# [R] Multiplying two vectors of the same size to give a third vector of the same size

@vi@e@gross m@iii@g oii gm@ii@com @vi@e@gross m@iii@g oii gm@ii@com
Wed Jun 21 04:46:35 CEST 2023

```I was rushing out Phil so let me amend what I wrote. As others noted, this is fairly beginner stuff. If you have more such questions, besides reading up, please consider sending questions to the Tutor mailing list where there is more patience. 😉

You wanted to change selected small values to 0.0.

So you do not want the code I supplied as illustration as  it removes small elements making a smaller  vector.

Assume this test:

A <- c(NA, 0.3, 0.6, NA, 0.9)
B <- c(NA, 0.2, 0.6, 0.9, 0.9)

C <- A * B
print(C)

The result at this point is:

   NA 0.06 0.36   NA 0.81

As expected, anything with an NA in either vector will generate an NA in the componentwise multiplication. Note the second item at 0.06 is under your threshold of 0.1.

What you want is not this:

Result <- C[C < 0.1]
> Result
   NA 0.06   NA

That threw away anything above the threshold.

What you want may be this:

C[C < 0.1] <- 0.0
print(C)

This returns

   NA 0.00 0.36   NA 0.81

Everything that is NA or at or above 0.1 is kept and anything below 0.1 is zeroed and kept.

Of course, if you do not want to keep the NA, that can trivially be removed:

C[!is.na(C)]
 0.00 0.36 0.81

-----Original Message-----
From: Philip Rhoades <phil using pricom.com.au>
Sent: Tuesday, June 20, 2023 1:04 PM
To: avi.e.gross using gmail.com
Cc: r-help using r-project.org
Subject: Re: [R] Multiplying two vectors of the same size to give a third vector of the same size

avi,

On 2023-06-21 01:55, avi.e.gross using gmail.com wrote:
> Phil,
>
> What have you tried. This seems straightforward enough.
>
> Could you clarify what you mean by NULL?

I guess in R in would just be an empty cell? - ie NOT a zero.

> In R, it is common to use NA or a more specific version of it.

Ah yes, that would be it I think.

> So assuming you have two vectors containing floats with some NA, then:
>
> C <- A*B
>
> Will give you the products one at a time if the lengths are the same.
> NA
> times anything is NA.

Right - yes that works! - thanks!

> Your second condition is also simple as you want anything below a
> threshold
> to be set to a fixes value.
>
>
> threshold <- 0.1
> C < threshold
>
> The last line returns a Boolean vector you can use to index C to get
> just
> the ones you select as TRUE and thus can change:
>
> Result <- C[C < threshold]

Ah, I see . .

> And you can of course do all the above as a one-liner.

Yes.

> Is that what you wanted?

Exactly except I meant:

Result <- C[C > threshold]

Thanks!

Phil.

> -----Original Message-----
> From: R-help <r-help-bounces using r-project.org> On Behalf Of Philip Rhoades
> via
> R-help
> Sent: Tuesday, June 20, 2023 11:38 AM
> To: r-help using r-project.org
> Subject: [R] Multiplying two vectors of the same size to give a third
> vector
> of the same size
>
> People,
>
> I am assuming that what I want to do is easier in R than say Ruby.
>
> I want to do what the Subject says ie multiply the cells in the same
> position of two vectors (A and B) to give a result in the same position
> in a third vector (C) BUT:
>
> - The values in the cells of A and B are floats between 0.0 and 1.0 or
> NULL
>
> - If there is a NULL in the multiplication, then the result in the cell
> for C is also a NULL
>
> - If there is a value less than (say) 0.01 in the multiplication, then
> the result in the cell for C is 0.0
>
> Any suggestions appreciated!
>
> Phil.
>
> --
>
> PO Box 896
> Cowra  NSW  2794
> Australia
> E-mail:  phil using pricom.com.au
>
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--