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comp.lang.ruby

The inject function

Shendelzare Silkwood

3/26/2008 2:19:00 PM

I want to add together numbers in an array with the inject command. I
want only the even numbers using only inject.

So, something like [1,2,3,4].inject.....but don't know what to do so
help!

Second, I need to reverse an array using only the inject command. So,
something like above with the result [4,3,2,1]

I'm confused...help!
--
Posted via http://www.ruby-....
10 Answers

Eustaquio 'TaQ' Rangel

3/26/2008 2:33:00 PM

0

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| I want to add together numbers in an array with the inject command. I
| want only the even numbers using only inject.
| So, something like [1,2,3,4].inject.....but don't know what to do so
| help!

Use select before inject:

[1,2,3,4,5,6].select {|i| i%2==0}.inject {|m,v| m+v}

You could use a test to sum the number only when it's even on inject, but

[1,2,3,4,5,6].inject {|m,v| m + (v%2==0?v:0) }

results to 13, because seems that the value of m is automatically the first
element of the array. On this case, you can tell it to start with 0:

[1,2,3,4,5,6].inject(0) {|m,v| m + (v%2==0?v:0) }

| Second, I need to reverse an array using only the inject command. So,
| something like above with the result [4,3,2,1]

Why don't you use reverse?

[1,2,3,4,5,6].reverse

Regards,
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Radoslaw Bulat

3/26/2008 2:36:00 PM

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Robert Dober

3/26/2008 2:39:00 PM

0

On Wed, Mar 26, 2008 at 3:19 PM, Shendelzare Silkwood
<zerruellx@yahoo.com> wrote:
> I want to add together numbers in an array with the inject command. I
> want only the even numbers using only inject.
>
> So, something like [1,2,3,4].inject.....but don't know what to do so
> help!
>
> Second, I need to reverse an array using only the inject command. So,
> something like above with the result [4,3,2,1]
Hmm making your homework here?

inject([]){ |a,e| [e]+a }
that of course is a st****d way to reverse an array.

HTH
Robert
>
> I'm confused...help!
> --
> Posted via http://www.ruby-....
>
>



--
http://ruby-smalltalk.blo...

---
Whereof one cannot speak, thereof one must be silent.
Ludwig Wittgenstein

James Gray

3/26/2008 2:42:00 PM

0

On Mar 26, 2008, at 9:35 AM, Rados=B3aw Bu=B3at wrote:

>> Second, I need to reverse an array using only the inject command. So,
>> something like above with the result [4,3,2,1]
>
> Use .reverse method. inject doesn't fit in.

I agree that reverse() is the correct tool for this job, but just to =20
fully answer the question:

>> [1, 2, 3, 4].inject([]) { |rev, item| [item] + rev }
=3D> [4, 3, 2, 1]

James Edward Gray II=

Dave Thomas

3/26/2008 2:45:00 PM

0


On Mar 26, 2008, at 9:35 AM, Rados=B3aw Bu=B3at wrote:
>> Second, I need to reverse an array using only the inject command. So,
>> something like above with the result [4,3,2,1]
>
> Use .reverse method. inject doesn't fit in.

I'm guessing this is a homework assignment (otherwise why would inject =20=

be required). You can reverse using inject if the accumulator is an =20
array...


Dave=

Eustaquio 'TaQ' Rangel

3/26/2008 2:52:00 PM

0

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| I agree that reverse() is the correct tool for this job, but just to
| fully answer the question:
| >> [1, 2, 3, 4].inject([]) { |rev, item| [item] + rev }

Just to use the same approach but with insert:

|> [1,2,3,4,5,6].inject([]){|m,v| m.insert(0,v)}
=> [6, 5, 4, 3, 2, 1]
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Robert Klemme

3/26/2008 6:35:00 PM

0

On 26.03.2008 15:32, Eustáquio 'TaQ' Rangel wrote:
> You could use a test to sum the number only when it's even on inject, but
>
> [1,2,3,4,5,6].inject {|m,v| m + (v%2==0?v:0) }
>
> results to 13, because seems that the value of m is automatically the first
> element of the array. On this case, you can tell it to start with 0:
>
> [1,2,3,4,5,6].inject(0) {|m,v| m + (v%2==0?v:0) }

Actually these two are not equivalent! You should usually use the
second form because the first form will return nil for an empty Array.

Btw, Radoslav's solution omits unnecessary additions of 0:

[1,2,3,4,5,6].inject(0) {|m,v| v % 2 == 0 ? m : m+v }

> | Second, I need to reverse an array using only the inject command. So,
> | something like above with the result [4,3,2,1]
>
> Why don't you use reverse?

I assume it is some kind of task assignment - which is why I regret that
solutions were posted so quickly.

Kind regards

robert

Robert Klemme

3/26/2008 6:35:00 PM

0

On 26.03.2008 15:52, Eustáquio 'TaQ' Rangel wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> | I agree that reverse() is the correct tool for this job, but just to
> | fully answer the question:
> | >> [1, 2, 3, 4].inject([]) { |rev, item| [item] + rev }
>
> Just to use the same approach but with insert:
>
> |> [1,2,3,4,5,6].inject([]){|m,v| m.insert(0,v)}
> => [6, 5, 4, 3, 2, 1]

While we're improving...

irb(main):001:0> [1,2,3,4,5,6].inject([]) {|a,x| a.unshift x}
=> [6, 5, 4, 3, 2, 1]

Oh, I just see Dan has it already.

Kind regards

robert

Eustaquio 'TaQ' Rangel

3/26/2008 7:05:00 PM

0

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Robert Klemme wrote:
| On 26.03.2008 15:32, Eustáquio 'TaQ' Rangel wrote:
|> You could use a test to sum the number only when it's even on inject, but
|> [1,2,3,4,5,6].inject {|m,v| m + (v%2==0?v:0) }
|> results to 13, because seems that the value of m is automatically the
|> first
|> element of the array. On this case, you can tell it to start with 0:
|> [1,2,3,4,5,6].inject(0) {|m,v| m + (v%2==0?v:0) }
| Actually these two are not equivalent! You should usually use the
| second form because the first form will return nil for an empty Array.

Hi Robert!

Yeah, and I just put the first form because at first glance they look equivalent
for newbies on the language and can make some confusion for "where are the 1
value coming from". :-)

Regards,
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Thufir Hawat

4/2/2008 8:25:00 AM

0

On Thu, 27 Mar 2008 03:34:55 +0900, Robert Klemme wrote:


> I assume it is some kind of task assignment - which is why I regret that
> solutions were posted so quickly.


I for one am curious about who assigned it :)


-Thufir