我不明白為什么這個循環不是無限的

Question

要停止循環， foundAtPosition需要等於-1 ，那是怎么回事？

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

我不明白為什么這個循環不是無限的

如果不傳遞foundAtPosition的第二個參數，它將是無限的

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

但是，由於您傳遞了此參數，因此第二次以后，它將從（在foundAtPosition索引之后）開始foundAtPosition ，最終它將具有-1

Answer 2

while循環執行while條件評估為true 。 當在字符串中找不到給定的單詞時， indexOf返回索引-1 。

Answer 3

關鍵在於這兩行

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

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

該算法正在遍歷您的String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

結果： wroxCount === 1

資源

• String.prototype.indexOf()