CODEWITHSUNDEEP
c++rrcpp
Keith
Keith

Reputation: 71

In Rcpp, why can't I assign a subsetted vector to another subsetted vector

I'm currently porting some R code into Rcpp for a speed improvement (which has been successful so far), but I'm trying to understand why Rcpp doesn't allow certain types of assignment when it comes to subsetting.

For example, in R if I have three vectors A, B, C, and if for some condition of C i want to fill A with the values of B, then i can write:

A[C < 3] = B[C< 3]

However I get an error when i try something similar in Rcpp.

Rccp seems to be happy to assign things like:

A[C < 3] = X

...and...

Y = B[C< 3]

...but not...

A[C < 3] = B[C< 3]

Therefore I first assign the RHS to another vector first, and then assign that vector to the LHS, it works fine!?

Here is some example code that works:

cppFunction("
    NumericVector valuesOverThree(NumericVector b){

        NumericVector a(b.size());

        NumericVector temp = b[b > 3];
        a[b > 3] = temp;
        return a;
    }
")
valuesOverThree(1:6)
# returns: [0, 0, 0, 4, 5, 6]

And here is some that gives an error:

cppFunction("
    NumericVector valuesOverThree(NumericVector b){
        NumericVector a(b.size());

        a[b > 3] = b[b > 3];
        return a;
    }
")
valuesOverThree(1:6)
# returns: error message

Is there any reason for this, or anyway I can make this work on one line?

As shown, there is a workaround, but it would be nicer if i could write everything on one line.

Many thanks

Upvotes: 3

Views: 668

Answers (2)

Qiang Kou
Qiang Kou

Reputation: 522

Just a simple remainder.

You can do exactly what you want with latest Rcpp.

> cppFunction("
+     NumericVector valuesOverThree(NumericVector b){
+         NumericVector a(b.size());
+ 
+         a[b > 3] = b[b > 3];
+         return a;
+     }
+ ")
> valuesOverThree(1:6)
[1] 0 0 0 4 5 6
>

Upvotes: 1

Dirk is no longer here
Dirk is no longer here

Reputation: 368231

We have a Rcpp Gallery article on indexing / subsetting.

I have in the past used RcppArmadillo, there is also an Rcpp Gallery post besides the normal (and excellent) Armadillo documentation.

I also would not worry too much about getting everything onto one line. The compiler is pretty good at removing temporaries. Rather, I try to write code I can still read in a week.

Upvotes: 2

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