Summary of operations on numbers in js (code)

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1. Get the value of the bit-th bit in the binary form of the number num. Note:
1. Bit starts from 1
2. Returns 0 or 1
3. Example: the binary number of 2 is 10, the first bit is 0, and the second bit is 1

function valueAtBit(num, bit) {
  var s = num.toString(2);     
  return s[s.length - bit];
 }
 function valueAtBit(num, bit) {
    //toString转化为二进制，split将二进制转化为数组，reverse()将数组颠倒顺序
    var arr = num.toString(2).split("").reverse();    
    return arr[bit-1];
}
function valueAtBit(num, bit) {
    return (num >> (bit -1)) & 1;
}

2. Given a binary string, convert it into the corresponding decimal number

function base10(str) {
    return parseInt(str,2);
}

3. Convert the given number into a binary string. If the string length is less than 8 digits, add 0 to the front to reach 8 digits.

function convertToBinary(num) {
     //转换为2进制格式
     var s = num.toString(2);     
     //获得2进制数长度
     var l = s.length;     
     if(l<8){         
     //声明一个字符串用于补满0
         var s1 = "0000000";         
         var s2 = s1.slice(0,8-l);
         s = s2+s; 
     }     return s;
 }
 function convertToBinary(num) {
    var str = num.toString(2);    
    while(str.length < 8) {
        str = "0" + str;
    }    
    return str;
}
function convertToBinary(num) {
    var s =  num.toString(2);    
    return &#39;00000000&#39;.slice(s.length) + s;
}
function convertToBinary(num) {
  return (&#39;00000000&#39; + num.toString(2)).slice(-8); //从倒数第八个开始取}

4. Find the value of multiplying a and b. A and b may be decimals. You need to pay attention to the accuracy of the result.

//根据两个书中精度较高的一个来确定，先将其转换成字符串，然后根据小数点的位置确定小数位数，
//字符串长度减去‘.'的位置后还要再减去1得到正确的小数位数，两个取其大，然后用toFixed()函数确定 
//结果的小数位数
function multiply(a, b) {
   //求两个数中精度大的一个
    var stra=a.toString();    
    var strb=b.toString();    
    var len=Math.max(stra.length-stra.indexOf('.')-1,strb.length-strb.indexOf('.')-1);    //
    return parseFloat(a*b).toFixed(len);
}

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