. Por exemplo, o número decimal 9 tinha como representação binária de 1001. Operadores bit-a-bit realizam as operações em tais representações binárias, mas retornam valores numéricos no padrão Javascript.
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{{EmbedInteractiveExample("pages/js/expressions-bitwiseoperators.html")}}
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A seguinte tabela resume os Operadores bit-a-bit:

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
OperadorUsoDescrição
Bitwise ANDa & bRetorna 1 em cada posição de bit para à qual o bit correspondente de ambos eram 1s.
Bitwise ORa | b +

Retorna 1 para cada posição de bit para à qual o correspondente de um ou ambos eram 1s.

+
Bitwise XORa ^ bRetorna 1 para cada posição de bit para à qual o bit correspondente de um mas não ambos eram 1s.
Bitwise NOT~ aInverte os bits de seus operandos.
Left shifta << bJogam a  em representação binária b (< 32) bits à esquerda, mudando de zeros à diretia.
Sign-propagating right shifta >> bJogam a  em representação binária b (< 32) bits à direita, descartando bits que foram tornados off.
Zero-fill right shifta >>> b  Jogam a  em representação binária b (< 32) bits à direita, descartando bits que foram tornados off, e jogando 0s para à esquerda.
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Inteiros assinados em 32-bit

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Os operandos de todos os operadores bit-a-bit são assinados como inteiros de 32-bit em duas formas complementares. Duas formas complementares significa que uma negativa contrapartida (e.g. 5 vs. -5) são todos os bits daqueles números invertidos (bit-a-bit NOT de um número, a.k.a. complementos de um número) mais um. Por example, os seguintes encodes inteiros são 314:

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00000000000000000000000100111010
+
+ +

Os seguintes encodes ~314, i.e. são os únicos complementos de  314:

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11111111111111111111111011000101
+
+ +

Finalmente, os seguintes encodes -314, i.e. são dois complementos de 314:

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11111111111111111111111011000110
+
+ +

As duas garantias complementares daquele bit mais à esquerda que é zero quando o número é positivo e 1 quando o número é negativo. Aliás, isto é chamado de sign bit ou bit assinalado.

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O número 0 é o inteiro composto completamente de 0 bits.

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0 (base 10) = 00000000000000000000000000000000 (base 2)
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+ +

O número -1 é o inteiro que é composto completamente de 1 bits.

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-1 (base 10) = 11111111111111111111111111111111 (base 2)
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+ +

O número -2147483648 (representação hexadecimal: -0x80000000) é o inteiro completamente composto de 0 bits exceto o primeiro (left-most) único.

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-2147483648 (base 10) = 10000000000000000000000000000000 (base 2)
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+ +

O número 2147483647 (representação hexadecimal: 0x7fffffff) é o inteiro composto completamente por bits 1, exceto pelo primeiro (o mais à esquerda).

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2147483647 (base 10) = 01111111111111111111111111111111 (base 2)
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Os números -2147483648 e 2147483647 são, respectivamente, o minimo e o máximo inteiro representáveis atráves de um número de 32 bits assinados.

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Operadores lógico bit-abit

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Conceitualmente, os operadores lógicos bit-abit funcionam da seguinte forma:

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& (Bitwise AND)

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Performa a operação AND em cada par de bits. a AND b retorna 1, apenas quando a e b são 1. A tabela verdade para a operação AND é:

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
aba AND b
000
010
100
111
+ +
.    9 (base 10) = 00000000000000000000000000001001 (base 2)
+    14 (base 10) = 00000000000000000000000000001110 (base 2)
+                   --------------------------------
+14 & 9 (base 10) = 00000000000000000000000000001000 (base 2) = 8 (base 10)
+
+ +

Performar a operação AND bit-a-bit de qualquer número x com 0 retornará 0. Performar a operação AND bit-a-bit de qualquer número x com -1 retornará x.

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| (Bitwise OR)

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Performa a operação OR em cada par de bits. a OR b retorna 1 se pelo menos a ou pelo menos b é 1. As tabela versão para a operação OR é:

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
aba OR b
000
011
101
111
+ +
.    9 (base 10) = 00000000000000000000000000001001 (base 2)
+    14 (base 10) = 00000000000000000000000000001110 (base 2)
+                   --------------------------------
+14 | 9 (base 10) = 00000000000000000000000000001111 (base 2) = 15 (base 10)
+
+ +

Performar a operação OR de qulalquer número x com 0 retornará 0. Performar a operação OR de qualquer número X com -1 retornará -1.

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^ (Bitwise XOR)

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Performs the XOR operation on each pair of bits. a XOR b yields 1 if a and b are different. The truth table for the XOR operation is:

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
aba XOR b
000
011
101
110
+ +
.    9 (base 10) = 00000000000000000000000000001001 (base 2)
+    14 (base 10) = 00000000000000000000000000001110 (base 2)
+                   --------------------------------
+14 ^ 9 (base 10) = 00000000000000000000000000000111 (base 2) = 7 (base 10)
+
+ +

Bitwise XORing any number x with 0 yields x. Bitwise XORing any number x with -1 yields ~x.

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~ (Bitwise NOT)

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Performs the NOT operator on each bit. NOT a yields the inverted value (a.k.a. one's complement) of a. The truth table for the NOT operation is:

+ + + + + + + + + + + + + + + + +
aNOT a
01
10
+ +
 9 (base 10) = 00000000000000000000000000001001 (base 2)
+               --------------------------------
+~9 (base 10) = 11111111111111111111111111110110 (base 2) = -10 (base 10)
+
+ +

Bitwise NOTing any number x yields -(x + 1). For example, ~-5 yields 4.

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Note that due to using 32-bit representation for numbers both ~-1 and ~4294967295 (232-1) results in 0.

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Operadores de deslocamento bit a bit

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The bitwise shift operators take two operands: the first is a quantity to be shifted, and the second specifies the number of bit positions by which the first operand is to be shifted. The direction of the shift operation is controlled by the operator used.

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Shift operators convert their operands to 32-bit integers in big-endian order and return a result of the same type as the left operand. The right operand should be less than 32, but if not only the low five bits will be used.

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<< (Left shift)

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This operator shifts the first operand the specified number of bits to the left. Excess bits shifted off to the left are discarded. Zero bits are shifted in from the right.

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For example, 9 << 2 yields 36:

+ +
.    9 (base 10): 00000000000000000000000000001001 (base 2)
+                  --------------------------------
+9 << 2 (base 10): 00000000000000000000000000100100 (base 2) = 36 (base 10)
+
+ +

Bitwise shifting any number x to the left by y bits yields x * 2 ** y.

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>> (Sign-propagating right shift)

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This operator shifts the first operand the specified number of bits to the right. Excess bits shifted off to the right are discarded. Copies of the leftmost bit are shifted in from the left. Since the new leftmost bit has the same value as the previous leftmost bit, the sign bit (the leftmost bit) does not change. Hence the name "sign-propagating".

+ +

For example, 9 >> 2 yields 2:

+ +
.    9 (base 10): 00000000000000000000000000001001 (base 2)
+                  --------------------------------
+9 >> 2 (base 10): 00000000000000000000000000000010 (base 2) = 2 (base 10)
+
+ +

Likewise, -9 >> 2 yields -3, because the sign is preserved:

+ +
.    -9 (base 10): 11111111111111111111111111110111 (base 2)
+                   --------------------------------
+-9 >> 2 (base 10): 11111111111111111111111111111101 (base 2) = -3 (base 10)
+
+ +

>>> (Zero-fill right shift)

+ +

This operator shifts the first operand the specified number of bits to the right. Excess bits shifted off to the right are discarded. Zero bits are shifted in from the left. The sign bit becomes 0, so the result is always non-negative.

+ +

For non-negative numbers, zero-fill right shift and sign-propagating right shift yield the same result. For example, 9 >>> 2 yields 2, the same as 9 >> 2:

+ +
.     9 (base 10): 00000000000000000000000000001001 (base 2)
+                   --------------------------------
+9 >>> 2 (base 10): 00000000000000000000000000000010 (base 2) = 2 (base 10)
+
+ +

However, this is not the case for negative numbers. For example, -9 >>> 2 yields 1073741821, which is different than -9 >> 2 (which yields -3):

+ +
.     -9 (base 10): 11111111111111111111111111110111 (base 2)
+                    --------------------------------
+-9 >>> 2 (base 10): 00111111111111111111111111111101 (base 2) = 1073741821 (base 10)
+
+ +

Examples

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Flags and bitmasks

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The bitwise logical operators are often used to create, manipulate, and read sequences of flags, which are like binary variables. Variables could be used instead of these sequences, but binary flags take much less memory (by a factor of 32).

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Suppose there are 4 flags:

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These flags are represented by a sequence of bits: DCBA. When a flag is set, it has a value of 1. When a flag is cleared, it has a value of 0. Suppose a variable flags has the binary value 0101:

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var flags = 5;   // binary 0101
+
+ +

This value indicates:

+ + + +

Since bitwise operators are 32-bit, 0101 is actually 00000000000000000000000000000101, but the preceding zeroes can be neglected since they contain no meaningful information.

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A bitmask is a sequence of bits that can manipulate and/or read flags. Typically, a "primitive" bitmask for each flag is defined:

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var FLAG_A = 1; // 0001
+var FLAG_B = 2; // 0010
+var FLAG_C = 4; // 0100
+var FLAG_D = 8; // 1000
+
+ +

New bitmasks can be created by using the bitwise logical operators on these primitive bitmasks. For example, the bitmask 1011 can be created by ORing FLAG_A, FLAG_B, and FLAG_D:

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var mask = FLAG_A | FLAG_B | FLAG_D; // 0001 | 0010 | 1000 => 1011
+
+ +

Individual flag values can be extracted by ANDing them with a bitmask, where each bit with the value of one will "extract" the corresponding flag. The bitmask masks out the non-relevant flags by ANDing with zeroes (hence the term "bitmask"). For example, the bitmask 0100 can be used to see if flag C is set:

+ +
// if we own a cat
+if (flags & FLAG_C) { // 0101 & 0100 => 0100 => true
+   // do stuff
+}
+
+ +

A bitmask with multiple set flags acts like an "either/or". For example, the following two are equivalent:

+ +
// if we own a bat or we own a cat
+// (0101 & 0010) || (0101 & 0100) => 0000 || 0100 => true
+if ((flags & FLAG_B) || (flags & FLAG_C)) {
+   // do stuff
+}
+
+ +
// if we own a bat or cat
+var mask = FLAG_B | FLAG_C; // 0010 | 0100 => 0110
+if (flags & mask) { // 0101 & 0110 => 0100 => true
+   // do stuff
+}
+
+ +

Flags can be set by ORing them with a bitmask, where each bit with the value one will set the corresponding flag, if that flag isn't already set. For example, the bitmask 1100 can be used to set flags C and D:

+ +
// yes, we own a cat and a duck
+var mask = FLAG_C | FLAG_D; // 0100 | 1000 => 1100
+flags |= mask;   // 0101 | 1100 => 1101
+
+ +

Flags can be cleared by ANDing them with a bitmask, where each bit with the value zero will clear the corresponding flag, if it isn't already cleared. This bitmask can be created by NOTing primitive bitmasks. For example, the bitmask 1010 can be used to clear flags A and C:

+ +
// no, we don't have an ant problem or own a cat
+var mask = ~(FLAG_A | FLAG_C); // ~0101 => 1010
+flags &= mask;   // 1101 & 1010 => 1000
+
+ +

The mask could also have been created with ~FLAG_A & ~FLAG_C (De Morgan's law):

+ +
// no, we don't have an ant problem, and we don't own a cat
+var mask = ~FLAG_A & ~FLAG_C;
+flags &= mask;   // 1101 & 1010 => 1000
+
+ +

Flags can be toggled by XORing them with a bitmask, where each bit with the value one will toggle the corresponding flag. For example, the bitmask 0110 can be used to toggle flags B and C:

+ +
// if we didn't have a bat, we have one now,
+// and if we did have one, bye-bye bat
+// same thing for cats
+var mask = FLAG_B | FLAG_C;
+flags = flags ^ mask;   // 1100 ^ 0110 => 1010
+
+ +

Finally, the flags can all be flipped with the NOT operator:

+ +
// entering parallel universe...
+flags = ~flags;    // ~1010 => 0101
+
+ +

Conversion snippets

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Convert a binary String to a decimal Number:

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var sBinString = '1011';
+var nMyNumber = parseInt(sBinString, 2);
+alert(nMyNumber); // prints 11, i.e. 1011
+
+ +

Convert a decimal Number to a binary String:

+ +
var nMyNumber = 11;
+var sBinString = nMyNumber.toString(2);
+alert(sBinString); // prints 1011, i.e. 11
+
+ +

Automate Mask Creation

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You can create multiple masks from a set of Boolean values, like this:

+ +
function createMask() {
+  var nMask = 0, nFlag = 0, nLen = arguments.length > 32 ? 32 : arguments.length;
+  for (nFlag; nFlag < nLen; nMask |= arguments[nFlag] << nFlag++);
+  return nMask;
+}
+var mask1 = createMask(true, true, false, true); // 11, i.e.: 1011
+var mask2 = createMask(false, false, true); // 4, i.e.: 0100
+var mask3 = createMask(true); // 1, i.e.: 0001
+// etc.
+
+alert(mask1); // prints 11, i.e.: 1011
+
+ +

Reverse algorithm: an array of booleans from a mask

+ +

If you want to create an Array of Booleans from a mask you can use this code:

+ +
function arrayFromMask(nMask) {
+  // nMask must be between -2147483648 and 2147483647
+  if (nMask > 0x7fffffff || nMask < -0x80000000) {
+    throw new TypeError('arrayFromMask - out of range');
+  }
+  for (var nShifted = nMask, aFromMask = []; nShifted;
+       aFromMask.push(Boolean(nShifted & 1)), nShifted >>>= 1);
+  return aFromMask;
+}
+
+var array1 = arrayFromMask(11);
+var array2 = arrayFromMask(4);
+var array3 = arrayFromMask(1);
+
+alert('[' + array1.join(', ') + ']');
+// prints "[true, true, false, true]", i.e.: 11, i.e.: 1011
+
+ +

You can test both algorithms at the same time…

+ +
var nTest = 19; // our custom mask
+var nResult = createMask.apply(this, arrayFromMask(nTest));
+
+alert(nResult); // 19
+
+ +

For the didactic purpose only (since there is the Number.toString(2) method), we show how it is possible to modify the arrayFromMask algorithm in order to create a String containing the binary representation of a Number, rather than an Array of Booleans:

+ +
function createBinaryString(nMask) {
+  // nMask must be between -2147483648 and 2147483647
+  for (var nFlag = 0, nShifted = nMask, sMask = ''; nFlag < 32;
+       nFlag++, sMask += String(nShifted >>> 31), nShifted <<= 1);
+  return sMask;
+}
+
+var string1 = createBinaryString(11);
+var string2 = createBinaryString(4);
+var string3 = createBinaryString(1);
+
+alert(string1);
+// prints 00000000000000000000000000001011, i.e. 11
+
+ +

Specifications

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SpecificationStatusComment
{{SpecName('ES1')}}{{Spec2('ES1')}}Initial definition.
{{SpecName('ES5.1', '#sec-11.7')}}{{Spec2('ES5.1')}}Defined in several sections of the specification: Bitwise NOT operator, Bitwise shift operators, Binary bitwise operators
{{SpecName('ES6', '#sec-bitwise-shift-operators')}}{{Spec2('ES6')}}Defined in several sections of the specification: Bitwise NOT operator, Bitwise shift operators, Binary bitwise operators
{{SpecName('ESDraft', '#sec-bitwise-shift-operators')}}{{Spec2('ESDraft')}}Defined in several sections of the specification: Bitwise NOT operator, Bitwise shift operators, Binary bitwise operators
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Browser compatibility

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{{Compat("javascript.operators.bitwise")}}

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See also

+ + -- cgit v1.2.3-54-g00ecf