Question

我正在尝试为一周中的几天创建一个数组，但如果输入的数字未分配给值，我希望它显示错误。这就是我所拥有的，但它在我的if语句之后说“今天未定义”。我认为这是因为阵列。有没有办法把if语句作为数组的一部分，所以只弹出一条消息？

var dayNum=prompt('enter as number from 0 to 6', ' ');
if (dayNum >='7');
{
alert("invalid input received, try again")
} 
var weekdays = ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", " "];
alert("Today is " + weekdays[dayNum]);

Answer 1

因为分号在这里终止了陈述

if (dayNum >='7');

删除分号。此外，如果输入无效，则第二种情况仍在运行。而是将其放入else语句中。

＆＃13;

var dayNum = Number( prompt('enter as number from 0 to 6', ' ') );
if ( isNaN(dayNum) || dayNum >= 7 )
{
  alert("invalid input received, try again")
} else {
  var weekdays = ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"];
  alert("Today is " + weekdays[dayNum]);
}

＆＃13;

最后，将字符串（这是提示的返回类型）转换为数字类型然后将它们视为一个类型也是明智之举。在这种情况下，JavaScript知道数组的索引只能是一个数字，因此会自动尝试为您转换类型。只要用户键入有效数字，就可以将其用作索引而不会出现问题。但是，字符串按字母顺序进行比较。这意味着(string >= '7')可能并不总是按照您的预期行事：字母大于数字，但特殊字符较少。

Answer 2

;

之后无需if (dayNum >=7)

var dayNum=prompt('enter as number from 0 to 6', ' ');


if (dayNum >=7)
{
alert("invalid input received, try again")
} 
var weekdays = ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", " "];
alert("Today is " + weekdays[dayNum]);
document.getElementById("demo").innerHTML = weekdays[dayNum];

Answer 3

您的代码中几乎没有错误。首先是在if条件之后的分号，而另一个是使用字符串比较而不是数字。在if条件中从数字中删除引号。并且数组的索引也是错误的。你提供一个字符串。它应该是int类型。

<script>
var dayNum=parseInt(prompt('enter as number from 0 to 6', ' '));
if (dayNum >= 7)
{
alert("invalid input received, try again");

} 
else{
var weekdays = ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", " "];
alert("Today is " + weekdays[dayNum]);

}
</script>

Answer 4

实际上，如果你使用0，你会希望输入为1-7，并且要声明一个假错误：

var weekdays = ["Error", "Sun", "Mon", ..., "Sat"];

然后你需要创建一个循环来将字符串转换为数字：

for(i=0;i<=7;i++){
   if(dayNum==i){
      dayNum=i;
   }
}

这将使用近似值为您提供整数而不是字符串。你的钓鱼代码如下：

var dayNum=prompt('enter as number from 0 to 6', ' ');
for(i=0;i<=7;i++){
    if(dayNum==i){
        dayNum=i;
    }
}
if (dayNum <=7)
{
    alert("invalid input received, try again");
}
else if(dayNum === 0)
{
    alert("invalid input received, try again");
}
else
{
var weekdays = ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", " "];
alert("Today is " + weekdays[dayNum]);
}

Answer 5

＆＃13;

var dayNum=prompt('enter as number from 0 to 6', ' ');
for(i=0;i<=7;i++){
    if(dayNum==i){
        dayNum=i;
    }
}
if (dayNum >=7)
{
    alert("invalid input received, try again");
}
else if(dayNum === 0)
{
    alert("invalid input received, try again");
}
else
{
var weekdays = ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", " "];
alert("Today is " + weekdays[dayNum]);
}

＆＃13;

javascript数组if语句

5 个答案: