我不明白为什么这个循环不是无限的

Question

要阻止循环foundAtPosition需要等于-1，那该怎么办？

var myString = "Welcome to Wrox books. ";
myString = myString + "The Wrox website is www.wrox.com. ";
myString = myString + "Visit the Wrox website today. Thanks for buying Wrox";
var foundAtPosition = 0;
var wroxCount = 0;
while (foundAtPosition != -1) {
    foundAtPosition = myString.indexOf("Wrox", foundAtPosition);
    if (foundAtPosition != -1) {
        wroxCount++;
        foundAtPosition++;
    }
}
document.write("There are " + wroxCount + " occurrences of the word Wrox");

Answer 1

  

我不明白为什么这个循环不是无限的

如果你没有传递第二个参数foundAtPosition

，那将是无限的

foundAtPosition = myString.indexOf("Wrox", foundAtPosition);

但是，既然你传递了这个参数，那么第二次它会从（在foundAtPosition索引之后）查看，最终会有-1

Answer 2

while循环正在执行，而的条件评估为 true 。如果在字符串中找不到给定的单词，则indexOf会返回索引 -1 。

Answer 3

密钥依赖于这两行

- This line
|
v
foundAtPosition = myString.indexOf("Wrox", foundAtPosition);

if (foundAtPosition != -1) {
    wroxCount++;
    foundAtPosition++; <- And this line
}

算法正在移动字符串的位置：

例如：

var phrase = "The Wrox website is www.wrox.com." <- Iteration [1]
                  ^
                  |_ First occurence at position [4] -> foundAtPosition++ = 5 -> wroxCount = 1


"The Wrox website is www.wrox.com." <- Iteration [2] -> The algorithm won't find the word because the `indexOf` function finds the index from position [5] til `phrase.length`.
                                  ^
                                  |_ No more occurences, therefore, foundAtPosition = -1

结果： wroxCount === 1

资源

• String.prototype.indexOf()