for vs each in Ruby

Question

I just had a quick question regarding loops in Ruby  Is there a difference between these two ways of iterating through a collection     way 1  collection each do  item      do whatever end    way 2 for item in  collection     do whatever end   Just wondering if these are exactly the same or if maybe there s a subtle difference  possibly when  collection is nil

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Never ever use for it may cause almost untraceable bugs   Don t be fooled  this is not about idiomatic code or style issues  Ruby s implementation of for has a serious flaw and should not be used   Here is an example where for introduces a bug   class Library   def initialize      ary        end   def method with block  amp block       ary  lt  lt  block   end   def method that uses these blocks      ary map  amp  call    end end  lib   Library new  for n in  w foo bar quz    lib method with block   n   end  puts lib method that uses these blocks   Prints  quz quz quz   Using  w foo bar quz  each    n        prints  foo bar quz   Why   In a for loop the variable n is defined once and only and then that one definition is use for all iterations  Hence each blocks refer to the same n which has a value of quz by the time the loop ends  Bug   In an each loop a fresh variable n is defined for each iteration  for example above the variable n is defined three separate times  Hence each block refer to a separate n with the correct values

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This is the only difference   each   irb gt   1 2 3  each    x        gt   1  2  3  irb gt  x NameError  undefined local variable or method  x  for main Object     from  irb  2     from  0   for   irb gt  for x in  1 2 3   end     gt   1  2  3  irb gt  x     gt  3   With the for loop  the iterator variable still lives after the block is done  With the each loop  it doesn t  unless it was already defined as a local variable before the loop started   Other than that  for is just syntax sugar for the each method   When  collection is nil both loops throw an exception      Exception  undefined local variable or method   collection  for main Object

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1  4  each    i       a   9 if i  3    puts a       nil  nil  9  nil  for i in 1  4    a   9 if i  3    puts a  end  nil  nil  9  9   In  for  loop  local variable is still lives after each loop  In  each  loop  local variable refreshes after each loop

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As far as I know  using blocks instead of in-language control structures is more idiomatic

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I just want to make a specific point about the for in loop in Ruby  It might seem like a construct similar to other languages  but in fact it is an expression like every other looping construct in Ruby  In fact  the for in works with Enumerable objects just as the each iterator   The collection passed to for in can be any object that has an each iterator method  Arrays and hashes define the each method  and many other Ruby objects do  too  The for in loop calls the each method of the specified object  As that iterator yields values  the for loop assigns each value  or each set of values  to the specified variable  or variables  and then executes the code in body   This is a silly example  but illustrates the point that the for in loop works with ANY object that has an each method  just like how the each iterator does   class Apple   TYPES    w red green yellow    def each     yield TYPES pop until TYPES empty    end end  a   Apple new for i in a do   puts i end yellow green red   gt  nil   And now the each iterator   a   Apple new a each do  i    puts i end yellow green red   gt  nil   As you can see  both are responding to the each method which yields values back to the block  As everyone here stated  it is definitely preferable to use the each iterator over the for in loop  I just wanted to drive home the point that there is nothing magical about the for in loop  It is an expression that invokes the each method of a collection and then passes it to its block of code  Hence  it is a very rare case you would need to use for in  Use the each iterator almost always  with the added benefit of block scope

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Your first example    collection each do  item      do whatever end   is more idiomatic  While Ruby supports looping constructs like for and while  the block syntax is generally preferred   Another subtle difference is that any variable you declare within a for loop will be available outside the loop  whereas those within an iterator block are effectively private

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It looks like there is no difference  for uses each underneath     irb  gt  gt  for x in nil  gt  gt  puts x  gt  gt  end NoMethodError  undefined method  each  for nil NilClass     from  irb  1  gt  gt  nil each   x  puts x  NoMethodError  undefined method  each  for nil NilClass     from  irb  4   Like Bayard says  each is more idiomatic  It hides more from you and doesn t require special language features  Per Telemachus s Comment  for     in    sets the iterator outside the scope of the loop  so   for a in  1 2    puts a end   leaves a defined after the loop is finished  Where as each doesn t  Which is another reason in favor of using each  because the temp variable lives a shorter period

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See  The Evils of the For Loop  for a good explanation  there s one small difference considering variable scoping     Using each is considered more idiomatic use of Ruby

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One more different     number     one    two    three      gt    one    two    three     loop1      loop2       number each do  c    loop1  lt  lt  Proc new   puts c   end    gt    one    two    three     for c in number   loop2  lt  lt  Proc new   puts c   end    gt    one    two    three     loop1 1  call two    gt  nil   loop2 1  call three    gt  nil    source  http   paulphilippov com articles enumerable-each-vs-for-loops-in-ruby  for more clear  http   www ruby-forum com topic 179264 784884

[ruby] "for" vs "each" in Ruby

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