From: Ben Giddings <bg-rubytalk@...>
Date: 2009-08-20T09:08:04+09:00
Subject: Re: if defined on var

On Aug 19, 2009, at 19:49, Derek Smith wrote:
> Your advice is always welcome and respected.  But Iam still confused,
> see below.
> The table derek does NOT exist of course.
>
> table_exists = db.execute("select name from sqlite_master where
> type='table' and tbl_name='derek'")
>
> if table_exists
>   p "yes", "#{table_exists}"
> end
> if table_exists == ""
>    p "yes"
> else
>    p "no"
> end
>
> __OUTPUT__
>
> $ ruby mail_log_miner.rb
> "yes"

The variable "table_exists" is not nil and not false.  It is probably  
an empty array of rows, if you're using the sqlite database adapter.

> ""

The stringified version of the variable "table_exists" is an empty  
string.  This is what you would expect if "table_exists" is an empty  
array:

irb(main):001:0> [].to_s
=> ""

Putting "#{foo}" inside a string basically says "call .to_s on the  
object and concatenate the results with the beginning and end of this  
string.  In other words:

"#{table_exists}"

is the same as

"" + table_exists.to_s + ""

If you want to see a better representation of an object, just call "p  
object" or "puts object.inspect"

> "no"

No, "table_exists' is not literally an empty string.  You wouldn't  
expect that, however, since db.execute doesn't return strings, but  
arrays.

> With a table that does exist using same if clauses.
>
> ruby mail_log_miner.rb
> "yes"

Yes, table_exists is not false and not nil.

> "lines"

"#{table_exists}" is "lines", i.e. table_exists.to_s is lines.

table_exists is probably ["lines"], i.e. an array with 1 element, the  
string "lines".

irb(main):002:0> ["lines"].to_s
=> "lines"

> "no"

No, "table_exists" is not literally an empty string.

I think what you want is more along these lines:

# Don't call it "table_exists" because that implies the value is a
# boolean, when in fact it will be an array
table_array = db.execute("select name from sqlite_master where
type='table' and tbl_name='derek'")

if table_array.size > 0
   p "yes", table_array
end

if not table_array.empty?
    p "yes"
else
    p "no"
end

Ben