A second look at Array#sort

We have already used Array#sort to sort strings and integers. But how can we sort a more complex structure?

When the default behaviour is not what you want, Array#sort allows you to tell it how to sort the data structure. Let's start with an example simpler than the addressbook:

friends = [
    [ "Joe", "Smith" ],
    [ "Melissa", "Adams"],  
    [ "Sandy", "Koh" ]
]
                       

We know that, by defalt, Array#sort will sort by the first entry - in this case, first names. But say that we want to sort by last names instead. Array#sort is not an iterator, but just like iterators it allows you to give it a block of code. Like so:

friends.sort do |a,b|  
    ...
end
                       

What this code should do is:

• Return -1 if a is less than b
• Return 0 if a is equal to b
• Return 1 if a is greater than b

With this information, the sort function knows how to sort your array.

Return values

How do you "return" a value? The return value is simply the value of the last statement executed. Let's take another look at irb

>> num = 3
=> 3
>> 2 + 3
=> 5
                       

That => 3 and that => 5 are the return values of those expressions.

The <=> operator.

Sorting is so common, that there is an operator to simplyfy return values. The operator <=>irb returns the -1, 0, or 1 that you would normally have received. Try it in

>> 3 <=> 1  # 3 > 1 so <=> returns 1
=> 1
>> 3 <=> 3  # 3 == 3 so <=> returns 0
=> 0
>> 3 <=> 5  # 3 < 5 so <=> returns -1
=> -1
>> "andy" <=> "sam" # "andy" comes before "sam"  
=> -1
                       

Sorting by last name

So we have:

friends.sort do |a,b|  
    ...
end
                       

a and b are elements of the array friends. The usual "friends.sort" is equivalent to:

friends.sort do |a,b|  
    a[0] <=> b[0]   # Sort by first entry  
end
                       

But we want to sort by the second entry (last name). So, instead we do:

friends = friends.sort do |a,b|  
    a[1] <=> b[1]   # Sort by second entry  
end
# Now 'friends' is sorted by last name.
                       

Sorting the addressbook

Now that we understand Array#sort better, we are ready to use a customized sort of the addressbook. We have:

addressbook.sort do |person_a, person_b|  
    ...
end
                       

person_a and person_b are both person structures. We can, for instance, sort them alphabetically by first name.

# p_a == "person a"
addressbook.sort do |p_a, p_b|
    p_a["first name"] <=> p_b["first name"]  
end
                       

Exercises

• Write down the code to sort addressbook by full name. That is, first name, then last.