Javascript 循环性能 : Counting occurrences of a number in a finite series

Question

编写 javascript 循环 以计算 7 的出现次数 的最有效方法是什么(作为示例数字) 从1数到100会遇到什么？

示例:

function numberOccurences(targetNumber, minNumber, maxNumber) {

var count = 0;
for (i = minNumber; i < maxNumber; i++) {
    count = count + (i.toString().split(targetNumber).length - 1);     
}
return count;
}

var result = numberOccurences(7,1,100);

Answer 1

这将在不查看实际数字的情况下完成。对不起，没有循环，但你确实要求效率。如果你真的想使用循环，进行递归和迭代。

function digitOccurences(digit, min, max, base) {
    if (typeof base != "number") base = 10;
    return digitOccurencesPlus(digit, max, base, 1, 0) - digitOccurencesPlus(digit, min, base, 1, 0);

    function digitOccurencesPlus(digit, N, base, pow, rest) {
        if (N == 0) return 0;
        var lastDigit = N%base,
            prevDigits = (N-lastDigit)/base;
        var occsInLastDigit = pow*(prevDigits+(lastDigit>digit));
        var occsOfLastInRest = rest * (lastDigit==digit);
        // console.log(prevDigits+" "+lastDigit, rest, occsInLastDigit, occsOfLastInRest);
        return occsInLastDigit + occsOfLastInRest + digitOccurencesPlus(digit, prevDigits, base, pow*base, pow*lastDigit+rest);
     }
}