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javascript - 背包表示

转载 作者：行者123 更新时间：2023-11-30 00:27:05 25 4
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Use reduce to compute the area of the largest rectangle in an array of rectangles. Each rectangle has width and height properties, and if r is a rectangle, the area is r.width * r.height

If there are no rectangles, return 0, so the base case is 0. In the online instruction examples, reduce() was used to form the sum over a set of items. In this case you will want to take the maximum over a set of items (using JavaScript’s Math.max(a, b) function.)

You must use a function named area to compute and return the area of a rectangle. It takes one parameter which is a rectangle object.

You must also define a function to pass into reduce, analogous to the sum_value function in “The Real Reduce” sub-lessons within the “Knapsack: Representation” lesson. You must name this function biggest.

下面的代码是类(class)练习的解决方案。

To complete this exercise, you should modify the code in the window below. Make the following changes:

  • Define area(rect) to compute and return the area of a rectangle object.
  • Define biggest to be the third parameter to reduce (already defined).
  • Finish the definition of compute using a call to reduce.
<html>
    <head>
        <title>Using Reduce</title>
        <link rel="stylesheet" href="../css/exercise.css"></link>
        <script>
    function reduce(a, base, f) {
        var result = base;
        for (var i = 0; i < a.length; i++)
            result = f(result, a[i]);
        return result;
    }

    // these are the rectangles to consider
    r1  = {width:  9, height: 34};    // area is 306
    r2  = {width: 10, height: 31};    // area is 310
    r3  = {width: 11, height: 28};    // area is 308
    r4  = {width: 12, height: 25};    // area is 300
    r5  = {width: 13, height: 24};    // area is 312
    r6  = {width: 14, height: 22};    // area is 308
    r7  = {width: 15, height: 20};    // 300
    r8  = {width: 16, height: 19};    // 304
    r9  = {width: 17, height: 18};    // 306
    r10 = {width: 18, height: 17};    // 306
    r11 = {width: 19, height: 16};    // 304
    r12 = {width: 20, height: 15};    // 300
    r13 = {width: 22, height: 14};    // 308
    r14 = {width: 24, height: 12};    // 288
    r15 = {width: 28, height: 11};    // 308

    // form an array of all the rectangles
    rectangles = [r1, r2,  r3,  r4,  r5,  r6,  r7, r8,
                  r9, r10, r11, r12, r13, r14, r15];

    // define the function named area here:
    function area(r){

    return r.width * r.height;

    }

    // define the function named biggest here:
    function biggest(biggestYet, newRect){
    biggestYet = Math.max (biggestYet, newRect)
     return biggestYet;

    }

    // complete the definition using a call to reduce:
    function compute() {
      //  return -23; // replace with code to return the area of
                      //  the largest rectangle in rectangles
       var biggestYet = 0;

     for (x = 0 ; x < rectangles.length ; x++)
     {
       var rArea = area ( rectangles[x]);
       console.log ( "area "+ x+ "," + rArea );

       biggestYet = biggest (biggestYet, rArea);
       console.log ( "biggest "+ x +","+ biggestYet); 

     }
      return biggestYet;


    }

        </script>
    </head>
    <body>
        <!-- the output is displayed using HTML -->
        <p>The area of the largest rectangle is:
           <!-- the ? will be replaced with the answer -->
           <div id = "answer">?</div></p>
        <br>
        <!-- a button runs compute and puts the answer into the HTML -->
        <button id = "computeButton"
            onclick = "x = compute();
                <!-- find the document element named 'answer' -->
                where = document.getElementById('answer');
                <!-- insert result x as text into the HTML -->
                where.innerHTML = x.toString();">Run compute()
                        to compute the largest rectangle.</button>
       </body>
       </html>

最佳答案

您应该使用名为 reduce 的方法。您遇到的问题是因为 onclick 处理程序。你需要再调查一下。这是一个可能的解决方案:

function reduce(a, base, f) {
    var result = base;
    for (var i = 0; i < a.length; i++)
        result = f(result, a[i]);
    return result;
};

function area(rectangle) {
    return rectangle.width * rectangle.height;
};

function biggest(result, rectangle) {
    return Math.max(result, area(rectangle));
};

function compute(rectangles) {
    var x = reduce(rectangles, 0, biggest);
    var where = document.getElementById('answer');
        where.innerHTML = x.toString();
};

var r1  = {width:  9, height: 34};    // area is 306
var r2  = {width: 10, height: 31};    // area is 310
var r3  = {width: 11, height: 28};    // area is 308
var r4  = {width: 12, height: 25};    // area is 300
var r5  = {width: 13, height: 24};    // area is 312
var r6  = {width: 14, height: 22};    // area is 308
var r7  = {width: 15, height: 20};    // 300
var r8  = {width: 16, height: 19};    // 304
var r9  = {width: 17, height: 18};    // 306
var r10 = {width: 18, height: 17};    // 306
var r11 = {width: 19, height: 16};    // 304
var r12 = {width: 20, height: 15};    // 300
var r13 = {width: 22, height: 14};    // 308
var r14 = {width: 24, height: 12};    // 288
var r15 = {width: 28, height: 11};    // 308

// form an array of all the rectangles
var rectangles = [r1, r2,  r3,  r4,  r5,  r6,  r7, r8, r9, r10, r11, r12, r13, r14, r15];

document.getElementById('computeButton').onclick = function() {
    compute(rectangles);
};
        <!-- the output is displayed using HTML -->
        <p>The area of the largest rectangle is:
           <!-- the ? will be replaced with the answer -->
           <div id = "answer">?</div></p>
        <br>
        <!-- a button runs compute and puts the answer into the HTML -->
        <button id = "computeButton">Run compute()
                        to compute the largest rectangle.</button>

或者整个程序作为一个线程:

Math.max.apply(Math, rectangles.map(area));

关于javascript - 背包表示，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/31041236/

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