5

Eloquent Javascript一书第五章的三个练习

 2 years ago
source link: http://fantaghiro.github.io/study/2016/05/06/Practice-In-Eloquent-Javascript-Ch5.html
Go to the source link to view the article. You can view the picture content, updated content and better typesetting reading experience. If the link is broken, please click the button below to view the snapshot at that time.
neoserver,ios ssh client

Eloquent Javascript一书第五章的三个练习

2016-05-06

Eloquent Javascript一书第五章

##Mother-Child Age Difference一题，题目是这样的：

给定数据ancestry：

var ANCESTRY_FILE = "[\n  " + [
  '{"name": "Carolus Haverbeke", "sex": "m", "born": 1832, "died": 1905, "father": "Carel Haverbeke", "mother": "Maria van Brussel"}',
  '{"name": "Emma de Milliano", "sex": "f", "born": 1876, "died": 1956, "father": "Petrus de Milliano", "mother": "Sophia van Damme"}',
  '{"name": "Maria de Rycke", "sex": "f", "born": 1683, "died": 1724, "father": "Frederik de Rycke", "mother": "Laurentia van Vlaenderen"}',
  '{"name": "Jan van Brussel", "sex": "m", "born": 1714, "died": 1748, "father": "Jacobus van Brussel", "mother": "Joanna van Rooten"}',
  '{"name": "Philibert Haverbeke", "sex": "m", "born": 1907, "died": 1997, "father": "Emile Haverbeke", "mother": "Emma de Milliano"}',
  '{"name": "Jan Frans van Brussel", "sex": "m", "born": 1761, "died": 1833, "father": "Jacobus Bernardus van Brussel", "mother":null}',
  '{"name": "Pauwels van Haverbeke", "sex": "m", "born": 1535, "died": 1582, "father": "N. van Haverbeke", "mother":null}',
  '{"name": "Clara Aernoudts", "sex": "f", "born": 1918, "died": 2012, "father": "Henry Aernoudts", "mother": "Sidonie Coene"}',
  '{"name": "Emile Haverbeke", "sex": "m", "born": 1877, "died": 1968, "father": "Carolus Haverbeke", "mother": "Maria Sturm"}',
  '{"name": "Lieven de Causmaecker", "sex": "m", "born": 1696, "died": 1724, "father": "Carel de Causmaecker", "mother": "Joanna Claes"}',
  '{"name": "Pieter Haverbeke", "sex": "m", "born": 1602, "died": 1642, "father": "Lieven van Haverbeke", "mother":null}',
  '{"name": "Livina Haverbeke", "sex": "f", "born": 1692, "died": 1743, "father": "Daniel Haverbeke", "mother": "Joanna de Pape"}',
  '{"name": "Pieter Bernard Haverbeke", "sex": "m", "born": 1695, "died": 1762, "father": "Willem Haverbeke", "mother": "Petronella Wauters"}',
  '{"name": "Lieven van Haverbeke", "sex": "m", "born": 1570, "died": 1636, "father": "Pauwels van Haverbeke", "mother": "Lievijne Jans"}',
  '{"name": "Joanna de Causmaecker", "sex": "f", "born": 1762, "died": 1807, "father": "Bernardus de Causmaecker", "mother":null}',
  '{"name": "Willem Haverbeke", "sex": "m", "born": 1668, "died": 1731, "father": "Lieven Haverbeke", "mother": "Elisabeth Hercke"}',
  '{"name": "Pieter Antone Haverbeke", "sex": "m", "born": 1753, "died": 1798, "father": "Jan Francies Haverbeke", "mother": "Petronella de Decker"}',
  '{"name": "Maria van Brussel", "sex": "f", "born": 1801, "died": 1834, "father": "Jan Frans van Brussel", "mother": "Joanna de Causmaecker"}',
  '{"name": "Angela Haverbeke", "sex": "f", "born": 1728, "died": 1734, "father": "Pieter Bernard Haverbeke", "mother": "Livina de Vrieze"}',
  '{"name": "Elisabeth Haverbeke", "sex": "f", "born": 1711, "died": 1754, "father": "Jan Haverbeke", "mother": "Maria de Rycke"}',
  '{"name": "Lievijne Jans", "sex": "f", "born": 1542, "died": 1582, "father":null, "mother":null}',
  '{"name": "Bernardus de Causmaecker", "sex": "m", "born": 1721, "died": 1789, "father": "Lieven de Causmaecker", "mother": "Livina Haverbeke"}',
  '{"name": "Jacoba Lammens", "sex": "f", "born": 1699, "died": 1740, "father": "Lieven Lammens", "mother": "Livina de Vrieze"}',
  '{"name": "Pieter de Decker", "sex": "m", "born": 1705, "died": 1780, "father": "Joos de Decker", "mother": "Petronella van de Steene"}',
  '{"name": "Joanna de Pape", "sex": "f", "born": 1654, "died": 1723, "father": "Vincent de Pape", "mother": "Petronella Wauters"}',
  '{"name": "Daniel Haverbeke", "sex": "m", "born": 1652, "died": 1723, "father": "Lieven Haverbeke", "mother": "Elisabeth Hercke"}',
  '{"name": "Lieven Haverbeke", "sex": "m", "born": 1631, "died": 1676, "father": "Pieter Haverbeke", "mother": "Anna van Hecke"}',
  '{"name": "Martina de Pape", "sex": "f", "born": 1666, "died": 1727, "father": "Vincent de Pape", "mother": "Petronella Wauters"}',
  '{"name": "Jan Francies Haverbeke", "sex": "m", "born": 1725, "died": 1779, "father": "Pieter Bernard Haverbeke", "mother": "Livina de Vrieze"}',
  '{"name": "Maria Haverbeke", "sex": "m", "born": 1905, "died": 1997, "father": "Emile Haverbeke", "mother": "Emma de Milliano"}',
  '{"name": "Petronella de Decker", "sex": "f", "born": 1731, "died": 1781, "father": "Pieter de Decker", "mother": "Livina Haverbeke"}',
  '{"name": "Livina Sierens", "sex": "f", "born": 1761, "died": 1826, "father": "Jan Sierens", "mother": "Maria van Waes"}',
  '{"name": "Laurentia Haverbeke", "sex": "f", "born": 1710, "died": 1786, "father": "Jan Haverbeke", "mother": "Maria de Rycke"}',
  '{"name": "Carel Haverbeke", "sex": "m", "born": 1796, "died": 1837, "father": "Pieter Antone Haverbeke", "mother": "Livina Sierens"}',
  '{"name": "Elisabeth Hercke", "sex": "f", "born": 1632, "died": 1674, "father": "Willem Hercke", "mother": "Margriet de Brabander"}',
  '{"name": "Jan Haverbeke", "sex": "m", "born": 1671, "died": 1731, "father": "Lieven Haverbeke", "mother": "Elisabeth Hercke"}',
  '{"name": "Anna van Hecke", "sex": "f", "born": 1607, "died": 1670, "father": "Paschasius van Hecke", "mother": "Martijntken Beelaert"}',
  '{"name": "Maria Sturm", "sex": "f", "born": 1835, "died": 1917, "father": "Charles Sturm", "mother": "Seraphina Spelier"}',
  '{"name": "Jacobus Bernardus van Brussel", "sex": "m", "born": 1736, "died": 1809, "father": "Jan van Brussel", "mother": "Elisabeth Haverbeke"}'
].join(",\n  ") + "\n]";

var ancestry = JSON.parse(ANCESTRY_FILE);

给定两个函数：

function average(array) {
  function plus(a, b) { return a + b; }
  return array.reduce(plus) / array.length;
}

var byName = {};
ancestry.forEach(function(person) {
  byName[person.name] = person;
});

要求计算母亲与孩子年龄差（当孩子出生时母亲的年龄）的平均值。

以下是我实现的代码：

var ages = [];
ancestry.forEach(function(person, index){
  if(byName[person.mother]){
      ages.push(person.born - byName[person.mother].born);
    }
});

console.log(average(ages));
// → 31.2

实现思路非常的简单，就是遍历ancestry中的每一项，如果通过这一项的mother属性值，能够在ancestry找到对应的一项，也就是如果一个人的妈妈的数据也在ancestry中的话，那么就将妈妈女孩子的年龄差push到数组ages中。最后调用average函数打印出ages数组所有数值的平均值。

下面是针对同一个问题，作者提供的解决方法：

var differences = ancestry.filter(function(person) {
  return byName[person.mother] != null;
}).map(function(person) {
  return person.born - byName[person.mother].born;
});

console.log(average(differences));
// → 31.2

作者使用了两个高阶函数，第一步利用filter将有mother数据的person从ancestry中过滤出来，第二步利用map将过滤这来的这些person的母亲与他们的年龄差map出来，形成一个数字differences。最后利用average函数将differences的平均值得出。

##Historical Life Expectancy问题

继续使用ancestry的数据和average函数。现在要求计算出每个世纪的人们的平均寿命。其中这个人究竟归入哪个世纪是按照这个人的死亡年龄来看的。要求是哪个世纪，用Math.ceil(person.died / 100)。

以下是我实现的代码：

var averageAgeByCentury = {};
ancestry.forEach(function(person, index){
    var century = Math.ceil(person.died / 100) + "";
    if(averageAgeByCentury[century] == null){
      averageAgeByCentury[century] = [];
    }
    averageAgeByCentury[century].push(person.died - person.born);
});

for(century in averageAgeByCentury){
  console.log(century, ": ", average(averageAgeByCentury[century]));
}

// → 16: 43.5
//   17: 51.2
//   18: 52.8
//   19: 54.8
//   20: 84.7
//   21: 94

我的大致思路非常简单。定义一个名为averageAgeByCentury的空对象。然后遍历ancestry中的每个person，先求他的century值。如果求出来的century值不是averageAgeByCentury的一个属性，那么就定义一个属性，这个属性对应的值是个数组，并且将这个人的寿命push到这个数组中区。如果求得的century已经是averageAgeByCentury的属性值，那么不再创建新的数组，而是向对应数组直接push这个人的寿命。

得到averageAgeByCentury对象之后，在分别打印它的每个表示century属性对应的数组的平均值即可。

作者实现的方式与我相同，但是作者将把每个人按照century来分组的思路进行了抽象化，抽象出了groupBy这个函数，最终作者的代码如下：

function groupBy(array, groupOf) {
  var groups = {};
  array.forEach(function(element) {
    var groupName = groupOf(element);
    if (groupName in groups)
      groups[groupName].push(element);
    else
      groups[groupName] = [element];
  });
  return groups;
}

var byCentury = groupBy(ancestry, function(person) {
  return Math.ceil(person.died / 100);
});

for (var century in byCentury) {
  var ages = byCentury[century].map(function(person) {
    return person.died - person.born;
  });
  console.log(century + ": " + average(ages));
}

// → 16: 43.5
//   17: 51.2
//   18: 52.8
//   19: 54.8
//   20: 84.7
//   21: 94

##模拟数组的every和some函数的功能

我的思路是这样的，后续事实证明这样走不通。

//这是个错误案例
function every(arr, f){
  arr.forEach(function(item, index){
      if(!f(item)){
          return false;
        }
    });
    return true;
}

function some(arr, f){
  arr.forEach(function(item, index){
      if(f(item)){
            return true;
        }
    });
    return false;
}

经测试，这中算法会有错误。由于forEach的循环内，除非抛出异常，否则不会中途中断。因此forEach循环内用return是无用的。

一下是作者实现的代码：

function every(array, predicate) {
  for (var i = 0; i < array.length; i++) {
    if (!predicate(array[i]))
      return false;
  }
  return true;
}

function some(array, predicate) {
  for (var i = 0; i < array.length; i++) {
    if (predicate(array[i]))
      return true;
  }
  return false;
}

console.log(every([NaN, NaN, NaN], isNaN));
// → true
console.log(every([NaN, NaN, 4], isNaN));
// → false
console.log(some([NaN, 3, 4], isNaN));
// → true
console.log(some([2, 3, 4], isNaN));
// → false

report

Recommend

About Joyk


Aggregate valuable and interesting links.
Joyk means Joy of geeK