problems with simple sorting of hashes in array being ignored

Question

I have a simple array

my_array = [{id:1,priority_score:2},{id:3,priority_score:5},{id:4,priority_score:3.5}...]

I am trying to sort by the priority score with

sort_array = my_array.sort_by{|priority| priority[:priority_score]}

strangely, I am not getting a back any difference from my original array.

I create the priority score using

new_hash = {id:h.id,priority_score:(a.score+b.score)/2}

and have added all sorts of .to_f,.to_i in case the problem was that the priority score was not being recognized as a number, but that isn't the problem.

Any suggestions?

You're confusing sort and sort_by - see stackoverflow.com/questions/9104273/… — Frederick Cheung
– Frederick Cheung, Commented Mar 3, 2012 at 22:24

Selman Ulug · Accepted Answer · 2012-03-03 22:23:23Z

1

you can use sort_by like this.

my_array.sort_by {|(h, _)| h[:priority_score]}
# [{:id=>1, :priority_score=>2}, {:id=>4, :priority_score=>3.5}, {:id=>3, :priority_score=>5}]

answered Mar 3, 2012 at 22:23

Selman Ulug

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3 Comments

pedalpete Over a year ago

I haven't seen the _ before, what is that for? (I left the office, so i can't actually check this until tomorrow).

Selman Ulug Over a year ago

_ means not caring value. a, _ = [10, 20] it will set a to 10.

Selman Ulug Over a year ago

@pedalpete I reexamined my answer and it is wrong. edu's solution is correct. my_array.sort_by {|h| h[:priority_score]} and my solution same thing. may you remove accept I'll delete it.

Daniel Pittman · Accepted Answer · 2012-03-03 22:18:17Z

1

The Array#sort method takes a block with two arguments, a and b, and wants you to compare them by hand. Given how you invoke it, I think you wanted sort_by, which takes a block with each item in the array, performs some transformation, and sorts by the result.

answered Mar 3, 2012 at 22:18

Daniel Pittman

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1 Comment

pedalpete Over a year ago

thanks Daniel, I typed it wrong, i am using sort_by, corrected my question now. sorry for that.

Edu · Accepted Answer · 2012-03-03 23:17:41Z

1

The sort gets two elements and the sorting is done by the comparison result of these elements.

This comparison should return -1, 0, 1.

0 for equal, -1 first < second and +1 first > second.

In most of the cases you can use ruby built in function <=>

For example:

my_array = [{id:1,priority_score:2},{id:3,priority_score:5},{id:4,priority_score:3.5}]

# Sort ascending  
cr = my_array.sort {|a,b| a[:priority_score] <=> b[:priority_score]}
#[{:id=>1, :priority_score=>2}, {:id=>4, :priority_score=>3.5}, {:id=>3, :priority_score=>5}]

# Sort descending
de = my_array.sort {|a,b| b[:priority_score] <=> a[:priority_score]}
#[{:id=>3, :priority_score=>5}, {:id=>4, :priority_score=>3.5}, {:id=>1, :priority_score=>2}]

answered Mar 3, 2012 at 23:17

Edu

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1 Comment

pedalpete Over a year ago

thanks Edu, I typed the question wrong, I am using sort_by. sorry for the confusion, but thanks for explaining this so clearly.

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