Unexpected behavior in inject

Ruby
1

I ran across something that puzzled me today, and I thought I’d ask here
and
see what you guys/girls think. I’m by no means a ruby guru, so if this
is
obvious to everyone but me, I’ll take the learning experience.

Here’s the simple script I was testing out:

[1,2,3].inject(0) do |acc, num|
acc + num
end

It works perfectly fine when I run it. No surprises here. However, when
I
add a puts call after the accumulator addition, like so:

[1,2,3].inject(0) do |acc, num|
acc + num
puts acc
end

Ruby gives me this error:

bash ~ $ ruby test.rb
0
test.rb:2: undefined method +' for nil:NilClass (NoMethodError) from test.rb:5:ininject’
from test.rb:1:in each' from test.rb:1:ininject’
from test.rb:1

This seems pretty strange to me, as if puts is modifying the accumulator
by
resetting it to nil. Is this normal behavior? If so, I would greatly
appreciate it if someone could point me in the right direction as to why
this is the way it is. I also noticed that if I place the puts call
before
the addition, there is no error, which also strikes me as a little odd.
Here’s my ruby version (using cygwin on windows server 2003):

bash ~ $ ruby -v
ruby 1.8.6 (2007-09-24 patchlevel 111) [i386-mswin32]

Thanks,
Alex

2

On Tue, Jun 15, 2010 at 9:21 AM, Alex F. [email protected] wrote:

end

Let me give you a slightly different example to show how it works: You
pass
in the zero, and it goes into acc in the block. Whatever the block
returns
is then fed back into acc, for the next iteration. After the last
iteration,
whatever the block returns is returned by the inject method. I think of
it
as a number passing through an Enumerable and coming out the other side
modified.

(5…10).inject 0 do |sum,num|
sum + num
end

This should return 45 in the following manner:
The first time the block is passed 0 , 5 and it returns 5
The second time the block is passed 5 , 6 and it returns 11
The third time the block is passed 11 , 7 and it returns 18
The fourth time the block is passed 18 , 8 and it returns 26
The fourth time the block is passed 26 , 9 and it returns 35
The fourth time the block is passed 35 , 10 and it returns 45
The method then returns 45

So why doesn’t this work?

[1,2,3].inject(0) do |acc, num|

acc + num
puts acc
end

Because the puts method returns nil. It is also the last thing in the
block,
so the block returns nil. So nil is fed back into acc. Then nil + acc
raises
the NoMethodError.

To resolve this, think about where you could put the puts method that
won’t
cause the block to return nil.

If you are interested, here is a functional version (takes the
enumerable as
a parameter) of inject that I wrote, except I call ti “passthrough”
because
that is more meaningful for me.

def passthrough( enumerable , to_pass )
enumerable.each do |element|
to_pass = yield to_pass , element
end
to_pass
end

3

test.rb:2: undefined method +' for nil:NilClass (NoMethodError) from test.rb:5:ininject’
from test.rb:1:in each' from test.rb:1:ininject’
from test.rb:1

This seems pretty strange to me, as if puts is modifying the accumulator by
resetting it to nil. Is this normal behavior? If so, I would greatly
appreciate it if someone could point me in the right direction as to why
this is the way it is.

Yes, this is exactly as expected. The last statement you put in the
block
is the return value of the block. ‘puts’ always returns nil, so the
next
iteration acc is nil.

To get the behavior you are looking for:

[1,2,3].inject(0) do |acc, num|
sum = acc + num
puts sum
sum
end

Now since ‘sum’ is the final statement in the block, acc will be sum for
the
next iteration.

-Jonathan N.

4

On Tue, Jun 15, 2010 at 10:31 AM, Jonathan N. [email protected]
wrote:

test.rb:2: undefined method `+’ for nil:NilClass (NoMethodError)

end

Now since ‘sum’ is the final statement in the block, acc will be sum for the
next iteration.

I understand now. Thank you for clarifying that for me.

Alex

5

On Tue, Jun 15, 2010 at 10:35 AM, Josh C. [email protected]
wrote:

[1,2,3].inject(0) do |acc, num|

The fourth time the block is passed 35 , 10 and it returns 45

Because the puts method returns nil. It is also the last thing in the block,
so the block returns nil. So nil is fed back into acc. Then nil + acc raises
the NoMethodError.

To resolve this, think about where you could put the puts method that won’t
cause the block to return nil.

That’s where I was going wrong, I didn’t realize that inject passed
the value of the block, back to the accumulator, not just the
modified accumulator. I was thinking that ruby was somehow detecting
when I modified the accumulator, so I was sometimes writing code like
this:

[1,2,3].inject(0) {|acc, num| acc += num}

Now I get it. It’s all about the value of the block, and there’s
really no magic going on. Insight is a wonderful thing :slight_smile:

to_pass = yield to_pass , element
end
to_pass
end

Thanks again,
Alex