# Feeling sharp? Sort an array subset in place

Here’s the situation: I have an array and I’d like to sort certain
elements in place – without touching the others.

Unsorted array contents:
3 <-- sort me!
x
1 <-- sort me!
y
2 <-- sort me!

Sorted array contents
1
x
2
y
3

See how 1, 2, and 3 played musical chairs? That’s exactly what we
want. It seems like there should be an easy one, two or three-line way
to do this given an array and a test condition (for identifying
elements which need sorting.)

How would you do it?

(My solution was a dirty cheat, it involved building a hash of {from =>
to} moves, cloning the original array contents, then changing each
element one at a time. Blah. There’s got to be a sexier way.)

On Tue, Feb 07, 2006 at 05:28:21PM +0900, Michael J. wrote:

Sorted array contents
1
x
2
y
3

class Array
def sort_subset!
els, indices = [], []
each_with_index{|x,i| (els << x; indices << i) if yield(x) }
els.sort.each_with_index{|x,i| self[indices[i]] = x}
end
end

a = (0…10).sort_by{rand}
a # => [1, 8, 2, 10, 3,
0, 5, 4, 9, 7, 6]
a.sort_subset!{|x| x % 2 == 0} # => [0, 2, 4, 6, 8,
10]
a # => [1, 0, 2, 4, 3,
6, 5, 8, 9, 7, 10]
a.sort_subset!{|x| x % 2 == 1} # => [1, 3, 5, 7, 9]
a # => [1, 0, 2, 4, 3,
6, 5, 8, 7, 9, 10]

Michael J. wrote:

Sorted array contents

How would you do it?

(My solution was a dirty cheat, it involved building a hash of {from =>
to} moves, cloning the original array contents, then changing each
element one at a time. Blah. There’s got to be a sexier way.)

a = [ 3, :x, 1, :y, 2 ]
f=nil
tmp = a.partition{ f=!f }
p tmp.first.sort.zip( tmp.last ).flatten.compact

Michael J. wrote:

Sorted array contents

How would you do it?

I’d change the app design to not have these arrays. Seriously. It
seems
as if you were storing things together that do not belong together. Why
do you need that?

Kind regards

``````robert
``````

irb(main):001:0> a=[3,“y”,1,“x”,2]
=> [3, “y”, 1, “x”, 2]
irb(main):002:0> s=a.select {|x| Integer === x}.sort
=> [1, 2, 3]
irb(main):003:0> i=0
=> 0
irb(main):004:0> a.map! {|e| Integer === e ? (r=s[i]; i+=1; r) : e}
=> [1, “y”, 2, “x”, 3]

Michael J. wrote:

Sorted array contents

How would you do it?

I’m definitely not feeling sharp (10 AM here and I haven’t slept yet)
but here’s my try:

arr = [3,:x,1,:y,2] # => [3, :x, 1, :y, 2]
sorted = arr.select { |v| v.class == Fixnum }.sort # => [1, 2, 3]
arr.map { |v| v.class == Fixnum ? sorted.shift : v } # => [1, :x, 2,
:y, 3]

Of course, substitute conditions as appropriate.

• Peter

On Tue, 2006-02-07 at 17:28 +0900, Michael J. wrote:

Here’s the situation: I have an array and I’d like to sort certain
elements in place – without touching the others.

I agree with Robert that you should probably think about why your data
is kept this way. But here’s one I’ve used:

``````class Array
def select_sort!(sort_proc = nil)
t = []
map! { |x| if yield x then t << x; nil else x end }
t.sort!(&sort_proc)
map! { |x| x or t.shift }
end
end
``````

Which works like:

a = [2,‘a’,3,‘b’,‘c’,1]

# => [2, “a”, 3, “b”, “c”, 1]

a.select_sort! { |x| x.is_a? Fixnum }

p a

# => [1, “a”, 2, “b”, “c”, 3]

a.select_sort!(lambda { |x,y| y <=> x }) { |x| x.is_a? Fixnum }

p a

# => [3, “a”, 2, “b”, “c”, 1]

But it breaks down if your array contains nil elements beforehand, and
has that nasty two-block look about it.