[英]How do you evaluate z = x- - == y + 1;
given that鉴于
int w = 1;
int x = 6;
int y = 5;
int z = 0;
z = !z || !x && !y;
printf("%d\n", z);
z = x-- == y + 1;
printf("%d\n", z);
Could someone explain how the line below would evaluate to 1 if x-- is 5 and y+1 is 6?如果 x-- 是 5 并且 y+1 是 6,有人可以解释下面的行如何评估为 1 吗?
z = x-- == y + 1;
The expression x--
evaluated to the value of x
before being decremented.表达式x--
在递减之前计算为x
的值。
So x-- == y + 1
is the same as 6 == 5 + 1
which is true, then the value 1 is assigned to z
.所以x-- == y + 1
与6 == 5 + 1
相同,这是真的,然后将值 1 分配给z
。
The expression表达方式
z = x-- == y + 1
will be parsed as将被解析为
z = ((x--) == (y + 1))
IOW, you are comparing the result of x--
to the result of y + 1
, and assigning the result of that comparison to z
.督察,你的结果比较x--
到的结果, y + 1
,然后将其赋值比较的结果z
。
The result of x--
is the current value of x
;的结果x--
是的当前值x
; the side effect is that x
is decremented.副作用是x
递减。 So, given所以,给定
x == 6
y == 5
then然后
x-- == 6
y + 1 == 6
(x-- == y + 1) == 1
so 1
is assigned to z
;所以1
被分配给z
; after this evaluation, x
will equal 5
.在此评估之后, x
将等于5
。
Note that C does not force left-to-right evaluation in this case - y + 1
may be evaluated before x--
.请注意,在这种情况下,C 不会强制从左到右求值 - y + 1
可以在x--
之前求x--
。 Also be aware that the side effect of the --
operator on x
does not have to be applied immediately after evaluation - the whole thing could be evaluated as另请注意,不必在评估后立即应用--
运算符对x
的副作用 - 整个事情可以评估为
t1 <- y + 1
t2 <- x
z <- t2 == t1
x <- x - 1
or要么
t1 <- x
t2 <- y + 1
x <- x - 1
z <- t1 == t2
or any other order.或任何其他订单。 The update to x
and the assignment to z
can happen in any order, even simultaneously (either interleaved or in parallel).对x
的更新和对z
的分配可以以任何顺序发生,甚至同时发生(交错或并行)。
While idiomatic C is often a little head-scratchy, this takes things a bit too far.虽然惯用的 C 通常有点令人头疼,但这有点太过分了。 As an academic exercise for learning about operator precedence and side effects it's okay (not great), but anyone who did this in production code could expect some grief over it in a code review.作为学习运算符优先级和副作用的学术练习,这很好(不是很好),但是任何在生产代码中这样做的人都可能会在代码审查中对此感到悲伤。 If nothing else it doesn't scan well - just adding some parens (like I did above) would greatly help with readability.如果没有其他问题,它不能很好地扫描 - 只需添加一些括号(就像我上面所做的那样)将极大地帮助提高可读性。
From the C Standard (6.5.2.4 Postfix increment and decrement operators)来自 C 标准(6.5.2.4 后缀自增和自减运算符)
2 The result of the postfix ++ operator is the value of the operand. 2 后缀++运算符的结果是操作数的值。 As a side effect, the value of the operand object is incremented (that is, the value 1 of the appropriate type is added to it).作为副作用,操作数对象的值会增加(即,将适当类型的值 1 添加到其中)。
3 The postfix -- operator is analogous to the postfix ++ operator, except that the value of the operand is decremented (that is, the value 1 of the appropriate type is subtracted from it) 3 后缀--运算符与后缀++运算符类似,只是操作数的值是递减的(即减去相应类型的值1)
Thus in this expression statement因此在这个表达式语句中
z = x-- == y + 1;
that may be equivalently rewritten like可以等效地重写为
z = ( x-- ) == ( y + 1 );
the value of the postfix decrement expression x--
is the value of the variable x
before decrementing.后缀递减表达式x--
的值是递减前变量x
的值。 That is the value of the expression is equal to 6
.即表达式的值等于6
。
The value of the expression y + 1
is also equal to 6
because the value of y
is equal to 5
.表达式y + 1
值也等于6
因为y
的值等于5
。
And at last (the C Standard, 6.5.9 Equality operators)最后(C 标准,6.5.9 相等运算符)
3 The == (equal to) and != (not equal to) operators are analogous to the relational operators except for their lower precedence.108) Each of the operators yields 1 if the specified relation is true and 0 if it is false. 3 ==(等于)和 !=(不等于)运算符类似于关系运算符,只是它们的优先级较低。108)如果指定的关系为真,则每个运算符都产生 1,如果指定的关系为假,则产生 0。 The result has type int.结果的类型为 int。 For any pair of operands, exactly one of the relations is true对于任何一对操作数,只有一个关系为真
So as the value of the expression x--
is equal to the value of the expression y + 1
then the variable z
gets the value 1
.因此,当表达式x--
的值等于表达式y + 1
的值时,变量z
的值为1
。
You could get the expected by you result keeping the postfix decrement operator the following way您可以通过以下方式获得预期的结果保持后缀递减运算符
z = ( x--, x == y + 1 );
by inserting the comma operator.通过插入逗号运算符。 In this case after the comma operator there is a sequence point that means that in second operand of the comma operator x == y + 1
x
will be already equal to 5.在这种情况下,在逗号运算符之后有一个序列点,这意味着在逗号运算符的第二个操作数中x == y + 1
x
将已经等于 5。
On the other hand, if you will write for example the expression like this另一方面,如果你会写这样的表达式
z = --x == y + 1;
where instead of the postfix decrement operator there is used unary decrement operator -- then the value of the expression --x will be equal to 5
(the value of the unary decrement operator is the value of its operand after decrementing).其中使用一元减量运算符代替后缀减量运算符——那么表达式 --x 的值将等于5
(一元减量运算符的值是其减量后的操作数的值)。 In this case the variable z gets the value 0
because 5
is not equal to 6
(the value of the expression y + 1
).在这种情况下,变量 z 的值为0
因为5
不等于6
(表达式y + 1
的值)。
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