Beginner question: Bubble sort using array.each_index

Ruby
1

Hi,

I’m a beginner going through Chris P.'s online book & exercises.

One of them required me to alphabetize a list of words WITHOUT using the
.sort method.

I discovered the following bubble sort algorithm online and understand
it except for one thing.

How is it able to determine that array|first| points to position 0 on
the array, and array[second] points to position 1, in order to make the
comparison?

To me, it looks like both array[first] and array[second] would equal one
another.

array.each_index do |first|
array.each_index do |second|
if array[first] < array[second]
array[first], array[second] = array[second], array[first]
end
end
end

puts array

2

Hi,

You misunderstand the code. “first” and “second” aren’t fixed values.
They are the block parameters belonging to the two iterators, so their
value will change on each iteration.

Have a look at the outer iterator first:

array.each_index do |first|

end

This iterator loops over the indices of “array”. On each iteration, the
current index is assigned to the “first” parameter. So if array is [‘x’,
‘z’, ‘y’] for example, the parameter will be assigned the values 0, 1
and 2 subsequently.

You can test this:

array.each_index do |first|
puts first
end

Then there is also an inner iterator, which also loops over the indices
of array. This means: While the outer iterator is going through the
indices 0, 1, 2, 3, …, the inner iterator will do the same on every
step.

See for yourself:

array.each_index do |first|
array.each_index do |second|
puts “first (outer): #{first}, second (inner): #{second}”
end
end

By the way, the code you posted is not bubble sort. I do not even know
its name, but it is definitely a bad algorithm, since it involves a lot
of
unnecessary element swapping.

3

Michael S. wrote in post #1063963:

If you have a better way I could accomplish this without using the
previously described algorithm, I would very much appreciate it. It’s
something I’ve been struggling with all night.

Well, you can use any sorting algorithm. You may also stick to the one
above. I didn’t say it’s wrong, I just said it’s inefficient (and not
bubble sort).

Typical and simple sorting algorithms are insertion sort, selection sort
and bubble sort (the actual one). You’ll find good explanations at
wikipedia:

And if you’re looking for implementations, this is a good source:

http://en.wikibooks.org/wiki/Algorithm_Implementation/Sorting

4

Thank you for clearing that up.

The reason I google’d and used that algorithm is because Chris P.
states that we should try to alphabetize an array of user-input words,
without using the sort method: Arrays and Iterators - Learn to Program

Thus, I wrote:

word = ‘word’
listOfWords = []

while word != ‘’
puts 'Enter a word and press enter: ’
word = gets.chomp.downcase
listOfWords.push word
end

listOfWords.pop

But became stuck at how I could alphabetize the array, using only the
things we have learned so far.

If you have a better way I could accomplish this without using the
previously described algorithm, I would very much appreciate it. It’s
something I’ve been struggling with all night.