Retrieve data using for loop in Ruby

Question

I have a question about using foreach loops in Ruby.

I want to display documents and am using a foreach loop in order to display these documents. It returns an error with the i variable inside of data["response"]["docs"][i]["topic"] which is a JSON string I am iterating over.

I do not understand why that is. Can anyone tell me what I am doing wrong?

If I simply do data["response"]["docs"][0]["topic"] it works fine but not with the i. Why is that?

<%
(0..10).each do |i|
%>
<%= i %> <br/>
<%= data["response"]["docs"][i]["topic"] %> 
<%
end
%>

Rather than (0..10) consider using n.times where n is the number of times you want to loop. It's much more readable and understandable. — the Tin Man
– the Tin Man, Commented Jul 21, 2016 at 18:30

the Tin Man · Accepted Answer · 2016-07-21 18:19:03Z

1

My question is, how many items are there in data["response"]["docs"]? Are there exactly 11? Either way I would use the following code instead:

<% data["response"]["docs"].each_with_index do |item, index| %>
<%= index %> 
<br/>
<%= item["topic"] %> 
<% end %>

This iterates over the data["response"]["docs"] no matter how many there are (whether is is 1 doc or 20 docs) and stores the value in the variable named item. The each_with_index function gives you the index as well, stored in index, so you can display it later. If you only want the first 11 use:

<% data["response"]["docs"].first(11).each_with_index do |item, index| %>

This will grab a maximum of 11 doc items.

edited Jul 21, 2016 at 18:19

the Tin Man

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answered Jul 21, 2016 at 18:14

Greg Answer

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1 Comment

Benny Over a year ago

It consists of hundreds of documents, but I would only like to show the first 10 for starters and this works like a charm! I didn't know you can create the loop like this, thanks!

jktin12 · Accepted Answer · 2016-07-21 20:54:32Z

0

It's hard to tell what might be going wrong because you haven't posted the error, but if you're using a 10-element array, you want to do:

(0..9).each do |i|

With 0-based indexes, you should only use the range from 0-9, rather than 0-10. You may be getting an error because you're trying to access an element that isn't there (i.e. at index 10).

Even better is:

<% data["response"]["docs"].each do |document| %>
<%= document["topic"] %>
<% end %>

or if you need to print the index:

<% data["response"]["docs"].each_with_index do |document, index| %>
<%= index %> <br/>
<%= document["topic"] %>
<% end %>

edited Jul 21, 2016 at 20:54

answered Jul 21, 2016 at 18:15

jktin12

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3 Comments

the Tin Man Over a year ago

It's preferable to not use .... The 3-dot version is harder to read and is unknown to a lot of developers, leading to the possibility of bugs being injected during maintenance. It's better/idiomatic to use the 2-dot version, .. with the trailing number being reduced by 1.

jktin12 Over a year ago

Interesting, I'll start doing that instead. Thanks Tin Man. Answer has been edited.

the Tin Man Over a year ago

Consider this also: It's easier to understand if you do 10.times do |i| than it is (0..9).each do |i|. i will still start at 0 and count to 9, but it's more obvious the code loops 10 times.

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