Eloquent Javascript一书第五章的三个练习
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