Subtract values of a array

ruby-talk
1

Hello,

I changed from ruby koans to rubymonks.

Now I have to make a method which can substract any number of numbers.

So I tried this

def subtract(*numbers)
  numbers.inject(0) { |sum, number| sum - number }
end

but that won't work subtract ( 4,5) gives -9 when -1 is the right answer.

Can anyone give me a tip for a better answer.

Roelof

2

Do you mean you want a method which subtracts some numbers from a given one
(for example: subtract(10,3,4) should give 10 - 3 - 4)? In this case, I'd say
you need a required parameter, which is the number to subtract from and which
you pass as first argument to inject.

To see why your method doesn't do what you want, check the documentation for
inject and, in particular, what the parameter means.

I hope this helps

Stefano

···

On Friday 28 September 2012 Roelof Wobben wrote

Hello,

I changed from ruby koans to rubymonks.

Now I have to make a method which can substract any number of numbers.

So I tried this

def subtract(*numbers)
  numbers.inject(0) { |sum, number| sum - number }
end

but that won't work subtract ( 4,5) gives -9 when -1 is the right answer.

Can anyone give me a tip for a better answer.

Roelof

3

inject(0) will pass 0 as the first accumulator. If you don't pass the
0, then inject will use the first element in the array as the first
accumulator, which is what you want I think.

Jesus.

···

On Thu, Sep 27, 2012 at 5:36 PM, Roelof Wobben <rwobben@hotmail.com> wrote:

Hello,

I changed from ruby koans to rubymonks.

Now I have to make a method which can substract any number of numbers.

So I tried this

def subtract(*numbers)
  numbers.inject(0) { |sum, number| sum - number }
end

but that won't work subtract ( 4,5) gives -9 when -1 is the right answer.

Can anyone give me a tip for a better answer.

4

You're overthinking it. Look at a simpler usage of inject.

I find it useful to think of inject as injecting the operation between
each element of the array it's given, and if it's given an argument as
well (not an operation, one to treat as a starting value), then
between that and the start as well. So what you've done there is tell
it to do, essentially, 0 - 4 - 5. All you want is 4 - 5. How would
you eliminate the "0 -" part? Hint: the argument is optional.

-Dave

···

On Thu, Sep 27, 2012 at 11:36 AM, Roelof Wobben <rwobben@hotmail.com> wrote:

def subtract(*numbers)
  numbers.inject(0) { |sum, number| sum - number }
end

but that won't work subtract ( 4,5) gives -9 when -1 is the right answer.

--
Dave Aronson, T. Rex of Codosaurus, LLC... aka
Available Secret-Cleared Ruby/Rails Freelancer
(NoVa/DC/Remote); see http://www.Codosaur.us/\.

5

Thanks for all the help.
And overthinking is a well known problem for me.

Roelof

···

Date: Fri, 28 Sep 2012 00:44:26 +0900
From: rubytalk2dave@davearonson.com
Subject: Re: subtract values of a array
To: ruby-talk@ruby-lang.org

On Thu, Sep 27, 2012 at 11:36 AM, Roelof Wobben <rwobben@hotmail.com> wrote:

> def subtract(*numbers)
> numbers.inject(0) { |sum, number| sum - number }
> end
>
> but that won't work subtract ( 4,5) gives -9 when -1 is the right answer.

You're overthinking it. Look at a simpler usage of inject.

I find it useful to think of inject as injecting the operation between
each element of the array it's given, and if it's given an argument as
well (not an operation, one to treat as a starting value), then
between that and the start as well. So what you've done there is tell
it to do, essentially, 0 - 4 - 5. All you want is 4 - 5. How would
you eliminate the "0 -" part? Hint: the argument is optional.

-Dave

--
Dave Aronson, T. Rex of Codosaurus, LLC... aka
Available Secret-Cleared Ruby/Rails Freelancer
(NoVa/DC/Remote); see http://www.Codosaur.us/\.

6

You're welcome. One big thing to get used to in Ruby is that it tries
to make things easy for you. C tries to make things easy for the
compiler writers. Java tries to make it easy for the machine. Ruby
is a refreshing change. This becomes even more important if you start
doing Rails -- go with what it provides for you, and it will be easy,
but try to deviate from its conventions, and you will have a much
harder time.

-Dave

···

On Thu, Sep 27, 2012 at 11:53 AM, Roelof Wobben <rwobben@hotmail.com> wrote:

Thanks for all the help.
And overthinking is a well known problem for me.

--
Dave Aronson, T. Rex of Codosaurus, LLC... aka
Available Secret-Cleared Ruby/Rails Freelancer
(NoVa/DC/Remote); see http://www.Codosaur.us/\.