A brief discussion on some basic arithmetic problems of characters and arrays in js

function reverseString(str) {
 str = str.split('').reverse().join('');
  
 return str;
}
 
reverseString("hello");

function palindrome(str) {
 // Good luck!
  
 str=str.replace(/[\ |\~|\`|\!|\@|\#|\$|\%|\^|\&|\*|\(|\)|\-|\_|\+|\=|\||\\|\[|\]|\{|\}|\;|\:|\"|\&#39;|\,|\<|\.|\>|\/|\?]/g,""); //去除标点符号,这是我百度的，js的正则不是很熟悉
 str = str.replace(/\s+/g);
 str = str.toLowerCase();
 var arr = str.split(&#39;&#39;);
   arr = arr.reverse();
  var str1 = arr.join("");
 if(str === str1){
 return true;}
 return false;
}
 
 
 
palindrome("eye");
 
/*
 
palindrome("eye") 应该返回一个布尔值
palindrome("eye") 应该返回 true.
palindrome("race car") 应该返回 true.
palindrome("not a palindrome") 应该返回 false.
palindrome("A man, a plan, a canal. Panama") 应该返回 true.
palindrome("never odd or even") 应该返回 true.
palindrome("nope") 应该返回 false.
palindrome("almostomla") 应该返回 false.
palindrome("My age is 0, 0 si ega ym.") 应该返回 true.
palindrome("1 eye for of 1 eye.") 应该返回 false.
palindrome("0_0 (: /-\ :) 0-0") 应该返回 true.
 
 
 
*/

/*这题很简单，主要是要明白 split() 是把字符串分割成数组
join() 是把 数组变成字符串
toLowerCase() toUpperCase() 大小写转换，注意，只对字母有效，其他字符（eg:/,!@）无效
*/
 
function titleCase(str) {
 str = str.split(" ");//按照空格把字符串分割成数组
  for (var i = 0; i < str.length; i++) {
    str[i] = str[i].toLowerCase();
    str[i] = str[i].substring(0, 1).toUpperCase() + str[i].substring(1);
  }
  return str.join(" ");//通过空格把数组连接成字符串
}
 
titleCase("I&#39;m a little tea pot");

function confirmEnding(str, target) {
 // "Never give up and good luck will find you."
 // -- Falcor
  
 return str.substr(str.length-target.length) == target ? true:false;
}
 
confirmEnding("Bastian", "n");
confirmEnding("He has to give me a new name", "na");
/*
confirmEnding("Bastian", "n") 应该返回 true.
confirmEnding("Connor", "n") 应该返回 false.
confirmEnding("Walking on water and developing software from a specification are easy if both are frozen", "specification") 应该返回 false.
confirmEnding("He has to give me a new name", "name") 应该返回 true.
confirmEnding("He has to give me a new name", "me") 应该返回 true.
confirmEnding("He has to give me a new name", "na") 应该返回 false.
confirmEnding("If you want to save our world, you must hurry. We dont know how much longer we can withstand the nothing", "mountain") 应该返回 false.
*/

function repeat(str, num) {
 // repeat after me
 var newstr = str;
 if(num >1){
  for(var i = 1; i< num ; i ++){
   str +=newstr;
  }
  return str;
 }else if(num == 1){
  return str;
 }else{
  return "";
 }
  
}
 
repeat("abc", 3);
repeat("*", 3);

function chunk(arr, size) {
 // Break it up.
var arr1 = [];
  for (var i = 0; i < arr.length; i = i + size) {
    var arr2 = arr;
    arr1.push(arr2.slice(i, i + size));
  }
  return arr1;
}
 
chunk(["a", "b", "c", "d"], 2);

/*
这题注意是对filter的理解，这是我最开始的代码，写的不是很好，没多大参考价值
还要注意 NaN 比较。 自己不等于自己（NaN ！= NaN ）
*/
 
function bouncer(arr) {
 // Don't show a false ID to this bouncer.
 var arr1 =[];
  var j = 0;
  arr.filter(function(val, index) {
    if (val === false || val === null || val === 0 || val === "" || val === undefined || val !== val) {
      arr1.push(index);
    }
  });
  var len = arr1.length;
 
  for(var i = 0; i < len ; i++){
     arr.splice(arr1[i]-j,1);
     j++;
  }
 return arr;
}
 
bouncer([7, "ate", "", false, 9]);

function destroyer(arr) {
 // Remove all the values
 var tempArguments = arguments;
 return arr.filter(function(entry) {
  for(var i = 1; i< tempArguments.length; i++) {
   if (entry == tempArguments[i]) {
    return false;
   }
  }
  return true;
 });
}
 
destroyer([1, 2, 3, 1, 2, 3], 2, 3);

function where(arr, num) {
 // Find my place in this sorted array.
 //注意sort() 排序规则
 arr.sort(function(a,b){
   return a- b;
 });
 
 for(var i =0;i<arr.length;i++){
    
  if(arr[i]>num | arr[i] == num){
     
   return i;
  }
 }
 return arr.length;
}
 
 
 
where([5, 3, 20, 3], 5);

function rot13(str) { // LBH QVQ VG!
  var arr = str.toUpperCase().split(" ");
  var str1 = [];
  for (var i = 0; i < arr.length; i++) {
    var arr1 = arr[i].split("");
    for (var j = 0; j < arr1.length; j++) {
      var num = arr1[j].charCodeAt();
      if (num >= 65 && num <= 90) {
        arr1[j] = num + 13 > 90 ? String.fromCharCode(64 + (num + 13 - 90)):String.fromCharCode(num + 13); //64 + (num + 13 - 90) 要明白为什么是64 ，
      }
 
    }
    str1.push(arr1.join(""));
  }
  return str1.join(" ");
}
 
// Change the inputs below to test
rot13("SERR PBQR PNZC");