From: Florian Gilcher <flo@...>
Date: 2009-12-03T10:45:39+09:00
Subject: Re: Increment outerloop value in innerloop in each do Syntax


On Dec 2, 2009, at 9:28 PM, Nag Raj wrote:

> Hi All,
>
> I am new to RUBY. Please help me in solving the below problem.
> I have a c code and I am trying to do the same in RUBY.
>
>   for(int i = 0; i < 10;i++)
>   {
>      for(int j = 0; j< 5; j++)
>      {
>         if(i == j)
>         {
>            printf("%d - its J value\n",j);
>            i++;
>         }
>      }
>      printf("%d - its I value\n",i);
>   }

Wow, is there any use case for this pattern?

>
> I want to do the same in RUBY. I have written following script
>
> (0..9).each do |i|
>   (0..5).each do |j|
>      if( i == j)
>         puts "#{j} J Value"
>      end
>   end
>   puts "#{i} I Value"
> end

Be aware that i or j are in no way the same as i and j in
your C program. Here, you iterate over the set of natural
numbers from 0..9 and 0..5, binding i and j to those values
for every step of the iteration. So i and j are not controlling
your iteration - each does.
So any change to i and j will be lost at the end of the do-block.

But, with a bit of trickery, you can achieve this:

   (0..9).each do |i|
     if j = (0..5).find{ |j| i==j }
       puts "#{j} J Value"
     else
       puts "#{i} I Value"
     end
   end

#find finds the first element of the range that satisfies
the condition given in the block. If there is none, it returns
nil, which is not true in a boolean sense. As the statement in
the condition can be any ruby expression (because the always return
a value, there is no void), we can immediately assign that to j.

As j has a true value (everything but nil and false) if the find
condition matches, the first branch of the if is used. Otherwise,
j is nil and i is printed.

Again: you iterate over a set, this is no counting loop. This might
take a bit to get used to, but in the end, you can use such nice
things as #find, #map, #inject on it. Learn them, they are a rubyists
mightiest tools.

Have a lot of fun.

Regards,
Florian Gilcher