Common Operators

Operator operations are the operations using all kinds of operators. This article will introduce common operators in Go. More operators will be introduced in other articles later.

About Some Descriptions In Operator Explanations

This article will only introduce arithmetic operators, bitwise operators, comparison operators, boolean operators and string concatenation operator. These operators are either binary operators or unary operators. A binary operator operation takes two operands and a unary operator operation takes only one operand.

All the operator operations introduced in this articles each returns one result.

This article doesn't pursue the accuracy of some descriptions. For example, when it says that a binary operator requires the types of its two operands must be the same, what it means is:

Similarly, when it says an operator, either a binary operator or a unary operator, requires the type of one of its operands must be of a certain type, what it means is:

Constant Expressions

Before introducing all kinds of operators, we should know what are constant expressions and a fact in the evaluations of constant expressions. Expressions will get explained in a later article expressions and statements. At present, we just should know that most of the operations mentioned the current article are expressions.

If all the operands involved in an expression are constants, then this expression is called a constant expression. All constant expressions are evaluated at compile time. The evaluation result of a constant expression is still a constant.

Only if one operand in an expression is not a constant, the expression is called a non-constant expression.

Arithmetic Operators

Go supports five basic binary arithmetic operators:

Operator Name Requirements For The Two Operands
+ addition The two operands must be both values of the same basic numeric type.
- subtraction
* multiplication
/ division
% remainder The two operands must be both values of the same basic integer type.

The five operators are also often called sum, difference, product, quotient and modulo operators, respectively. Go 101 will not explain how these operator operations work in detail.

Go supports six bitwise binary arithmetic operators:

Operator Name Requirements For The Two Operands And Mechanism Explanations
& bitwise and

The two operands must be both values of the same integer type.

Mechanism explanations (a value with the subscript 2 is the binary literal form of the value):
  • 11002 & 10102 results 10002
  • 11002 | 10102 results 11102
  • 11002 ^ 10102 results 01102
  • 11002 &^ 10102 results 01002
| bitwise or
^ bitwise xor
&^ bitwise clear
<< bitwise left shift

The left operand must be an integer and the right operand must be an unsigned integer (or an untyped integer constant which is representable as an uint value).

Mechanism explanations:
  • 11002 << 3 results 11000002
  • 11002 >> 3 results 12

If the left operand is (or is viewed as) a signed integer, then the sign bit (the highest bit) in the left operand will be always kept in the result value. For example. if the left operand is an int8 value -128, or 100000002 in the binary literal form, then 100000002 << 1 results 110000002, a.k.a., -64.

>> bitwise right shift

Go also supports three unary arithmetic operators:

Operator Name Explanations
+ positive +n is equivalent to 0 + n.
- negative -n is equivalent to 0 - n.
^ bitwise complement
(bitwise not)
^n is equivalent to m ^ n, where m is a value all of which bits are 1. For example, if the type of n is int8, then m is -1, and if the type of n is uint8, then m is 0xFF.
Note,

Example:
func main() {
	var (
		a, b float32 = 12.0, 3.14
		c, d int16   = 15, -6
		e	uint8   = 7
	)

	// The ones compile okay.
	_ = 12 + 'A' // two untyped operands (both are numeric)
	_ = 12 - a   // one untyped operand and one typed operand
	_ = a * b	// two typed operands
	_ = c % d
	_, _ = c + int16(e), uint8(c) + e
	_, _, _, _ = a / b, c / d, -100 / -9, 1.23 / 1.2
	_, _, _, _ = c | d, c & d, c ^ d, c &^ d
	_, _, _, _ = d << e, 123 >> e, e >> 3, 0xF << 0
	_, _, _, _ = -b, +c, ^e, ^-1

	// The following ones fail to compile.
	_ = a % b   // error: a and b are not integers
	_ = a | b   // error: a and b are not integers
	_ = c + e   // error: type mismatching
	_ = b >> 5  // error: b is not an integer
	_ = c >> -5 // error: -5 is not representable as uint
	_ = e << c  // error: c is not an unsigned integer
}

About The Results Of Arithmetic Operator Operations

Except bitwise shift operations, the result of a binary arithmetic operator operation The rules for the result of a bitwise shift operator operation is a little complicated. Firstly, the result value is always an integer value. Whether it is typed or untyped depends on specific scenarios. Example:
func main() {
	const X, Y, Z = 2, 'A', 3i // three untyped values.
	                           // Default types: int, rune, complex64.

	var a, b int = X, Y // two typed values.

	d := X + Y // the type of d is the default type of Y: rune (int32).
	e := Y - a // the type of e is the type of a: int.
	f := a * b // the type of f is the types of a and b: int.
	g := Z * Y // the type of g is the default type of Z: complex64.

	println(X, Y, Z)    // 2 65 (+0.000000e+000+3.000000e+000i)
	println(d, e, f, g) // 67 63 130 (+0.000000e+000+1.950000e+002i)
}

Another example (bitwise shift operations):
const N = 2
const A = 3.0 << N       // A == 6, its default type is int.
const B = int8(3.0) << N // B == 6, it is typed and its type is int8.

var m = uint(32)
// The following three lines are equivalent to each other.
var x int64 = 1 << m  // the type of 1 is deduced as int64, not int.
var y = int64(1 << m) // the type of 1 is deduced as int64, not int.
var z = int64(1) << m

// The following two lines fail to compile.
/*
var _ = 1.23 << m // error: shift of type float64
const _ = 1 << B  // error: the right operand must be unsigned.
*/

The last rule for bitwise shift operator operation is to avoid some operations returning different results on different architectures. For example, the bitwise operation at line 8 (or line 7<) will return different results between 32-bit architectures and 64-bit architectures if the operand 1 is deduced as int instead of int64, which may produce some bugs hard to detect.

One intresting consequence of the last rule for bitwise shift operator operation is shown in the following code snippet:
const n = uint(2)
var m = uint(2)

// The following two lines compile okay.
var _ float64 = 1 << n
var _ = float64(1 << n)

// The following two lines fail to compile.
var _ float64 = 1 << m
var _ = float64(1 << m)
The reason of the last two lines failing to compile is they are both equivalent to the followings two line:
var _ = float64(1) << m
var _ = 1.0 << m // error: shift of type float64

About Overflows

Overflows are not allowed for typed constant values but are allowed for non-constant and untyped constant values, either the values are intermediate or final results. Overflows will be truncated (or wrapped around) for non-constant values, but overflows (for default types) on untyped constant value will not be truncated (or wrapped around).

Example:
// Results are non-constants.
var a, b uint8 = 255, 1
var c = a + b  // okay: higher overflowed bits are truncated. c == 0
var d = a << b // okay: higher overflowed bits are truncated. d == 254

// Results are untyped constants.
const X = 0x1FFFFFFFF * 0x1FFFFFFFF // okay, though X overflows int.
const R = 'a' + 0x7FFFFFFF		    // okay, though R overflows rune.

// Operation results or conversion results are typed values.
// These lines all fail to compile.
var e = X                // error: untyped constant X overflows int.
var h = R                // error: constant 2147483744 overflows rune.
const Y = 128 - int8(1)  // error: 128 overflows int8
const Z = uint8(255) + 1 // error: the result 256 overflow uint8.

About Integer Division And Remainder Operations

Assume x and y are two operands of the same integer type, the integer quotient q (= x / y) and remainder r (= x % y) satisfy x == q*y + r, where |r| < |y|. If r is not zero, its sign is the same as x (the dividend). The result of x / y is truncated towards zero.

If the divisor y is a constant, it must not be zero. If the divisor is zero at run time and it is an integer, a run-time panic occurs. Panics are like exceptions in some other languages. We can learn more about panics in this article.

Example:
println( 5/3,   5%3)  // 1 2
println( 5/-3,  5%-3) // -1 2
println(-5/3,  -5%3)  // -1 -2
println(-5/-3, -5%-3) // 1 -2

println(5.0 / 3.0)	 // 1.666667
println((1-1i)/(1+1i)) // -1i

var a, b = 1.0, 0.0
println(a/b, b/b) // +Inf NaN

_ = int(a)/int(b) // compile okay but panic at run time.

// The following two lines fail to compile.
println(1.0/0.0) // error: division by zero
println(0.0/0.0) // error: division by zero

Using op= For Binary Arithmetic Operators

For a binary arithmetic operator op, x = x op y can be shortened to x op= y. In the short form, x will be only evaluated once.

Example:
var a, b int8 = 3, 5
a += b
println(a) // 8
a *= a
println(a) // 64
a /= b
println(a) // 12
a %= b
println(a) // 2
b <<= uint(a)
println(b) // 20

The Increment ++ And Decrement -- Operators

Like many other popular languages, Go also supports the increment ++ and decrement -- operators. However, operations using the two operators don't return any results, so such operations can not be used as expressions. The only operand involved in such an operation must be a numeric value, the numeric value must not be a constant, and the ++ or -- operator must follow the operand.

Example:
package main

func main() {
	a, b, c := 12, 1.2, 1+2i
	a++ // okay
	b-- // okay
	c++ // okay

	// The following three lines fail to compile.
	/*
	_ = a++
	_ = b--
	_ = c++
	++a
	--b
	++c
	*/
}

String Concatenation Operator

As above has mentioned, the addition operator can also be used as string concatenation.
Operator Name Requirements For The Two Operands
+ string concatenation The two operands must be both values of the same string type.

The op= form also applies for the string concatenation operator.

Example:
println("Go" + "lang") // Golang
var a = "Go"
a += "lang"
println(a) // Golang

If one of the two operands of a string concatenation operation is a typed string, then the type of the result string is the same as the type of the typed string. If both the of the two operands are untyped (constant) strings, the result is also an untyped string value.

Boolean Operators

Go supports two boolean binary operators and one boolean unary operator:

Operator Name Requirements For Operand(s)
&& boolean and (binary) The two operands must be both values of the same boolean type.
|| boolean or (binary)
! boolean not (unary) The type of the only operand must be a boolean type.

We can use the != operator introduced in the next sub-section as the boolean xor operator.

Mechanism explanations:
// x    y       x && y   x || y   !x      !y
true    true    true     true     false   false
true    false   false    true     false   true
false   true    false    true     true    false
false   false   false    false    true    true

If one of the two operands is a typed boolean, then the type of the result boolean is the same as the type of the typed boolean. If both the of the two operands are untyped booleans, the result is also an untyped boolean value.

Comparison Operators

Go supports six comparison binary operators:

Operator Name Requirements For The Two Operands
== equal to Generally, the types of its two operands must be the same. For detailed rules, please read comparison rules in Go.
!= not equal to
< less than The two operands must be both values of the same integer type, floating-point type or string type.
<= less than or equal to
> larger than
>= larger than or equal to

The type of the result of any comparison operation is always an untyped boolean value. If both of the two operands of a comparison operation are constant, the result is also a constant (boolean) value.

Later, if we say two values are comparable, we mean they can be compared with the == and != operators. We will learn that values of which types are not comparable later. Values of basic types are all comparable.

Please note that, not all real numbers can be accurately represented in memory, so comparing two floating-point (or complex) values may be not reliable. We should check whether or not the absolution of the difference of two floating-point values is smaller than a small threshold to judge whether or not the two floating-point values are equal.

Operator Precedences

The operator precedences may be also some different to other languages. Here are the operator precedences in Go. Top ones have higher precedences. The precedences of operators in the same line are same. Like many other languages, () can be used to promote precedences.

*   /   %   <<  >>  &   &^
+   -   |   ^
==  !=  <   <=  >   >=
&&
||

One obvious difference to some other popular languages is that the precedences of << and >> are higher than + and - in Go.

More About Constant Expressions

The following declared variable will be initialized as 2.2 instead of 2.7. The reason is the division operation has a higher precedence than the addition operation, and in the division operation, both 3 and 2 are viewed as integers. The evaluation result of 3/2 is 1.
var x = 1.2 + 3/2

The two named constants declared in the following program are not equal. In the first declaration, both 3 and 2 are viewed as integers, however, they are both viewed as floating-point numbers in the second declaration.
package main

const x = 3/2*0.1
const y = 0.1*3/2

func main() {
	println(x) // +1.000000e-001
	println(y) // +1.500000e-001
}

More Operators

Same as C/C++, there are two pointer related operators, * and &. Yes the same operator symbols as the multiplication and bitwise-and operators. & is used to take the address of an addressable value, and * is used to deference a pointer value. Unlike C/C++, in Go, values of pointer types don't support arithmetic operations. For more details, please read pointers in Go later.

There are some other operators in Go. They will be introduced and explained in other Go 101 articles.


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