为什么最后一行代码是必要的？

Question

我从我正在阅读的一本书上的一个示例中理解了以下所有代码。除了注释行。我在想，没有它，循环就永远不会结束？不过，我不明白它背后的逻辑。

var drink = "Energy drink";
var lyrics = "";
var cans = "99";

while (cans > 0) {
    lyrics = lyrics + cans + " cans of " + drink + " on the wall <br>";
    lyrics = lyrics + cans + " cans of " + drink + " on the wall <br>";
    lyrics = lyrics + "Take one down, pass it around, <br>";

    if (cans > 1) {
        lyrics = lyrics + (cans-1) + " cans of" + drink + " on the wall  <br>";
    }
    else {
        lyrics = lyrics + "No more cans of" + drink + " on the wall<br>";
    }
    cans = cans - 1;  // <-- This line I don't understand
}

document.write(lyrics);

Answer 1

这是一个从99 (var cans = "99")开始的循环，然后向后计数到0。突出显示的行是“减一”的行。如果不是这一行，它将永远循环并添加99 cans of Energy drink on the wall。

顺便说一句，document.write是不好的，var cans = "99"应该是var cans = 99。当然，这可能不是你的代码，只是说说而已。我的建议是:继续读下去。

Answer 2

就像D·斯特劳特说的，这是一个循环。基本上，你的代码是说“当变量cans大于0时，执行这个操作”。如果cans的值永远不会改变，那么它将失败或无限重复。所以基本上你是从99罐开始的。对于每个罐头，它将罐头数量减去1，直到其为0，此时循环终止。

Answer 3

D·斯特劳特是对的。

但仅供参考-因为你正在学习-你也可以用'for‘循环而不是'while’循环做同样的事情。

如下所示：

// store the div in a variable to use later - at the bottom 
var beer_holder = document.getElementById("beer_holder");

// Non-alcoholic to keep it PG
var drink = "O'Doul's";

var lyrics = "";

var cans = "99";

// Create a variable called 'i' and set it to the cans variable amount. 
// Check if i is greater than 0,
// if it is, than do what is between the curly brackets, then subtract 1 from cans
for(var i=cans; i > 0; i--){

    lyrics = i + " cans of " + drink + " on the wall, " + i + " cans of " + drink +
        " take one down, pour it down the sink, " + (i - 1) + " cans of O'Doul's on the wall."

    // take the paragraph tag and put the lyrics within it
    //  the <br/> tags make sure each lyric goes on a seperate line
    beer_holder.innerHTML += lyrics + "<br/><br/>";

}

如果您想使用它，这里有一个指向工作示例的链接：http://jsfiddle.net/phillipkregg/CHyh2/

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