Accessing a hash -- Beginner

Ruby
1

I want to access a hash via a gets command. A simplified
example, for a hash named, “test”:

print 'hash name: ’
hash_var = gets.chomp # enter ‘test’ here

puts hash_var # <-- nil
puts test # <-- the contents of the hash

One problem I see is hash_var is class String, while test is
class Hash. I cannot figure out how to make hash_var class
Hash. I’m not even sure if I am on the right track. I
would appreciate any help.

Thanks in advance,

Don

2

Hi –

On Sat, 4 Nov 2006, don wrote:

class Hash. I cannot figure out how to make hash_var class
Hash. I’m not even sure if I am on the right track. I
would appreciate any help.

Do you want to set a hash key, or retrieve a value using an existing
key? For example:

test = { “one” => 1, “two” => 2 }
hash_key = gets.chomp
puts test[hash_key]

If you input “one”, the program will print 1.

David

3

don wrote:

class Hash. I cannot figure out how to make hash_var class
Hash. I’m not even sure if I am on the right track. I
would appreciate any help.

The trick is that you want the variable named test, while the return
from gets is a string. You can use eval to evaluate a string in the
current context, so this works (but is VERY dangerous, read on):

test = {:foo => ‘bar’}
p eval(‘test’)
#=> {:foo=>“bar”}

This approach will execute any Ruby code that happens to be typed in by
the user, which is obviously bad:

p eval(‘test = nil’)
p test
#=> nil

One approach might be to gsub out any non word characters (but this
still makes me a bit queasy):

p eval(‘test’.gsub(/\W/, ‘’))
#=> {:foo=>“bar”}

p eval(‘test = nil’.gsub(/\W/, ‘’))
NameError: undefined local variable or method `testnil’ for main:Object
from (irb):11
from (irb):11

Hope this helps.

Tom

4

Tom W. wrote:

class Hash. I cannot figure out how to make hash_var class
Hash. I’m not even sure if I am on the right track. I
would appreciate any help.

One approach might be to gsub out any non word characters (but this
still makes me a bit queasy):

p eval(‘test’.gsub(/\W/, ‘’))
#=> {:foo=>“bar”}

On further consideration, even this is futile. Consider:

p eval(‘exit’)

Which leaves you back on the command line. What you want to do is create
a Hash that contains the hash you are interested in:

test = {:foo => ‘bar’}
shelf = {‘test’ => test}

Now you can look up the Hash of interest by name:

p shelf[‘test’]
#=> #=> {:foo=>“bar”}

eval is dangerous. VERY DANGEROUS.

Tom

5

don wrote:

class Hash. I cannot figure out how to make hash_var class
Hash. I’m not even sure if I am on the right track. I
would appreciate any help.

There is not a single Hash instance in your code. Also “test” is not
set, hence it’s interpreted as a method call:

$ ruby -e " print 'hash name: ’

hash_var = gets.chomp # enter ‘test’ here

puts hash_var # <-- nil
puts test # <-- the contents of the hash
"
hash name: foo
foo
-e:5:in `test’: wrong number of arguments (ArgumentError)
from -e:5

What is your problem? What are you trying to achieve?

Regards

robert
6

On 2006-11-03, [email protected] wrote:

I want to access the hash, then print part of the contents.
I am going to restate my problem – hopefully a little
clearer – later.

Thanks for the response, David.

Don

7

On 2006-11-03, Robert K. wrote:

[…]

hash name: foo
foo
-e:5:in `test’: wrong number of arguments (ArgumentError)
from -e:5

What is your problem? What are you trying to achieve?

Robert, thanks for the response. Part of my problem is I
did not communicate my problem very well. I will try again
a little later.

Don

8

I think I did a poor job of communicating my problem. I do
not think it is a complicated one, although it might be. I
know very little about programming.

I have 32 hashes. I would like to key in a hash name and
extract data from it. Like this:

Enter team: (Enter ‘ari’, for example)
team_var = gets.chomp
puts team_var[‘coach’]

Above does not work. For one thing, ari is actually $ari.
team_var = ‘$’ + team_var does not work either.

So simply put, how do you call a hash by name interactively?

Below is an example of what my code would look like:

$ari = {
‘name’ => ‘Arizona Cardinals’,
‘team_notes’ => ‘skank team’,
‘qb’ => ‘Matt Leinart, Kurt Warner’,
‘qb_rank’ => ‘’,
‘cbs’ => ‘’,
‘espn’ => ‘’,
‘players’ => ‘rb Edgerrin James’,
‘coach’ => ‘Denny Green’,
‘weekly_notes’ => ’ wk01 Fitzgerald 133 yds.’
}

print 'team: ’
team = gets.chomp
team = ‘$’ + team
puts team[‘coach’]

Thanks in advance for your help.

Don

9

On 2006-11-04, don [email protected] wrote:

I have 32 hashes. I would like to key in a hash name and
extract data from it. Like this:

Enter team: (Enter ‘ari’, for example)
team_var = gets.chomp
puts team_var[‘coach’]

You could stick (or create from the beginning) those hashes into another
hash, say “teams,” and then you’ll be able to get them by indexing
“teams” with user input.

‘team_notes’ => ‘skank team’,
‘qb’ => ‘Matt Leinart, Kurt Warner’,
‘qb_rank’ => ‘’,
‘cbs’ => ‘’,
‘espn’ => ‘’,
‘players’ => ‘rb Edgerrin James’,
‘coach’ => ‘Denny Green’,
‘weekly_notes’ => ’ wk01 Fitzgerald 133 yds.’
}

teams = {} # Create a new hash to store the individual team hashes
teams[‘ari’] = $ari

print 'team: ’
team = gets.chomp
then change the above^ to
team = teams[gets.chomp]
and you can proceed with

puts team[‘coach’]

You’ll still want some error-checking (what if the user enters
‘akaskdfj’?), of course.

You can also now access the elements of the individual teams like
“teams[‘ari’][‘coach’]” if you want, although it will often be less
cluttered-looking if you pull a reference to the individual team out
before you go to work on it, as “team = teams[gets.chomp]” does.

10

On 2006-11-03, Jim Marshall wrote:

hash, say “teams,” and then you’ll be able to get them by indexing
“teams” with user input.

[…]

You’ll still want some error-checking (what if the user enters
‘akaskdfj’?), of course.

You can also now access the elements of the individual teams like
“teams[‘ari’][‘coach’]” if you want, although it will often be less
cluttered-looking if you pull a reference to the individual team out
before you go to work on it, as “team = teams[gets.chomp]” does.

Excellent, Jim. That’s what I will do.

Thak you. Take care.

Don

11

On 2006-11-03, Tom W. wrote:

Tom W. wrote:

[…]

Which leaves you back on the command line. What you want to do is create
eval is dangerous. VERY DANGEROUS.

Tom

Thanks Tom. I will skip eval for now. I am going to repose
my question as I did not do a very good job the first time.

Don