C语言 有符号整数和无符号整数有什么区别

What is the difference between signed and unsigned int

提问by Moumita Das

What is the difference between signed and unsigned int?

有符号整数和无符号整数有什么区别？

回答by Bill Evans at Mariposa

As you are probably aware, ints are stored internally in binary. Typically an intcontains 32 bits, but in some environments might contain 16 or 64 bits (or even a different number, usually but not necessarily a power of two).

您可能知道，ints 在内部以二进制形式存储。通常 anint包含 32 位，但在某些环境中可能包含 16 或 64 位（甚至是不同的数字，通常但不一定是 2 的幂）。

But for this example, let's look at 4-bit integers. Tiny, but useful for illustration purposes.

但是对于这个例子，让我们看看 4 位整数。很小，但对于说明目的很有用。

Since there are four bits in such an integer, it can assume one of 16 values; 16 is two to the fourth power, or 2 times 2 times 2 times 2. What are those values? The answer depends on whether this integer is a signed intor an unsigned int. With an unsigned int, the value is never negative; there is no sign associated with the value. Here are the 16 possible values of a four-bit unsigned int:

由于这样的整数有 4 位，因此它可以采用 16 个值之一；16 是 2 的四次方，或 2 乘以 2 乘以 2。这些值是多少？答案取决于这个整数是 asigned int还是 an unsigned int。使用unsigned int，值永远不会为负；没有与该值相关联的符号。以下是四位的 16 个可能值unsigned int：

bits  value
0000    0
0001    1
0010    2
0011    3
0100    4
0101    5
0110    6
0111    7
1000    8
1001    9
1010   10
1011   11
1100   12
1101   13
1110   14
1111   15

... and Here are the 16 possible values of a four-bit signed int:

...这里是四位的 16 个可能值signed int：

bits  value
0000    0
0001    1
0010    2
0011    3
0100    4
0101    5
0110    6
0111    7
1000   -8
1001   -7
1010   -6
1011   -5
1100   -4
1101   -3
1110   -2
1111   -1

As you can see, for signed ints the most significant bit is 1if and only if the number is negative. That is why, for signed ints, this bit is known as the "sign bit".

如您所见，对于signed ints，最高有效位是1当且仅当数字为负时。这就是为什么，对于signed ints，这个位被称为“符号位”。

回答by Keith Thompson

intand unsigned intare two distinct integer types. (intcan also be referred to as signed int, or just signed; unsigned intcan also be referred to as unsigned.)

int和unsigned int是两种不同的整数类型。（int也可称为signed int，或仅称为signed；unsigned int也可称为unsigned。）

As the names imply, intis a signedinteger type, and unsigned intis an unsignedinteger type. That means that intis able to represent negative values, and unsigned intcan represent only non-negative values.

顾名思义，int是有符号整数类型，并且unsigned int是无符号整数类型。这意味着它int能够表示负值，并且unsigned int只能表示非负值。

The C language imposes some requirements on the ranges of these types. The range of intmust be at least -32767.. +32767, and the range of unsigned intmust be at least 0.. 65535. This implies that both types must be at least 16 bits. They're 32 bits on many systems, or even 64 bits on some. inttypically has an extra negative value due to the two's-complement representation used by most modern systems.

C 语言对这些类型的范围提出了一些要求。的范围int必须至少为-32767.. +32767，范围unsigned int必须至少为0.. 65535。这意味着两种类型都必须至少为 16 位。它们在许多系统上是 32 位，在某些系统上甚至是 64 位。int由于大多数现代系统使用二进制补码表示，因此通常具有额外的负值。

Perhaps the most important difference is the behavior of signed vs. unsigned arithmetic. For signed int, overflow has undefined behavior. For unsigned int, there is no overflow; any operation that yields a value outside the range of the type wraps around, so for example UINT_MAX + 1U == 0U.

也许最重要的区别是有符号和无符号算术的行为。对于 signed int，溢出具有未定义的行为。对于unsigned int，没有溢出；任何产生超出类型范围的值的操作都会环绕，例如UINT_MAX + 1U == 0U.

Any integer type, either signed or unsigned, models a subrange of the infinite set of mathematical integers. As long as you're working with values within the range of a type, everything works. When you approach the lower or upper bound of a type, you encounter a discontinuity, and you can get unexpected results. For signed integer types, the problems occur only for very large negative and positive values, exceeding INT_MINand INT_MAX. For unsigned integer types, problems occur for very large positive values and at zero. This can be a source of bugs. For example, this is an infinite loop:

任何整数类型，无论是有符号还是无符号，都对无限数学整数集的子范围进行建模。只要您使用类型范围内的值，一切都会正常。当您接近类型的下限或上限时，您会遇到不连续性，并且可能会得到意想不到的结果。对于有符号整数类型，只有非常大的负值和正值，超过INT_MIN和时才会出现问题INT_MAX。对于无符号整数类型，非常大的正值和零会出现问题。这可能是错误的来源。例如，这是一个无限循环：

for (unsigned int i = 10; i >= 0; i --) [
    printf("%u\n", i);
}

because iis alwaysgreater than or equal to zero; that's the nature of unsigned types. (Inside the loop, when iis zero, i--sets its value to UINT_MAX.)

因为i是始终大于或等于零; 这就是无符号类型的本质。（在循环内部，当i为零时，i--将其值设置为UINT_MAX。）

回答by imran

Sometimes we know in advance that the value stored in a given integer variable will always be positive-when it is being used to only count things, for example. In such a case we can declare the variable to be unsigned, as in, unsigned int num student;. With such a declaration, the range of permissible integer values (for a 32-bit compiler) will shift from the range -2147483648 to [phone] to range 0 to 4294967295. Thus, declaring an integer as unsigned almost doubles the size of the largest possible value that it can otherwise hold.

有时我们事先知道存储在给定整数变量中的值总是正的——例如，当它仅用于计数时。在这种情况下，我们可以将变量声明为无符号变量，如 , unsigned int num student;。使用这样的声明，允许的整数值范围（对于 32 位编译器）将从 -2147483648 到 [phone] 的范围转移到 0 到 4294967295 的范围。因此，将整数声明为无符号几乎会使最大可能的大小增加一倍否则它可以持有的价值。

回答by user2977636

In laymen's terms an unsigned int is an integer that can not be negative and thus has a higher range of positive values that it can assume. A signed int is an integer that can be negative but has a lower positive range in exchange for more negative values it can assume.

用外行的话来说，unsigned int 是一个不能为负的整数，因此它可以假设的正值范围更大。有符号 int 是一个整数，它可以是负数，但具有较低的正范围以换取它可以假设的更多负值。

回答by nano

In practice, there are two differences:

在实践中，有两个区别：

1. printing(eg with coutin C++ or printfin C): unsigned integer bit representation is interpreted as a nonnegative integer by print functions.
2. ordering: the ordering depends on signed or unsigned specification.

1. 打印（例如cout在 C++ 或printfC 中）：无符号整数位表示被打印函数解释为非负整数。
2. ordering：排序取决于有符号或无符号规范。

this code can identify the integer using ordering criterion:

此代码可以使用排序标准识别整数：

char a = 0;
a--;
if (0 < a)
    printf("unsigned");
else
    printf("signed");