# Easy task, Sum array element... Why doesn't it work?

**URL:** <https://www.ruby-forum.com/t/easy-task-sum-array-element-why-doesnt-it-work/185930>\
**Category:** Ruby\
**Created:** [March 29, 2010, 12:26pm UTC](https://www.ruby-forum.com/t/easy-task-sum-array-element-why-doesnt-it-work/185930 "2010-03-29T12:26:15Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![walle](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/5daacb/32.png) [@walle](https://www.ruby-forum.com/u/walle)\
**Post date:** [March 29, 2010, 12:26pm UTC](https://www.ruby-forum.com/t/easy-task-sum-array-element-why-doesnt-it-work/185930/1 "2010-03-29T12:26:15Z")

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I have done it many times before, but this time I can’t get it too work.  
I’m starting to get a bit frustrating.  
So, I’m trying to sum all elements from the second element in a Array  
using inject. The problem is that inject returns nil every time. What am  
I doing wrong?

parameters.each {|element| print element + “,”} --\> ?faq,using,rtorrent  
parameters.each {|element| print element.class.to\_s + “,”} --\>  
String,String,String  
parameters.class --\> Array

Doesn’t work.  
(parameters[1]…parameters[-1]).inject {|result, element| result +  
element} --\> nil  
Neither does.  
(parameters[1]…parameters[-1]).to\_a.inject {|result, element| result +  
element} --\> nil

//Walle

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<div class="post-metadata">

**Author:** ![walle](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/5daacb/32.png) [@walle](https://www.ruby-forum.com/u/walle)\
**Post date:** [March 29, 2010, 12:29pm UTC](https://www.ruby-forum.com/t/easy-task-sum-array-element-why-doesnt-it-work/185930/2 "2010-03-29T12:29:55Z")

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Walle W. wrote:

> Doesn’t work.  
> (parameters[1]…parameters[-1]).inject {|result, element| result +

That’s creating a new Range object: (“faq”…“rtorrent”)

What you want is an array slice:

parameters[1…-1].inject …

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<div class="post-metadata">

**Author:** ![walle](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/5daacb/32.png) [@walle](https://www.ruby-forum.com/u/walle)\
**Post date:** [March 29, 2010, 12:39pm UTC](https://www.ruby-forum.com/t/easy-task-sum-array-element-why-doesnt-it-work/185930/3 "2010-03-29T12:39:11Z")

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Brian C. wrote:

> Walle W. wrote:
> 
> > Doesn’t work.  
> > (parameters[1]…parameters[-1]).inject {|result, element| result +
> 
> That’s creating a new Range object: (“faq”…“rtorrent”)
> 
> What you want is an array slice:
> 
> parameters[1…-1].inject …

Thanks, it worked.

I did a small test in IRB, and it seems like my method should work.  
Strange.  
a = [“a”, “b”, “c”]  
=\> [“a”, “b”, “c”]

> > (a[1]…a[-1]).inject {|result, element| entry + result}  
> > =\> “bc”

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<div class="post-metadata">

**Author:** ![walle](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/5daacb/32.png) [@walle](https://www.ruby-forum.com/u/walle)\
**Post date:** [March 29, 2010, 12:58pm UTC](https://www.ruby-forum.com/t/easy-task-sum-array-element-why-doesnt-it-work/185930/4 "2010-03-29T12:58:06Z")

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Thanks Jesus, I get it now.

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<div class="post-metadata">

**Author:** ![walle](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/5daacb/32.png) [@walle](https://www.ruby-forum.com/u/walle)\
**Post date:** [March 29, 2010, 12:53pm UTC](https://www.ruby-forum.com/t/easy-task-sum-array-element-why-doesnt-it-work/185930/5 "2010-03-29T12:53:46Z")

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On Mon, Mar 29, 2010 at 12:39 PM, Walle W. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> Thanks, it worked.
> 
> I did a small test in IRB, and it seems like my method should work.  
> Strange.  
> a = [“a”, “b”, “c”]  
> =\> [“a”, “b”, “c”]
> 
> > > (a[1]…a[-1]).inject {|result, element| entry + result}  
> > > =\> “bc”

As Brian has explained, (a[1]…[a-1]) creates a range. When you  
iterate a range, it calls succ starting on the first element until it  
reaches the last one. In your example with a, b, c:

irb(main):006:0\> (“a”…“c”).each {|s| p s}  
“a”  
“b”  
“c”

You get the three elements of the array, by chance. Try changing that  
to something else, for example:

irb(main):013:0\> array = [“a”, “d”, “f”]  
=\> [“a”, “d”, “f”]  
irb(main):014:0\> (array[1]…array[-1]).inject {|result, element|  
result + element}  
=\> “def”

In your original example, the words were quite far apart:

irb(main):005:0\> (“faq”…“rtorrent”).each\_with\_index {|s, i| p s;  
break if i \> 20}  
“faq”  
“far”  
“fas”  
“fat”  
“fau”  
“fav”  
“faw”  
“fax”  
“fay”  
“faz”  
“fba”  
“fbb”  
“fbc”  
“fbd”  
“fbe”  
“fbf”  
“fbg”  
“fbh”  
“fbi”  
“fbj”  
“fbk”  
“fbl”

and so on until “rtorrent”.

Jesus.

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<div class="post-metadata">

**Author:** ![walle](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/5daacb/32.png) [@walle](https://www.ruby-forum.com/u/walle)\
**Post date:** [March 29, 2010, 4:15pm UTC](https://www.ruby-forum.com/t/easy-task-sum-array-element-why-doesnt-it-work/185930/6 "2010-03-29T16:15:09Z")

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On Mon, Mar 29, 2010 at 4:26 AM, Walle W. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> 

Rails has this by default:  
[http://api.rubyonrails.org/classes/Enumerable.html#M002571](http://api.rubyonrails.org/classes/Enumerable.html#M002571)

module Enumerable  
def sum(identity = 0, &block)  
return identity unless size \> 0

```
if block_given?
  map(&block).sum
else
  inject { |sum, element| sum + element }
end

```

end  
end

[‘a’,‘b’,‘c’].sum # =\> “abc”  
[1,2,3].sum # =\> 6  
[[0],[‘0’],[:‘0’],[/0/]].sum # =\> [0, “0”, :“0”, /0/]  
[].sum(/abc/) # =\> /abc/  
[1,2,3].sum(/abc/) # =\> 6  
[‘a’,‘b’,‘c’].sum(&:upcase) # =\> “ABC”

That last one is 1.9 only (or another monkey patch 😜 )

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<div class="post-metadata">

**Author:** ![walle](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/5daacb/32.png) [@walle](https://www.ruby-forum.com/u/walle)\
**Post date:** [March 29, 2010, 9:28pm UTC](https://www.ruby-forum.com/t/easy-task-sum-array-element-why-doesnt-it-work/185930/7 "2010-03-29T21:28:20Z")

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2010/3/29 Jesús Gabriel y Galán [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

> On Mon, Mar 29, 2010 at 12:39 PM, Walle W. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> In your original example, the words were quite far apart:
> 
> irb(main):005:0\> (“faq”…“rtorrent”).each\_with\_index {|s, i| p s;  
> break if i \> 20}

Actually in his original example parameters appears to be [“?faq” ,  
“using” , “rtorrent”]

so

(parameters[1]…parameters[-1])

is

(“using”…“rtorrent”)

and since “using” \> “rtorrent” the range is empty which is why inject  
returns nil.

–  
Rick DeNatale

Blog: [http://talklikeaduck.denhaven2.com/](http://talklikeaduck.denhaven2.com/)  
Twitter: [http://twitter.com/RickDeNatale](http://twitter.com/RickDeNatale)  
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