为什么它会为代码段建议括号？ [英] Why does it suggest parentheses for the code snippet?
问题描述
第2行的代码是否正确？
a.c：2：警告：建议围绕&&的括号在||
int isleap（int year）{
if（year％4 == 0&& year％100！= 0 ||年％400 == 0）/ *行
2 * /
返回1;
返回0;
}
2月14日17:16，lovecreatesbea ... @ gmail.com
< lovecreatesbea ... @ gmail.comwrote：
第2行的代码是否正确？
是的。这是正确的，但它并不难看。
a.c：2：警告：建议括号围绕&&在||
您的编译器有很好的品味:-)它是哪一个？
int isleap（int year ）{
if（year％4 == 0&& year％100！= 0 || year％400 == 0）/ * line
2 * /
返回1;
返回0;
}
返回（！（年份％4）&&（年％100）||！（年％
400））的奖励积分;"当然？
文章< 11 ********************** @ a34g2000cwb .googlegroups .com>，
lovecreatesbea ... @ gmail.com< lo *************** @ gmail.comwrote：
< blockquote class =post_quotes>
>第2行的代码是否正确？
> a.c：2：警告：建议围绕&&的括号在||
> int isleap（int year）{
if（year％4 == 0&& year; ％100！= 0 ||年％400 == 0）/ *行
2 * /
返回1;
返回0;
}
&&绑定比||更紧密，因此测试等同于
（（（年％4 == 0）&&（年％100！= 0））|| （年％400 == 0））
然而，人们错误地想到
&&和||由于具有相同的优先级，因此人们从左到右阅读运算符是很常见的。在这种特殊情况下，它不会产生差异，但是如果你已经写过，那么如果（年％400 == 0 ||年），则需要
％4 == 0&& year％100！= 0）
然后人们倾向于（错误地）将其视为
if（（年％400 == 0 ||年％4 == 0）&&年％100！= 0）
真的是什么时候
if（年％400 == 0 ||（年％4 == 0&年％100！= 0））
警告正在改变您对常见误读的可能性。
-
有什么可以说的，看，这是新的吗？它已经过时了，这已经是我们面前的了。 - Ecclesiastes
" lovecreatesbea ... @ gmail.com" < lo *************** @ gmail.comwrites：
第2行的代码是否正确？
ac：2：警告：建议围绕&&的括号在||
int isleap（int year）{
if（year％4 == 0&& year％100！= 0 ||年％400 == 0）/ *行
2 * /
返回1;
返回0;
}
编号这是在FAQ中。
20.32：2000年是闰年吗？是（年％4 == 0）准确的测试
闰年？
答：是和否分别。目前
格里高利历的完整表达是
年％4 == 0&& （年％100！= 0 ||年％400 == 0）
有关详细信息，请参阅优秀的天文年历或其他参考资料。
（要预防一个永恒的辩论：声称存在4000年规则的参考文献是错误的。）参见问题
13.14和13.14b。
-
Peter Seebach在C99：
[F]或大部分功能已添加，未删除。这听起来很棒
很棒，直到你试着在一天内完成大小的标准打印输出
。
Isn''t code at line 2 correct?
a.c:2: warning: suggest parentheses around && within ||
int isleap(int year){
if (year % 4 == 0 && year % 100 != 0 || year % 400 == 0) /*line
2*/
return 1;
return 0;
}
On 14 Feb, 17:16, "lovecreatesbea...@gmail.com"
<lovecreatesbea...@gmail.comwrote:Isn''t code at line 2 correct?Yeah. It''s correct, but it''s not ugly.
a.c:2: warning: suggest parentheses around && within ||Your compiler has good taste :-) Which one is it?
int isleap(int year){
if (year % 4 == 0 && year % 100 != 0 || year % 400 == 0) /*line
2*/
return 1;
return 0;
}Bonus points for "return(!(year % 4) && (year % 100) || !(year %
400));" surely?
In article <11**********************@a34g2000cwb.googlegroups .com>,
lovecreatesbea...@gmail.com <lo***************@gmail.comwrote:>Isn''t code at line 2 correct?
>a.c:2: warning: suggest parentheses around && within ||
>int isleap(int year){
if (year % 4 == 0 && year % 100 != 0 || year % 400 == 0) /*line
2*/
return 1;
return 0;
}&& binds more tightly than ||, so the test is equivilent to
(((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0))
However, it is relatively common for people to mistakenly think of
&& and || as having the same precedences, and so it is common
for people to read the operators from left to right. It doesn''t
make a difference in this particular case, but if you had written,
if (year % 400 == 0 || year % 4 == 0 && year % 100 != 0)
then people would tend to (mistakenly) read this as
if ((year % 400 == 0 || year % 4 == 0) && year % 100 != 0)
when really it is
if (year % 400 == 0 || (year % 4 == 0 && year % 100 != 0))
The warning is altering you to the possibility of the common misreading.
--
Is there any thing whereof it may be said, See, this is new? It hath
been already of old time, which was before us. -- Ecclesiastes
"lovecreatesbea...@gmail.com" <lo***************@gmail.comwrites:
Isn''t code at line 2 correct?
a.c:2: warning: suggest parentheses around && within ||
int isleap(int year){
if (year % 4 == 0 && year % 100 != 0 || year % 400 == 0) /*line
2*/
return 1;
return 0;
}No. This is in the FAQ.
20.32: Will 2000 be a leap year? Is (year % 4 == 0) an accurate test
for leap years?
A: Yes and no, respectively. The full expression for the present
Gregorian calendar is
year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)
See a good astronomical almanac or other reference for details.
(To forestall an eternal debate: references which claim the
existence of a 4000-year rule are wrong.) See also questions
13.14 and 13.14b.
--
Peter Seebach on C99:
"[F]or the most part, features were added, not removed. This sounds
great until you try to carry a full-sized printout of the standard
around for a day."
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