两个语句的结果不同

pony2001mx

大家好：
为什么以下两个语句的输出结果不同？谢谢！

1. perl -e '$a=1000; ($a + 2 > 900) ?  $b=$a-90 : ($b=length($a)); print $b;'

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输出910

1. perl -e '$a=1000; ($a + 2 > 900) ?  $b=$a-90 : $b=length($a); print $b;'

复制代码

输出4

zhlong8

操作符优先级的问题 = 比 ?: 要低。说实话在 ?: 中用稍微复杂点的表达式我都分不清优先级的，所以我不这么用。摘抄 perlop

Conditional Operator

Ternary "?:" is the conditional operator, just as in C.

Scalar or list context propagates downward into the 2nd or 3rd argument, whichever is selected.

$x = $ok ? $y : $z;  # get a scalar
@x = $ok ? @y : @z;  # get an array
$x = $ok ? @y : @z;  # oops, that's just a count!

The operator may be assigned to if both the 2nd and 3rd arguments are legal lvalues (meaning that you can assign to them):

($x_or_y ? $x : $y) = $z;

Because this operator produces an assignable result, using assignments without parentheses will get you in trouble. For example, this:

$x % 2 ? $x += 10 : $x += 2

Really means this:

(($x % 2) ? ($x += 10) : $x) += 2

Rather than this:

($x % 2) ? ($x += 10) : ($x += 2)

That should probably be written more simply as:

$x += ($x % 2) ? 10 : 2;

pony2001mx

谢谢zhlong8的例子！

回复 2# zhlong8

104359176

虽然三元表达式很酷，但因此出现的错误更酷。我尽量不用三元表达式, 除非在函数返回值中面对很简单的情况：

1. $x > 10 ? return 0 ! return 1;

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rubyish

zhlong8 发表于 2015-04-25 07:50
操作符优先级的问题 = 比 ?: 要低。

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