Ruby method parameters

Tuesday, 17/11/2020

Tram Ho

When working with method in ruby , you may encounter cases where the parameters defined in the method will be of the form: *args , **kwargs , &block , … So what are they, today we will find Understand this issue via the ruby method parameters

Default parameters

When defining a method , we can specify a default value for them. When a method is called, if the arguments values are less than the previously declared parameters values then the default parameters will use those default assigned values.

 <span class="token keyword">def</span> <span class="token method-definition"><span class="token function">info</span></span> <span class="token punctuation">(</span> name <span class="token punctuation">,</span> sex <span class="token operator">=</span> <span class="token string">"male"</span> <span class="token punctuation">)</span>
        puts <span class="token string">"Name: <span class="token interpolation"><span class="token delimiter tag">#{</span> name <span class="token delimiter tag">}</span></span> , with sex: <span class="token interpolation"><span class="token delimiter tag">#{</span> sex <span class="token delimiter tag">}</span></span> "</span>
 <span class="token keyword">end</span>

 info <span class="token punctuation">(</span> <span class="token string">"huy"</span> <span class="token punctuation">,</span> <span class="token string">"female"</span> <span class="token punctuation">)</span> 
 <span class="token comment"># =&gt; Name: huy, with sex: female</span>
 info <span class="token punctuation">(</span> <span class="token string">"huy"</span> <span class="token punctuation">)</span> 
 <span class="token comment"># =&gt; Name: huy, with sex: male</span>

def info ( name , sex = "male" )

puts "Name: #{ name } , with sex: #{ sex } "

end

info ( "huy" , "female" )

# => Name: huy, with sex: female

info ( "huy" )

# => Name: huy, with sex: male

If there is more than one default parameter in the method definition, these parameters must be contiguous. For example, it is not possible to put another parameter between the two default parameter .

method has more than one default parameter when executed with a list of arguments passed in, these arguments will be valued for the default parameters from left to right:

  <span class="token keyword">def</span> <span class="token method-definition"><span class="token function">f</span></span> <span class="token punctuation">(</span> a <span class="token punctuation">,</span> b <span class="token operator">=</span> <span class="token number">1</span> <span class="token punctuation">,</span> c <span class="token operator">=</span> <span class="token number">2</span> <span class="token punctuation">)</span>
    puts a <span class="token punctuation">,</span> b <span class="token punctuation">,</span> c
  <span class="token keyword">end</span>
  
  f <span class="token punctuation">(</span> <span class="token number">10</span> <span class="token punctuation">)</span> 
  <span class="token comment"># =&gt; 10, 1, 2</span>
  f <span class="token punctuation">(</span> <span class="token number">10</span> <span class="token punctuation">,</span> <span class="token number">100</span> <span class="token punctuation">)</span> 
  <span class="token comment"># =&gt; 10, 100, 2</span>
  f <span class="token punctuation">(</span> <span class="token number">10</span> <span class="token punctuation">,</span> <span class="token number">100</span> <span class="token punctuation">,</span> <span class="token number">200</span> <span class="token punctuation">)</span> 
  <span class="token comment"># =&gt; 10, 100, 200</span>

def f ( a , b = 1 , c = 2 )

puts a , b , c

end

f ( 10 )

# => 10, 1, 2

f ( 10 , 100 )

# => 10, 100, 2

f ( 10 , 100 , 200 )

# => 10, 100, 200

Parameter with splat operator (*)

The array type parameter marked with the * in front is used when passing an array of any arguments:

  <span class="token keyword">def</span> <span class="token method-definition"><span class="token function">f</span></span> <span class="token punctuation">(</span> first <span class="token punctuation">,</span> <span class="token operator">*</span> rest <span class="token punctuation">)</span>
      puts rest <span class="token punctuation">,</span> rest <span class="token punctuation">.</span> <span class="token keyword">class</span>
  <span class="token class-name">end</span>

  f <span class="token punctuation">(</span> <span class="token number">1</span> <span class="token punctuation">,</span> <span class="token number">2</span> <span class="token punctuation">,</span> <span class="token number">3</span> <span class="token punctuation">,</span> <span class="token number">4</span> <span class="token punctuation">)</span> 
  <span class="token comment"># =&gt; [2, 3, 4], Array</span>

puts rest , rest . class

end

f ( 1 , 2 , 3 , 4 )

# => [2, 3, 4], Array

A method has no more than one parameter of this type, an array type parameter must be adjacent to the default parameters, and may precede other normal parameters:

  <span class="token keyword">def</span> <span class="token method-definition"><span class="token function">f</span></span> <span class="token punctuation">(</span> a <span class="token operator">=</span> <span class="token number">1</span> <span class="token punctuation">,</span> <span class="token operator">*</span> args <span class="token punctuation">,</span> b <span class="token punctuation">)</span>
      puts <span class="token string">"a: <span class="token interpolation"><span class="token delimiter tag">#{</span> a <span class="token delimiter tag">}</span></span> , args: <span class="token interpolation"><span class="token delimiter tag">#{</span> args <span class="token delimiter tag">}</span></span> , b: <span class="token interpolation"><span class="token delimiter tag">#{</span> b <span class="token delimiter tag">}</span></span> "</span>
  <span class="token keyword">end</span>

  f <span class="token punctuation">(</span> <span class="token number">10</span> <span class="token punctuation">,</span> <span class="token number">20</span> <span class="token punctuation">,</span> <span class="token number">30</span> <span class="token punctuation">,</span> <span class="token number">40</span> <span class="token punctuation">)</span> 
  <span class="token comment"># =&gt; a: 10, args: [20, 30], b: 40</span>

def f ( a = 1 , * args , b )

puts "a: #{ a } , args: #{ args } , b: #{ b } "

end

f ( 10 , 20 , 30 , 40 )

# => a: 10, args: [20, 30], b: 40

Additionally, * also used when we want to pass an array of arguments when we call the method :

    <span class="token keyword">def</span> <span class="token method-definition"><span class="token function">f</span></span> <span class="token punctuation">(</span> name <span class="token punctuation">,</span> age <span class="token punctuation">,</span> sex <span class="token punctuation">)</span>
        puts name <span class="token punctuation">,</span> age <span class="token punctuation">,</span> sex
    <span class="token keyword">end</span>
    
    user <span class="token operator">=</span> <span class="token punctuation">[</span> <span class="token string">"sherlock"</span> <span class="token punctuation">,</span> <span class="token number">20</span> <span class="token punctuation">,</span> <span class="token string">"male"</span> <span class="token punctuation">]</span>
    f <span class="token punctuation">(</span> <span class="token operator">*</span> user <span class="token punctuation">)</span>
    <span class="token comment"># =&gt; sherlock, 20, male</span>

puts name , age , sex

end

user = [ "sherlock" , 20 , "male" ]

f ( * user )

# => sherlock, 20, male

Mapping arguments to parameters

We will learn how ruby assigns arguments passed to the parameters defined in the method .

Suppose a method has o original parameters (normal parameters), d parameters are assigned default values, and 1 parameters are array type. method is called with a argument, then:

If a < o : an ArgumentError occurs
If o <= a <= o+d : ao default parameter will be assigned value, the rest, o+da default parameter will use the original default value.
If a > o+d : aod the aod argument will be assigned to the last array parameter

Using hash for named arguments

When a method requires more than 2 or 3 parameters, it is very difficult to execute this method , we need to remember the order of all these parameters when passing the arguments. To solve this problem, in ruby we can use the hash argument :

  <span class="token keyword">def</span> <span class="token method-definition"><span class="token function">f</span></span> <span class="token punctuation">(</span> opts <span class="token punctuation">)</span>
    puts opts
  <span class="token keyword">end</span>
  
  f <span class="token punctuation">(</span> <span class="token punctuation">{</span> name <span class="token punctuation">:</span> <span class="token string">"huy"</span> <span class="token punctuation">,</span> age <span class="token punctuation">:</span> <span class="token number">11</span> <span class="token punctuation">}</span> <span class="token punctuation">)</span> 
  <span class="token comment"># =&gt; {:name=&gt;"huy", :age=&gt;11}</span>

puts opts

end

f ( { name : "huy" , age : 11 } )

# => {:name=>"huy", :age=>11}

If in the method the hash is at the end (or is followed by a block ), we can omit the {} sign when calling the method :

  f <span class="token punctuation">(</span> name <span class="token punctuation">:</span> <span class="token string">"huy"</span> <span class="token punctuation">,</span> age <span class="token punctuation">:</span> <span class="token number">11</span> <span class="token punctuation">)</span>
  <span class="token comment"># =&gt; {:name=&gt;"huy", :age=&gt;11}</span>

f ( name : "huy" , age : 11 )

# => {:name=>"huy", :age=>11}

Parameter with double splat (**) operator

One more way to declare a parameter of type hash: use ** :

  <span class="token keyword">def</span> <span class="token method-definition"><span class="token function">f</span></span> <span class="token punctuation">(</span> <span class="token operator">*</span> <span class="token operator">*</span> hash <span class="token punctuation">)</span>
      puts hash
  <span class="token keyword">end</span>
  
  f <span class="token punctuation">(</span> a <span class="token punctuation">:</span> <span class="token number">1</span> <span class="token punctuation">,</span> b <span class="token punctuation">:</span> <span class="token number">2</span> <span class="token punctuation">)</span>
  <span class="token comment"># =&gt; {:a=&gt;1, :b=&gt;2}</span>
  f <span class="token punctuation">(</span> <span class="token punctuation">)</span>
  <span class="token comment"># Nếu không truyền đối số nào, mặc định cho `hash` sẽ là `{}`</span>
  <span class="token comment"># =&gt; {}</span>

puts hash

end

f ( a : 1 , b : 2 )

# => {:a=>1, :b=>2}

f ( )

# Nếu không truyền đối số nào, mặc định cho `hash` sẽ là `{}`

# => {}

Block arguments with (&)

A block may be passed when calling the method :

  <span class="token keyword">def</span> <span class="token method-definition"><span class="token function">f</span></span> <span class="token punctuation">(</span> name <span class="token punctuation">,</span> <span class="token operator">&amp;</span> block <span class="token punctuation">)</span>
      <span class="token keyword">yield</span> name
  <span class="token keyword">end</span>

  f <span class="token punctuation">(</span> <span class="token string">"sherlock"</span> <span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token operator">|</span> name <span class="token operator">|</span> puts name <span class="token punctuation">}</span>
  <span class="token comment"># =&gt; sherlock</span>

yield name

end

f ( "sherlock" ) { | name | puts name }

# => sherlock

If in the parameter list there is no & , the block can still be passed via {} or do; ;end

Note that the parameter with & is defined only once, and it must be placed at the bottom of the list:

  <span class="token keyword">def</span> <span class="token method-definition"><span class="token function">f</span></span> <span class="token punctuation">(</span> <span class="token operator">&amp;</span> block <span class="token punctuation">,</span> <span class="token operator">&amp;</span> other_block <span class="token punctuation">)</span>
    <span class="token punctuation">.</span> <span class="token punctuation">.</span> <span class="token punctuation">.</span>
   <span class="token keyword">end</span>
  <span class="token comment"># =&gt; syntax error, unexpected ',', expecting ')'</span>
  
  <span class="token keyword">def</span> <span class="token method-definition"><span class="token function">f</span></span> <span class="token punctuation">(</span> <span class="token operator">&amp;</span> block <span class="token punctuation">,</span> name <span class="token punctuation">)</span>
    <span class="token punctuation">.</span> <span class="token punctuation">.</span> <span class="token punctuation">.</span>
   <span class="token keyword">end</span>
   
   <span class="token comment"># =&gt; syntax error, unexpected ',', expecting ')'</span>

def f ( & block , & other_block )

. . .

end

# => syntax error, unexpected ',', expecting ')'

. . .

end

# => syntax error, unexpected ',', expecting ')'

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Source : Viblo

Ruby method parameters

Default parameters

Parameter with splat operator (*)

Mapping arguments to parameters

Using hash for named arguments

Parameter with double splat (**) operator

Block arguments with (&)

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