JS——思维拓展

1、阶乘求和：4的阶乘是1*2*3*4

<script>
    function jiechen(value) {
        var n = 1;
        for (var i = 1; i <= value; i++) {
            n *= i;
        }
        return n;
    }
    function sum(value) {
        var n = 0;
        for (var i = 1; i <= value; i++) {
            n += jiechen(i);
        }
        return n;
    }
    console.log(sum(3));//9
</script>

2、素数：又称质数，除了1和它本身，不能被其他数整除

<script>
    function isPrime(num) {
        for (var i = 2; i < num; i++) {
            if (num % i === 0) {
                return false;
            }
        }
        return true;
    }
    console.log(isPrime(9));//false
</script>

 3、将1-10中的奇数放入数组

<script>
    var arr = [];
    for (var i = 1; i <= 10; i++) {
        if (i % 2 !== 0) {
            arr[arr.length] = i;
        }
    }
    console.log(arr);//[1, 3, 5, 7, 9]
</script>

4、将1-10中的偶数放入数组

<script>
    var arr = [];
    for (var i = 1; i <= 10; i++) {
        if (i % 2 === 0) {
            arr[arr.length] = i;
        }
    }
    console.log(arr);//[2, 4, 6, 8, 10]
</script>

5、数组翻转

<script>
    var arr = ["刘备", "关羽", "张飞", "诸葛亮", "马超", "黄忠", "姜维"];
    console.log(arr);
    var newArr = [];
    for (var i = 0; i < arr.length; i++) {
        newArr[newArr.length] = arr[arr.length - 1 - i];//用新数组的长度作为索引值。
    }
    console.log(newArr);
</script>

<script>
    var arr = ["刘备", "关羽", "张飞", "诸葛亮", "马超", "黄忠", "姜维"];
    console.log(arr);
    var newArr = [];
    for (var i = 0; i < arr.length / 2; i++) {
        var temp = arr[i];
        arr[i] = arr[arr.length - 1 - i];//前面项和对应的后面项交换位置。（arr.length-1-i = 倒数第i+1项）
        arr[arr.length - 1 - i] = temp;
    }
    console.log(arr);
</script>

 5、递归法计算1-100的值

<script>
    function getSum(n) {
        if (n < 1) {
            return 0;
        }
        return n + getSum(n - 1);
    }
    console.log(getSum(100));//5050
</script>

 6、斐波那契数列：1、1、2、3、5、8、13、21、34、…

<script>
    var n1 = 1;
    var n2 = 1;
    function getNum(num) {
        for (var i = 3; i <= num; i++) {
            var temp = n2;
            n2 = n1 + n2;
            n1 = temp;
        }
        return n2;
    }
    console.log(getNum(6));//8
</script>

 7、判断闰年：四年一闰而且百年不闰或者四百年一润

<script>
    function isRun(year) {
        if (year % 4 === 0 && year % 100 !== 0 || year % 400 === 0) {
            return true;
        }
        return false;
    }
</script>

8、输入年月日，计算这天是当年的第多少天

<script>
    function isRun(year) {
        if (year % 4 === 0 && year % 100 !== 0 || year % 400 === 0) {
            return true;
        }
        return false;
    }
    function getDay(year, month, day) {
        var arr = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
        for (var i = 0; i < month - 1; i++) {
            day = day + arr[i];
        }
        if (month > 2 && isRun(year)) {
            day++;
        }
        return day;
    }
    console.log(getDay(2017,3,17));//76
</script>