我不明白输出("four
")是怎么来的?
$a = 2;
echo
$a == 1 ? 'one' :
$a == 2 ? 'two' :
$a == 3 ? 'three' :
$a == 5 ? 'four' :
'other'
;
// prints 'four'
我不明白为什么要打印"four
"。
您需要将三元条件词括起来:
<?php
for ($a=0; $a < 7; $a++) {
echo (
$a == 1 ? 'one' :
($a == 2 ? 'two' :
($a == 3 ? 'three' :
($a == 5 ? 'four' : 'other'))));
echo "'n";
// prints 'four'
}
exit;
?>
退货:
other
one
two
three
other
four
other
正如你所料。
请参阅PHP三元运算符帮助中"三元运算符"底部的注释。
正在从左到右对表达式求值。所以你实际上得到了:
echo (
((($a == 1 ? 'one' : $a == 2)
? 'two' : $a == 3) ? 'three' :
$a == 5) ? 'four' : 'other');
因此,对于$a=2
,您可以得到:
echo (
((($a==2) ? 'two' : $a == 3) ? 'three' :
$a == 5) ? 'four' : 'other');
然后
echo (
((true ? 'two' : $a == 3) ? 'three' :
$a == 5) ? 'four' : 'other');
然后
echo (
('two' ? 'three' : $a == 5) ? 'four' : 'other');
然后
echo (
'three' ? 'four' : 'other');
以及CCD_ 4。
请记住,PHP是动态类型化的,并将任何非零非null值视为TRUE。
在《PHP手册》的比较运算符页面上,他们解释说,当嵌套(堆叠)三元运算符时,PHP的行为是"不明显"。
你写的代码是这样的:
$a = 2;
echo
((($a == 1 ? 'one' :
$a == 2) ? 'two' :
$a == 3) ? 'three' :
$a == 5) ? 'four' :
'other'
;
// prints 'four'
由于$a
为2,并且'two'
和'three'
都为TRUE,因此您将得到"four
",因为如果'four'
为TRUE或否,则不再进行比较。
如果你想改变这一点,你必须把括号放在不同的地方[也由:BeingSimpler和MGwynne]:
$a = 2;
echo
($a == 1 ? 'one' :
($a == 2 ? 'two' :
($a == 3 ? 'three' :
($a == 5 ? 'four' :
'other'))))
;
// prints 'two'
分组条件有问题,只需要添加括号将它们隔开。
$a = 2;
echo (
$a == 1 ? 'one' :
($a == 2 ? 'two' :
($a == 3 ? 'three' :
($a == 5 ? 'four' : 'other'))));
echo "'n";
// prints 'four'
exit;
已解决。
这是我为帮助自己理解三元运算符的左与右结合性而提出的。
// PHP
$a = "T";
$vehicle = $a == "B" ? "bus" :
$a == "A" ? "airplane" :
$a == "T" ? "train" :
$a == "C" ? "car" :
$a == "H" ? "horse" : "feet";
// (as seen by the PHP interpreter)
// INITIAL EXPRESSION: ((((($a == "B" ? "bus" : $a == "A") ? "airplane" : $a == "T") ? "train" : $a == "C") ? "car" : $a == "H") ? "horse" : "feet");
// STEP 1: (((((FALSE ? "bus" : FALSE) ? "airplane" : TRUE) ? "train" : FALSE) ? "car" : FALSE) ? "horse" : "feet")
// STEP 2: ((((FALSE ? "airplane" : TRUE) ? "train" : FALSE) ? "car" : FALSE) ? "horse" : "feet")
// STEP 3: (((TRUE ? "train" : FALSE) ? "car" : FALSE) ? "horse" : "feet")
// STEP 4: (("train" ? "car" : FALSE) ? "horse" : "feet")
// STEP 5: ("car" ? "horse" : "feet")
// FINAL EVALUATION: ("horse")
// If you used the initial expression here (with the parenthesis) in a different language, it would also evaluate to "horse."
echo $vehicle; // gives us "horse"
这与相反
// EVERY OTHER LANGUAGE
var a = "T";
var vehicle = a == "B" ? "bus" :
a == "A" ? "airplane" :
a == "T" ? "train" :
a == "C" ? "car" :
a == "H" ? "horse" : "feet";
// (as seen by the other language's interpreter)
// INITIAL EXPRESSION: (a == "B" ? "bus" : (a == "A" ? "airplane" : (a == "T" ? "train" : (a == "C" ? "car" : (a == "H" ? "horse" : "feet")))));
// STEP 1: (FALSE ? "bus" : (FALSE ? "airplane" : (TRUE ? "train" : (FALSE ? "car" : (FALSE ? "horse" : "feet")))))
// STEP 2: (FALSE ? "bus" : (FALSE ? "airplane" : (TRUE ? "train" : (FALSE ? "car" : "feet"))))
// STEP 3: (FALSE ? "bus" : (FALSE ? "airplane" : (TRUE ? "train" : "feet")))
// STEP 4: (FALSE ? "bus" : (FALSE ? "airplane" : "train"))
// STEP 5: (FALSE ? "bus" : "train")
// FINAL EVALUATION: ("train")
// If you used the initial expression here (with the parenthesis) in PHP, it would also evaluate to "train."
console.log(vehicle); // gives us "train"
如果您注意到,在PHP示例中,最里面的表达式在左边,而在第二个示例中,最小的表达式在右边。每个步骤都计算下一个最里面的表达式,直到得到一个结果。如果要在PHP中嵌套三元运算,圆括号显然非常重要!