# Question about Vector class

**URL:** <https://www.ruby-forum.com/t/question-about-vector-class/64120>\
**Category:** Ruby\
**Created:** [June 22, 2006, 7:52am UTC](https://www.ruby-forum.com/t/question-about-vector-class/64120 "2006-06-22T07:52:55Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Nasir\_K](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/838e76/32.png) [@Nasir\_K](https://www.ruby-forum.com/u/Nasir_K)\
**Post date:** [June 22, 2006, 7:52am UTC](https://www.ruby-forum.com/t/question-about-vector-class/64120/1 "2006-06-22T07:52:55Z")

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Hi,  
I am extremely new to Ruby, just my first week so I apologize if this  
has  
already been discussed or is completely absurd.

I was using the library Vector class defined in matrix.rb in one of my  
programs, the [] method in the library class does not check for range  
out of  
bounds and the []= does not even exist. In my program I extended the  
Vector  
class as follows -

# 

* * *

require ‘matrix’

# Extend the Vector library class for []=

# defined in src/ruby-1.8.4/matrix.rb

class Vector  
include ExceptionForMatrix

# []= not defined in the library class

def []= (i, val)  
Vector.Raise ErrDimensionMismatch if i \>= @elements.size  
@elements[i] = val  
end

# Fix the [] in library Vector class

def   
Vector.Raise ErrDimensionMismatch if i \>= @elements.size  
@elements[i]  
end  
end  
#-------------------------------------------------------------------------------

Should this not be part of matrix.rb itself?

Best Regards  
Nasir

---

<div class="post-metadata">

**Author:** ![Nasir\_K](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/838e76/32.png) [@Nasir\_K](https://www.ruby-forum.com/u/Nasir_K)\
**Post date:** [June 22, 2006, 8:49am UTC](https://www.ruby-forum.com/t/question-about-vector-class/64120/2 "2006-06-22T08:49:11Z")

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Nasir K. wrote:

> I was using the library Vector class defined in matrix.rb in one of my  
> programs, the [] method in the library class does not check for range out of  
> bounds and the []= does not even exist. In my program I extended the Vector  
> class as follows -  
> …  
> class Vector  
> include ExceptionForMatrix

Once a module has been included, you don’t need to re-include it when  
you re-open the class.

> # []= not defined in the library class
> 
> def []= (i, val)  
> Vector.Raise ErrDimensionMismatch if i \>= @elements.size  
> @elements[i] = val  
> end

I haven’t a great deal of experience with matrix.rb, but from the  
quick look I just did, it seems that Vector instances are immutable  
(un-changeable), like the Numeric classes. Immutability means you can  
pass a vector or number into a method and know it won’t be  
unexpectedly altered. It means you can think of a vector as a fixed  
value, rather than a container.

So your proposed []= method is a bad idea. If you want to be able to  
tweak a vector one element at a time, consider using Array (especially  
if you’re not wanting a mathematical vector) or a method like the  
following, to keep Vector immutable:

def with\_element\_replaced (i, val)  
Vector.Raise ErrDimensionMismatch if i \>= @elements.size  
new\_vec = self.to\_a  
new\_vec[i] = val  
Vector[\*new\_vec]  
end

> # Fix the [] in library Vector class
> 
> def   
> Vector.Raise ErrDimensionMismatch if i \>= @elements.size  
> @elements[i]  
> end  
> end

I don’t see a strong argument for this over the existing behaviour.  
What you’ve done isn’t wrong, though, and if you find it handy, use  
it. You’ve gained early and descriptive failure, but lost a bit of  
flexibility in using negative indexes (vec[-1] for the last element).

Cheers,  
Dave