ruby - 如何在 Ruby 中按降序对数组进行排序

Question

我有一个哈希数组:

[
  { :foo => 'foo', :bar => 2 },
  { :foo => 'foo', :bar => 3 },
  { :foo => 'foo', :bar => 5 },
]

我试图根据每个散列中 :bar 的值按降序对这个数组进行排序。

我正在使用 sort_by 对以上数组进行排序:

a.sort_by { |h| h[:bar] }

但是，这会按升序对数组进行排序。如何使其按降序排序？

一种解决方案是执行以下操作:

a.sort_by { |h| -h[:bar] }

但是那个负号好像不太合适。

Answer 1

对各种建议的答案进行基准测试总是很有启发性的。这是我发现的:

#!/usr/bin/rubyrequire 'benchmark'ary = []1000.times {   ary << {:bar => rand(1000)} }n = 500Benchmark.bm(20) do |x|  x.report("sort")               { n.times { ary.sort{ |a,b| b[:bar] <=> a[:bar] } } }  x.report("sort reverse")       { n.times { ary.sort{ |a,b| a[:bar] <=> b[:bar] }.reverse } }  x.report("sort_by -a[:bar]")   { n.times { ary.sort_by{ |a| -a[:bar] } } }  x.report("sort_by a[:bar]*-1") { n.times { ary.sort_by{ |a| a[:bar]*-1 } } }  x.report("sort_by.reverse!")   { n.times { ary.sort_by{ |a| a[:bar] }.reverse } }end                          user     system      total        realsort                  3.960000   0.010000   3.970000 (  3.990886)sort reverse          4.040000   0.000000   4.040000 (  4.038849)sort_by -a[:bar]      0.690000   0.000000   0.690000 (  0.692080)sort_by a[:bar]*-1    0.700000   0.000000   0.700000 (  0.699735)sort_by.reverse!      0.650000   0.000000   0.650000 (  0.654447)

I think it's interesting that @Pablo's sort_by{...}.reverse! is fastest. Before running the test I thought it would be slower than "-a[:bar]" but negating the value turns out to take longer than it does to reverse the entire array in one pass. It's not much of a difference, but every little speed-up helps.

---

Please note that these results are different in Ruby 1.9

Here are results for Ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-darwin10.8.0]:

                           user     system      total        real
sort                   1.340000   0.010000   1.350000 (  1.346331)
sort reverse           1.300000   0.000000   1.300000 (  1.310446)
sort_by -a[:bar]       0.430000   0.000000   0.430000 (  0.429606)
sort_by a[:bar]*-1     0.420000   0.000000   0.420000 (  0.414383)
sort_by.reverse!       0.400000   0.000000   0.400000 (  0.401275)

这些是在旧的 MacBook Pro 上。更新或更快的机器将具有较低的值，但相对差异将保持不变。

---

这是在新硬件和 2.1.1 版 Ruby 上的一些更新版本:

#!/usr/bin/ruby

require 'benchmark'

puts "Running Ruby #{RUBY_VERSION}"

ary = []
1000.times {
  ary << {:bar => rand(1000)}
}

n = 500

puts "n=#{n}"
Benchmark.bm(20) do |x|
  x.report("sort")               { n.times { ary.dup.sort{ |a,b| b[:bar] <=> a[:bar] } } }
  x.report("sort reverse")       { n.times { ary.dup.sort{ |a,b| a[:bar] <=> b[:bar] }.reverse } }
  x.report("sort_by -a[:bar]")   { n.times { ary.dup.sort_by{ |a| -a[:bar] } } }
  x.report("sort_by a[:bar]*-1") { n.times { ary.dup.sort_by{ |a| a[:bar]*-1 } } }
  x.report("sort_by.reverse")    { n.times { ary.dup.sort_by{ |a| a[:bar] }.reverse } }
  x.report("sort_by.reverse!")   { n.times { ary.dup.sort_by{ |a| a[:bar] }.reverse! } }
end

# >> Running Ruby 2.1.1
# >> n=500
# >>                            user     system      total        real
# >> sort                   0.670000   0.000000   0.670000 (  0.667754)
# >> sort reverse           0.650000   0.000000   0.650000 (  0.655582)
# >> sort_by -a[:bar]       0.260000   0.010000   0.270000 (  0.255919)
# >> sort_by a[:bar]*-1     0.250000   0.000000   0.250000 (  0.258924)
# >> sort_by.reverse        0.250000   0.000000   0.250000 (  0.245179)
# >> sort_by.reverse!       0.240000   0.000000   0.240000 (  0.242340)

---

在较新的 Macbook Pro 上使用 Ruby 2.2.1 运行上述代码的新结果。同样，确切的数字并不重要，重要的是它们之间的关系:

Running Ruby 2.2.1
n=500
                           user     system      total        real
sort                   0.650000   0.000000   0.650000 (  0.653191)
sort reverse           0.650000   0.000000   0.650000 (  0.648761)
sort_by -a[:bar]       0.240000   0.010000   0.250000 (  0.245193)
sort_by a[:bar]*-1     0.240000   0.000000   0.240000 (  0.240541)
sort_by.reverse        0.230000   0.000000   0.230000 (  0.228571)
sort_by.reverse!       0.230000   0.000000   0.230000 (  0.230040)

---

在 2015 年中期的 MacBook Pro 上针对 Ruby 2.7.1 进行了更新:

Running Ruby 2.7.1
n=500     
                           user     system      total        real
sort                   0.494707   0.003662   0.498369 (  0.501064)
sort reverse           0.480181   0.005186   0.485367 (  0.487972)
sort_by -a[:bar]       0.121521   0.003781   0.125302 (  0.126557)
sort_by a[:bar]*-1     0.115097   0.003931   0.119028 (  0.122991)
sort_by.reverse        0.110459   0.003414   0.113873 (  0.114443)
sort_by.reverse!       0.108997   0.001631   0.110628 (  0.111532)

---

...the reverse method doesn't actually return a reversed array - it returns an enumerator that just starts at the end and works backwards.

Array#reverse 的来源是:

               static VALUE
rb_ary_reverse_m(VALUE ary)
{
    long len = RARRAY_LEN(ary);
    VALUE dup = rb_ary_new2(len);

    if (len > 0) {
        const VALUE *p1 = RARRAY_CONST_PTR_TRANSIENT(ary);
        VALUE *p2 = (VALUE *)RARRAY_CONST_PTR_TRANSIENT(dup) + len - 1;
        do *p2-- = *p1++; while (--len > 0);
    }
    ARY_SET_LEN(dup, RARRAY_LEN(ary));
    return dup;
}

做 *p2-- = *p1++; while (--len > 0); 如果我没记错我的 C，则以相反的顺序复制指向元素的指针，所以数组是颠倒的。