`to_sym` converts a string to a symbol.  For example, `&quot;a&quot;.to_sym` becomes `:a`.

It&#39;s not specific to Rails; vanilla Ruby has it as well.

It looks like in some versions of Ruby, a symbol could be converted to and from a Fixnum as well.  But `irb` from Ruby 1.9.2-p0, from ruby-lang.org, doesn&#39;t allow that unless you add your own `to_sym` method to Fixnum.  I&#39;m not sure whether Rails does that, but it doesn&#39;t seem very useful in any case.

Expanding with useful details on the accepted answer by @cHao:  
[`to_sym`](https://www.rubydoc.info/stdlib/core/String:to_sym) is used when the original `string` variable needs to be converted to a `symbol`.

In some cases, you can avoid `to_sym`, by creating the variable as `symbol`, not `string` in the first place. For example:

```
my_str1 = &#39;foo&#39;
my_str2 = &#39;bar baz&#39;

my_sym1 = my_str1.to_sym
my_sym2 = my_str2.to_sym

# Best:
my_sym1 = :foo
my_sym2 = :&#39;bar baz&#39;

```

or

```
array_of_strings = %w[foo bar baz]
array_of_symbols = array_of_strings.map(&amp;:to_sym)

# Better:
array_of_symbols = %w[foo bar baz].map(&amp;:to_sym)

# Best
array_of_symbols = %i[foo bar baz]
```

**SEE ALSO:**

https://stackoverflow.com/q/16621073/967621  
https://stackoverflow.com/q/16289455/967621  
https://stackoverflow.com/q/800122/967621  
[uppercase %I - Interpolated Array of symbols, separated by whitespace](https://stackoverflow.com/a/47992410/967621)  



I have the following MySQL query:

    SELECT p.*,
    	IF(COUNT(ms.PropertyID) &gt; 0,1,0) AS Contacted,
    	pm.MediaID,
    	date_format(p.AvailableFrom, &#39;%d %b %Y&#39;) AS &#39;AvailableFrom&#39;,
    	astext(pg.Geometry) AS Geometry
    FROM property p, propertygeometry pg
    	JOIN shortlist sl ON sl.PropertyID = p.id AND sl.MemberID = 384216
    	LEFT JOIN message ms ON ms.PropertyID = p.id AND ms.SenderID = 384216
    	LEFT JOIN property_media pm ON pm.PropertyID = p.id AND pm.IsPrimary = 1
    WHERE p.paused = 0
    	AND p.PropertyGeometryID = pg.id
    GROUP BY p.id

And I&#39;m getting this error:

**#1054 - Unknown column &#39;p.id&#39; in &#39;on clause&#39;**

As far as I can see the query looks right, any idea what could be wrong?

MySQL unknown column in ON clause

Is it possible to increase the width of a scrollbar on a `&lt;div&gt;` element placed inside the `&lt;body&gt;`?

I am not talking about the default scrollbar on the *browser itself*, this page runs in full screen mode and because the browser scrollbar never comes into picture, the inner `&lt;div&gt;` element has its own scrollbar.

How can I increase a scrollbar&#39;s width using CSS?

What does the `to_sym` method do? What is it used for?


What does the Ruby method &#39;to_sym&#39; do?

<p>What does the <code>to_sym</code> method do? What is it used for?</p>


How can I add space character in the ruby array if I want to use %w(a b c) syntax?


Space in the ruby array by %w

**How do I dynamically define a class in Ruby WITH a name?**

I know how to create a class dynamically **without** a name using something like:

    dynamic_class = Class.new do
      def method1
      end
    end

But you can&#39;t specify a class name. I want to create a class dynamically **with** a name. 

Here&#39;s an example of what I want to do but of course it doesn&#39;t actually work.  
(Note that I am not creating an instance of a class but a class definition)

    class TestEval
      def method1
        puts &quot;name: #{self.name}&quot;
      end
    end

    class_name = &quot;TestEval&quot;
    dummy = eval(&quot;#{class_name}&quot;)

    puts &quot;dummy: #{dummy}&quot;

    dynamic_name = &quot;TestEval2&quot;
    class_string = &quot;&quot;&quot;
    class #{dynamic_name}
      def method1
      end
    end
    &quot;&quot;&quot;
    dummy2 = eval(class_string)
    puts &quot;dummy2: #{dummy2}&quot; # doesn&#39;t work


Actual output:  

    dummy: TestEval
    dummy2: 

Desired output:

    dummy: TestEval
    dummy2: TestEval2


======================================================

**Answer: A totally dynamic solution using sepp2k&#39;s method**

    dynamic_name = &quot;TestEval2&quot;

    Object.const_set(dynamic_name, Class.new) # If inheriting, use Class.new( superclass )
    dummy2 = eval(&quot;#{dynamic_name}&quot;)
    puts &quot;dummy2: #{dummy2}&quot;



Dynamic Class Definition WITH a Class Name

How can I extract a substring from within a string in Ruby?

Example:

    String1 = &quot;&lt;name&gt; &lt;substring&gt;&quot;

I want to extract `substring` from `String1` (i.e. everything within the last occurrence of `&lt;` and `&gt;`).

Extract a substring from a string in Ruby using a regular expression

When I try to install the mysql2 gem, it fails with no apparent errors. Does anyone know what to do to work around this so mysql2 installs?

    $ sudo gem install mysql2
    Building native extensions.  This could take a while...
    ERROR:  Error installing mysql2:
    	ERROR: Failed to build gem native extension.
    
    /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby extconf.rb
    checking for rb_thread_blocking_region()... no
    checking for mysql_query() in -lmysqlclient... no
    checking for main() in -lm... yes
    checking for mysql_query() in -lmysqlclient... no
    checking for main() in -lz... yes
    checking for mysql_query() in -lmysqlclient... no
    checking for main() in -lsocket... no
    checking for mysql_query() in -lmysqlclient... no
    checking for main() in -lnsl... no
    checking for mysql_query() in -lmysqlclient... no
    checking for main() in -lmygcc... no
    checking for mysql_query() in -lmysqlclient... no
    *** extconf.rb failed ***
    Could not create Makefile due to some reason, probably lack of
    necessary libraries and/or headers.  Check the mkmf.log file for more
    details.  You may need configuration options.
    
    Provided configuration options:
    	--with-opt-dir
    	--without-opt-dir
    	--with-opt-include
    	--without-opt-include=${opt-dir}/include
    	--with-opt-lib
    	--without-opt-lib=${opt-dir}/lib
    	--with-make-prog
    	--without-make-prog
    	--srcdir=.
    	--curdir
    	--ruby=/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby
    	--with-mysql-config
    	--without-mysql-config
    	--with-mysql-dir
    	--without-mysql-dir
    	--with-mysql-include
    	--without-mysql-include=${mysql-dir}/include
    	--with-mysql-lib
    	--without-mysql-lib=${mysql-dir}/lib
    	--with-mysqlclientlib
    	--without-mysqlclientlib
    	--with-mlib
    	--without-mlib
    	--with-mysqlclientlib
    	--without-mysqlclientlib
    	--with-zlib
    	--without-zlib
    	--with-mysqlclientlib
    	--without-mysqlclientlib
    	--with-socketlib
    	--without-socketlib
    	--with-mysqlclientlib
    	--without-mysqlclientlib
    	--with-nsllib
    	--without-nsllib
    	--with-mysqlclientlib
    	--without-mysqlclientlib
    	--with-mygcclib
    	--without-mygcclib
    	--with-mysqlclientlib
    	--without-mysqlclientlib
    
    
    Gem files will remain installed in /Library/Ruby/Gems/1.8/gems/mysql2-0.2.6 for inspection.
    Results logged to /Library/Ruby/Gems/1.8/gems/mysql2-0.2.6/ext/mysql2/gem_make.out



Ruby gem mysql2 install failing

I have several gems including ruby-debug in a bundler group called :development. When I run the bundle command, these gems are ignored and it only installs the gems that are not in any group. How can I make sure bundler doesn&#39;t ignore the gems in the :development group? 

Edit: This is what my Gemfile looks like.

    source &#39;http://rubygems.org&#39;
    gem &#39;rails&#39;, &#39;3.0.1&#39;
    
    # Auth gems
    gem &quot;devise&quot;, &quot;1.1.3&quot;
    gem &quot;omniauth&quot;
    
    # Bundle Mongoid gems
    gem &quot;mongoid&quot;, &quot;2.0.0.beta.19&quot;
    gem &quot;bson_ext&quot;
    
    # Asset gems
    gem &#39;jquery-rails&#39;
    gem &quot;jammit&quot;
    
    # Controller gems
    gem &#39;inherited_resources&#39;, &#39;1.1.2&#39;
    
    # View gems
    gem &#39;haml&#39;
    gem &#39;formtastic&#39;, &#39;~&gt; 1.1.0&#39;
    
    # Nokogiri
    gem &quot;mechanize&quot;
    gem &quot;json&quot;
    
    
    group :development do
      gem &quot;ruby-debug&quot;
      gem &#39;compass&#39;
      gem &#39;compass-colors&#39;
      gem &#39;pickler&#39;
      gem &#39;haml-rails&#39;
      gem &#39;rails3-generators&#39;
      gem &quot;hpricot&quot;
      gem &quot;ruby_parser&quot;
      gem &#39;fog&#39;
    end



Rails bundler doesn&#39;t install gems inside a group

How can I check to see if a String contains a whitespace character, an empty space or &quot; &quot;.  If possible, please provide a Java example. 

For example: `String = &quot;test word&quot;;` 

How can I find whitespace in a String?

I need to convert a decimal to a string with N decimals (two or four) and NO thousand separator:

&#39;XXXXXXX (dot) DDDDD&#39;

The problem with `CultureInfo.InvariantCulture` is that is places &#39;,&#39; to separate thousands.

UPDATE

This should work for decimal and double types.

&lt;hr/&gt;

My previous question: *https://stackoverflow.com/questions/3971130/need-to-convert-double-or-decimal-to-string*



Converting double to string with N decimals, dot as decimal separator, and no thousand separator

Do you use &lt;code&gt;StringUtils.EMPTY&lt;/code&gt; instead of &lt;code&gt;&quot;&quot;&lt;/code&gt;?

I mean either as a return value or if you set a the value of a String variable. I don&#39;t mean for comparison, because there we use &lt;code&gt;StringUtils.isEmpty()&lt;/code&gt;

Is StringUtils.EMPTY recommended?

I have an array of strings:

    string[] remove = { &quot;a&quot;, &quot;am&quot;, &quot;p&quot;, &quot;pm&quot; };

And I have a textbox that a user enters text into. If they type any string in the `remove` array at the end of the text in the textbox, it should be removed. What is the easiest way to do this?

**EDIT** To clarify, I&#39;m making a time parser. When you give the function a string, it does its best to parse it into this format: `08:14pm` I have a textbox to test it. When the focus leaves the textbox, I need to get the text without the am/pm/a/p suffix so I can parse the number only segment.

C# removing substring from end of string

I&#39;m working on a project where all conversions from `int` to `String` are done like this:

    int i = 5;
    String strI = &quot;&quot; + i;

I&#39;m not familiar with Java. 

Is this usual practice or is something wrong, as I suppose?

How do I convert from int to String?

What does it mean when you add the static keyword to a method?

    public static void doSomething(){
       //Well, do something!
    }

Can you add the `static` keyword to class? What would it mean then?

What&#39;s a &quot;static method&quot; in C#?

What difference that `final` makes between the code below. Is there any advantage in declaring the arguments as `final`.

    public String changeTimezone( Timestamp stamp, Timezone fTz, Timezone toTz){  
        return ....
    }

    public String changeTimezone(final Timestamp stamp, final Timezone fTz, 
            final Timezone toTz){
        return ....
    }


Making java method arguments as final

I&#39;m writing a small piece of python as a homework assignment, and I&#39;m not getting it to run! I don&#39;t have that much Python-experience, but I know quite a lot of Java. 
I&#39;m trying to implement a Particle Swarm Optimization algorithm, and here&#39;s what I have: 


    class Particle:    
    
        def __init__(self,domain,ID):
           	self.ID = ID
        	self.gbest = None
        	self.velocity = []
        	self.current = []
        	self.pbest = []
        	for x in range(len(domain)):
         	    self.current.append(random.randint(domain[x][0],domain[x][1])) 
         	    self.velocity.append(random.randint(domain[x][0],domain[x][1])) 
                self.pbestx = self.current    		
        
        def updateVelocity():
    	for x in range(0,len(self.velocity)):
    	    self.velocity[x] = 2*random.random()*(self.pbestx[x]-self.current[x]) + 2 * random.random()*(self.gbest[x]-self.current[x])	
        
        	
        def updatePosition():    
            for x in range(0,len(self.current)):
        	    self.current[x] = self.current[x] + self.velocity[x]	
            	
        def updatePbest():
            if costf(self.current) &lt; costf(self.best):
                self.best = self.current        
        
        def psoOptimize(domain,costf,noOfParticles=20, noOfRuns=30):
            particles = []
        	for i in range(noOfParticles):    
        		particle = Particle(domain,i)    
        		particles.append(particle)    
        	
        	for i in range(noOfRuns):
         	    Globalgbest = []
        	    cost = 9999999999999999999
           	for i in particles:    
     	    if costf(i.pbest) &lt; cost:
         	        cost = costf(i.pbest)
        		Globalgbest = i.pbest
          		for particle in particles:
         		    particle.updateVelocity()
        		    particle.updatePosition()
        		    particle.updatePbest(costf)
        		    particle.gbest = Globalgbest  	
        	
        
        	return determineGbest(particles,costf)

Now, I see no reason why this shouldn&#39;t work. 
However, when I run it, I get this error:

**&quot;TypeError: updateVelocity() takes no arguments (1 given)&quot;**

I don&#39;t understand! I&#39;m not giving it any arguments!

Thanks for the help,

Linus




Python interpreter error, x takes no arguments (1 given)

Im just wondering whether there is a way to call a method where i build the name of the method on the fly with a string.

e.g. I have a method called loaddata
    
    -(void)loadData;

to call this i would normally call it like

    [self loadData];

But i want to be able to call it dynamically with a string e.g.

    NSString *methodName = [[NSString alloc] initWithString:@&quot;loadData&quot;];
    [self methodName];

This is a stupid example but i hope you get my point. I am using it for databinding classes that I am setting up for my IPad application. Hard to explain but to get it to fire I need to work out how to call a method with a string.

Any ideas?

Thanks


Objective C calling method dynamically with a string

In Python, I&#39;m trying to run a method in a class and I get an error:


    Traceback (most recent call last):
      File &quot;C:\Users\domenico\Desktop\py\main.py&quot;, line 8, in &lt;module&gt;
        fibo.f()
      TypeError: unbound method f() must be called with fibo instance 
      as first argument (got nothing instead)

Code: (swineflu.py)

    class fibo:
        a=0
        b=0
        
        def f(self,a=0):
            print fibo.b+a
            b=a;
            return self(a+1)

Script main.py

    import swineflu
    
    f = swineflu
    fibo = f.fibo
    
    fibo.f()            #TypeError is thrown here


What does this error mean? What is causing this error? 

unbound method f() must be called with fibo_ instance as first argument (got classobj instance instead)

What is the function of symbol in ruby? what&#39;s difference between string and symbol?
Why is it not a good idea to dynamically create a lot of symbols? 

Why is it not a good idea to dynamically create a lot of symbols in ruby (for versions before 2.2)?

How can I convert a symbol type into a string in Emacs lisp? 

I wasn&#39;t able to find a `symbol-to-string` function or anything similar.

Convert Symbol to a String in Elisp

While learning Ruby I&#39;ve come across the &quot;=&gt;&quot; operator on occasion. Usually I see it in the form of 

    :symbol =&gt; value

and it *seems* to be used frequently when passing values to functions. What exactly is that operator called? What does it do/mean? Is it built into Ruby or is it something that different frameworks like Rails and DataMapper add to the symbol class? Is it only used in conjunction with the symbol class? Thanks.

In Ruby what does &quot;=&gt;&quot; mean and how does it work?

I found these symbols in a function declaration several times, but I don&#39;t know what they mean.

**Example:**

    void raccogli_dati(double **&amp; V, double **p, int N) { 
      int ultimo = 3; 
      V = new double * [N/2]; 
      for(int i=0; i &lt; N/2; i++) { 
        V[i] = new double[N/2], std :: clog &lt;&lt; &quot;digita &quot; &lt;&lt; N/2 - i
                     &lt;&lt; &quot; valori per la parte superiore della matrice V: &quot;; 
        for(int j=i; j &lt; N/2; j++) 
          std :: cin &gt;&gt; V[i][j], p[ultimo++][0] = (V[i][j] /= sqrt(p[i][0]*p[j][0]));
      } 
      for(int i=1; i &lt; N/2; i++) 
        for(int j=0; j &lt; i; j++) 
           V[i][j] = V[j][i];
    }


Meaning of *&amp; and **&amp; in C++

When I say `{ :bla =&gt; 1, :bloop =&gt; 2 }`, what exactly does the `:` do? I read somewhere about how it&#39;s similar to a string, but somehow a symbol. 

I&#39;m not super-clear on the concept, could someone enlighten me?

Content Type	Original Author	Original Content on Stackoverflow
Question	keruilin	View Question on Stackoverflow
Solution 1 - Ruby	cHao	View Answer on Stackoverflow
Solution 2 - Ruby	Timur Shtatland	View Answer on Stackoverflow

What does the Ruby method 'to_sym' do?

Ruby Problem Overview

Ruby Solutions

Solution 1 - Ruby

Solution 2 - Ruby

How can I increase a scrollbar's width using CSS?

MySQL unknown column in ON clause

Attributions