Strange problem with "and" - "&&"

Hello

I have a strange problem with this code:

<%unless (@students.empty? and params[:commit].nil?) %>
<%=“test”%>
<%= render :partial => ‘results’ %>
<% end %>

Even though in some cases @students.empty? returns false and params
[:commit].nil? returns true (or the opposite), it displays the test
and the render area.

I put before and after the code the <%=params[:commit].nil?%> and <
%[email protected]?%> so I can see it (it shows false and true or the
opposite and always it shows the render area and the test). I also
tried && but it was the same problem.

The only way it worked was this:

<% unless @students.empty? %>
<% unless params[:commit].nil?%>

  <%="test"%>

<%= render :partial => ‘results’ %>

<% end %>
<% end %>

So why “and” - “&&” doesn’t work? I tried every combination with () or
without ()…

Thank you

On Oct 1, 6:00 pm, Yiannis [email protected] wrote:

Hello

I have a strange problem with this code:

<%unless (@students.empty? and params[:commit].nil?) %>
<%=“test”%>
<%= render :partial => ‘results’ %>
<% end %>

[snip]

The only way it worked was this:

<% unless @students.empty? %>
<% unless params[:commit].nil?%>

I think you have some misunderstanding when it comes to boolean logic

  • Your code that works is just fundamentally different to the first
    snippet you showed.

unless a
unless b

end
end

is equivalent to if !a && !b

but

unless a && b

end
is equivalent to if !( a && b) which in turn is equivalent to !a || !
b (see de morgan’s laws)

Fred

Change it to || (or).

Try this in IRB:

puts “Render” unless(true && false) # Prints Render

puts “Render” unless(true || false) # Does not print
puts “Render” unless(false || true) # Does not print
puts “Render” unless(true || true) # Does not print
puts “Render” unless(false || false) # Prints render (which is what
you want)

Quoting Y. [email protected]:

Hello

I have a strange problem with this code:

<%unless (@students.empty? and params[:commit].nil?) %>
<%=“test”%>
<%= render :partial => ‘results’ %>
<% end %>

unless both conditions are true, render the partial

the equivalent is:

if !@students.empty? || !params[:commit].nil?

I suspect you want an 'or" instead of the ‘and’ in the ‘unless’. I.e.
unless
either condition is true, render the partial.

unless @students.empty? or params[:commit].nil?

equivalent:

if !@students.empty? and !params[:commit].nil?

or:

if !@students.empty? and params[:commit]

so the partial will be rendered if both there are students and
params[:commit]
exists and is non-nil.

I personally have stopped using ‘unless’ with complex arguments entirely
and
am using it even with simple arguments less.

Lookup De Morgan’s Theorem
(http://en.wikipedia.org/wiki/De_Morgan’s_theorem)
and tattoo it on your arm or commit it to memory.

HTH,
Jeffrey

Oops! Yes indeed I need an “or” instead of “and”, how I missed
that?? :slight_smile: Thank you all for your help and the advices!

On Thu, Oct 1, 2009 at 2:52 PM, Colin L. [email protected]
wrote:

I think Jeffrey is right, “unless” with complex logic is just too
difficult for most of us to get our brains round easily.

You just insulted thousands of Perl programmers.

But seriously… it’s just the opposite of “if”.


Greg D.
http://destiney.com/

2009/10/1 Greg D. [email protected]:

On Thu, Oct 1, 2009 at 2:52 PM, Colin L. [email protected] wrote:

I think Jeffrey is right, “unless” with complex logic is just too
difficult for most of us to get our brains round easily.

You just insulted thousands of Perl programmers.

But seriously… it’s just the opposite of “if”.

Easy is the opposite of Difficult :slight_smile:

Maybe I should not have said ‘most of us’ but certainly it applies to
me. There is a similar problem with variables with names like
not_valid, it is too easy to overload the brain when included in a
logical expression.

Colin

2009/10/1 JohnDel [email protected]:

Oops! Yes indeed  I need an “or” instead of “and”, how I missed
that?? :slight_smile: Thank you all for your help and the advices!

I think Jeffrey is right, “unless” with complex logic is just too
difficult for most of us to get our brains round easily. I avoid it
except in trivially simple cases.

Colin

Colin L. wrote:

2009/10/1 Greg D. [email protected]:

On Thu, Oct 1, 2009 at 2:52 PM, Colin L. [email protected] wrote:

I think Jeffrey is right, “unless” with complex logic is just too
difficult for most of us to get our brains round easily.

You just insulted thousands of Perl programmers.

But seriously… it’s just the opposite of “if”.

Easy is the opposite of Difficult :slight_smile:

I agree. Even in Perl, many people consider unless poor style.

Maybe I should not have said ‘most of us’ but certainly it applies to
me. There is a similar problem with variables with names like
not_valid, it is too easy to overload the brain when included in a
logical expression.

Quite. Never forget to avoid omitting to eschew expressing things in
anything other than non-excessively negative form. :smiley:

Colin

Best,

Marnen Laibow-Koser
http://www.marnen.org
[email protected]

2009/10/2 Marnen Laibow-Koser [email protected]:

But seriously… it’s just the opposite of “if”.

Quite. Never forget to avoid omitting to eschew expressing things in
anything other than non-excessively negative form. :smiley:

It took me a couple of minutes to decide, but +1

Colin

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