Issue #16147 has been updated by sammomichael (Samuel Michael).


sammomichael (Samuel Michael) wrote:
> I also have added rudimentary support for nested comprehensions. 
>  
   

> # Build Matrix from nested loop
> `$l[[for x in 1..10 do end], for x in 2..20 do x end] ` #=> does (1..10).map{(2..20).map{|x|x}
> `$l[[for j in 1..5 do end], for i in 1..5 do end] #=> (1..5).map{(1..5).map(&:itself)}`
   
> # Flatten Matrix with nested loop
> `$l[for x in 1..10 do for x in 2..20 do x end end]` #=> does (1..10).map{(2..20).map{|x|x}.flatten`
   `$l[for x in 1..10 do for x in 2..20 do x+2 end end]` #=> [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22]



----------------------------------------
Feature #16147: List Comprehensions in Ruby
https://bugs.ruby-lang.org/issues/16147#change-81939

* Author: sammomichael (Samuel Michael)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
## List comprehensions are present in many languages and programmers are quite fond of their simplicity and power. Add to that the fact that Ruby has a for...in loop that is rarely used but could possibly be repurposed. 

### Currently we can already do a hack like this to make Ruby support list comprehension syntax:

``` ruby
S = [for x in 0...9 do $* << x*2 if x.even? end, $*][1]
# [0, 4, 8, 12, 16]
```

Still, it would be far nicer if the for...in loop would return the desired array automatically, this is one way to approach that taking advantage of lambda bracket invocation syntax:

``` ruby
c = -> x do $*.clear             
  if x['if'] && x[0] != 'f' .  
    y = x[0...x.index('for')]    
    x = x[x.index('for')..-1]
    (x.insert(x.index(x.split[3]) + x.split[3].length, " do $* << #{y}")
    x.insert(x.length, "end; $*")
    eval(x)
    $*)
  elsif x['if'] && x[0] == 'f'
    (x.insert(x.index(x.split[3]) + x.split[3].length, " do $* << x")
    x.insert(x.length, "end; $*")
    eval(x)
    $*)
  elsif !x['if'] && x[0] != 'f'
    y = x[0...x.index('for')]
    x = x[x.index('for')..-1]
    (x.insert(x.index(x.split[3]) + x.split[3].length, " do $* << #{y}")
    x.insert(x.length, "end; $*")
    eval(x)
    $*)
  else
    eval(x.split[3]).to_a
  end
end 

```

so basically we are converting a string to proper ruby syntax for loop then we can use python syntax in a string to do:

``` ruby

c['for x in 1..10']
c['for x in 1..10 if x.even?']
c['x**2 for x in 1..10 if x.even?']
c['x**2 for x in 1..10']

# [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
# [2, 4, 6, 8, 10]
# [4, 16, 36, 64, 100]
# [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

```




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