I think you want the `matrix-ref` function.

Sam
On Sun, Oct 7, 2018 at 12:30 AM Ricardo Iglesias
<[email protected]> wrote:
>
> Sorry for the confusing wording.
> Basically, All I want is to be able to do
> G[ row ][ col ] = value
>
> In Racket. I've been messing with Racket this afternoon and have an engine 
> that is able to index into matrices, and just implemented several common 
> matrix operations, such as transpose, matrix multiplication, etc..., so for 
> now, I'm good.
>
>
> On Saturday, October 6, 2018 at 8:47:41 PM UTC-7, cwebber wrote:
>>
>> Ricardo Iglesias writes:
>>
>> > Good afternoon. I'm trying to move away from things like Matlab and SciPy
>> > to do linear algebra work.
>> > Something I notice I do a lot is indexing operations, such as
>> > MATRIX[ row ] [ column ]
>> >
>> > I'm looking at the "math/matrix" library provided here:
>> > https://docs.racket-lang.org/math/matrices.html, and don't see anything of
>> > this sort; I only see things like
>> > (matrix-map-rows)
>> >
>> > and
>> > (matrix-map-cols)
>> >
>> > Being able to access and modify a particular entry in the matrix would be
>> > very useful in specifying operations to matrices. For example, given a
>> > matrix *G*, one might want to perform indexing operations to get the matrix
>> > attached.
>> >
>> > Is this possible with the math/matrix library?
>> >
>> > Additionally, would there be a way to implement a version of
>> > matrix-map-row/cols that uses an index?
>>
>> Not quite what you're asking, but someone at RacketCon suggested to me
>> that maybe we should get GNU Scientific Library bindings added to
>> Racket, so we can better compete with Python and R for scientific /
>> statistical work...
>
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