Cannot read property of undefined in comparator function

huangapple
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英文:

Cannot read property of undefined in comparator function

问题

这里的问题是你的比较器（loginComparator、tasksAmountComparator 和 fineComparator）返回了布尔值，而不是要求返回负数、零或正数来指示比较结果。在快速排序算法中，比较器通常返回负数表示第一个元素应该在第二个元素之前，零表示它们相等，正数表示第一个元素应该在第二个元素之后。

因此，你应该修改你的比较器函数，以返回适当的比较结果，例如：

  1. function loginComparator(firstParticipant, secondParticipant) {
  2.     if (firstParticipant.login < secondParticipant.login) {
  3.         return -1;
  4.     } else if (firstParticipant.login > secondParticipant.login) {
  5.         return 1;
  6.     } else {
  7.         return 0;
  8.     }
  9. }
  11. function tasksAmountComparator(firstParticipant, secondParticipant) {
  12.     if (firstParticipant.solved < secondParticipant.solved) {
  13.         return -1;
  14.     } else if (firstParticipant.solved > secondParticipant.solved) {
  15.         return 1;
  16.     } else {
  17.         return 0;
  18.     }
  19. }
  21. function fineComparator(firstParticipant, secondParticipant) {
  22.     if (firstParticipant.fine < secondParticipant.fine) {
  23.         return -1;
  24.     } else if (firstParticipant.fine > secondParticipant.fine) {
  25.         return 1;
  26.     } else {
  27.         return 0;
  28.     }
  29. }

这样修改后，你的比较器函数将返回正确的比较结果，适用于快速排序算法。

英文:

I have a quicksort algoritm (Hoare Partition Scheme) for objects of my class and three comparators.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

  1. class Participant {
  2.     login;
  3.     fine;
  4.     solved;
  5. }
  7. /*This function should convert array of strings - 
  8.       [&#39;alla 4 100&#39;,&#39;gena 6 1000&#39;,&#39;gosha 2 90&#39;,&#39;rita 2 90&#39;,&#39;timofey 4 80&#39;]
  9.       Into an array of Participant objects and return their names separated by &#39;\n&#39;
  10.       In this format:
  11.       gena
  12.       timofey
  13.       alla
  14.       gosha
  15.       rita
  16. */
  17. function effectiveSort(membersInfo) {
  18.     var sortedMembers = [];
  19.     for (let i = 0; i &lt; membersInfo.length; ++i) {
  20.         sortedMembers.push(new Participant);
  23.         sortedMembers[i].login = membersInfo[i].split(&#39; &#39;)[0];
  24.         sortedMembers[i].fine = Number(membersInfo[i].split(&#39; &#39;)[2]);
  25.         sortedMembers[i].solved = Number(membersInfo[i].split(&#39; &#39;)[1]);
  26.     }
  27.     sortedMembers = (quickSort(sortedMembers, 0, sortedMembers.length - 1, loginComparator));
  28.     sortedMembers = (quickSort(sortedMembers, 0, sortedMembers.length - 1, fineComparator));
  29.     sortedMembers = (quickSort(sortedMembers, 0, sortedMembers.length - 1, tasksAmountComparator));
  31.     return sortedMembers.map(a =&gt; a.login).join(&#39;\n&#39;);
  32. }
  34. //Comparators for each class field. That&#39;s where the problem starts
  36. function loginComparator(firstParticipant, secondParticipant) {
  37.     return firstParticipant.login &lt; secondParticipant.login;
  38. }
  40. function tasksAmountComparator(firstParticipant, secondParticipant) {
  41.     return firstParticipant.solved &gt;= secondParticipant.solved;
  42. }
  44. function fineComparator(firstParticipant, secondParticipant) {
  46.     return firstParticipant.fine &gt;= secondParticipant.fine;
  47. }
  49. //Hoare Partition Scheme
  51. function partition(arr, start, end, compar) {
  52.     const pivotVal = arr[Math.floor((start + end) / 2)];
  53.     while (start &lt;= end) {
  54.         while (compar(arr[start], pivotVal)) {
  55.             start++;
  56.         }
  57.         while (compar(pivotVal, arr[end])) {
  58.             end--;
  59.         }
  60.         if (start &lt;= end) {
  61.             let temp = arr[start];
  62.             arr[start] = arr[end];
  63.             arr[end] = temp;
  64.             start++;
  65.             end--;
  66.         }
  67.     }
  68.     return start;
  69. }
  71. function quickSort(arr, start = 0, end = arr.length - 1, compar) {
  72.     if (start &lt; end) {
  73.         const index = partition(arr, start, end, compar);
  74.         quickSort(arr, start, index - 1, compar);
  75.         quickSort(arr, index, end, compar);
  76.     }
  77.     return arr;
  78. }
  80. //Test data
  81. var real_mebmers = [&#39;alla 4 100&#39;, &#39;gena 6 1000&#39;, &#39;gosha 2 90&#39;, &#39;rita 2 90&#39;, &#39;timofey 4 80&#39;];
  82. var zero_members = [&#39;alla 0 0&#39;, &#39;gena 0 0&#39;, &#39;gosha 0 0&#39;, &#39;rita 0 0&#39;, &#39;timofey 0 0&#39;];
  83. console.log(effectiveSort(real_mebmers));

<!-- end snippet -->

The problem starts in these comparators
return (firstParticipant.solved &gt;= secondParticipant.solved); and return firstParticipant.fine &gt;= secondParticipant.fine;
JS says that.

> return (firstParticipant.solved >= secondParticipant.solved);
> ^ TypeError: Cannot read property 'solved' of undefined

Same goes for fineComparator. If I change these return statements to
return (firstParticipant.solved &gt; secondParticipant.solved); and here return firstParticipant.fine &gt; secondParticipant.fine;
Everthing seems fine. But why I can't check two properties for equality? I checked these properties and they don't have 'undefined' value, but JS somehow sees it...

答案1

得分: 1

我认为你需要这样做：

我调试了你的代码，看起来在迭代时，end 变量可能会变成 -1。

  1. console.log(list[0]); does work
  2. // console.log(list[-1]); // does not work
英文:

I think you need to do this:

  1. while (end &gt;= 0 &amp;&amp; compar(pivotVal, arr[end])) {

I debugged your code and it looks like when it iterates the end variable can become -1.

  1. list = [1,2,3];
  2. console.log(list[0]); does work
  3. // console.log(list[-1]); // does not work

huangapple
  • 本文由 发表于 2023年4月6日 21:35:21
  • 转载请务必保留本文链接：https://go.coder-hub.com/75950171.html
  • comparator
  • javascript
  • logical-operators
  • quicksort
  • undefined
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