Why am i getting invalid types float[int] for array subscript?

Question

So i've looked at other questions on stackoverflow that seemed to describe the same problem, but the problem in each of these cases seems to be a wrong reference, e.g. the object was not an array. I think i've referenced my array correctly, but today is my first day doing C++. Could anyone tell me what i'm doing wrong here?

#include <Rcpp.h>
#include <cmath>
using namespace Rcpp;

// [[Rcpp::export]]
float convolute2D1(float arr[], int skew) {
  int m, n, l, j;
  float o;
  if (skew < 0) {
    m = 1;
    n = 2;
    skew = abs(skew);
  } else {
    m = 2;
    n = 1;
  }
  l = *(&arr + 1) - arr;
  l = l / 2 - skew;
  for(int i = 1; i <= l; i++) {
    j = i + skew;
    o = o + abs(arr[m][j] - arr[n][j]);
  }
  return o / l;
}

// END OF SCRIPT

i'm getting the errors:

Line 31 invalid types 'float[int]' for array subscript
Line 31 invalid types 'float[int]' for array subscript
Line 41 expected ',' or '...' before 'SEXP'
Line 45 expected primary-expression before ']' token
line 46 'skewSEXP' was not declared in this scope
Line 47 expected primary-expression before ']' token

I recommend you look into RcppArmadillo for proper (C++) vectors and matrices made for linear algebra. There may even be a convolution implementation somewhere though I cannot immediately think of one. — Dirk is no longer here
– Dirk is no longer here, Commented Jul 2, 2021 at 19:57
@DirkEddelbuettel thank you! i'll look into RcppArmadillo right away. — JadaLovelace
– JadaLovelace, Commented Jul 2, 2021 at 19:59

anastaciu · Accepted Answer · 2021-07-02 19:58:04Z

2

You're passing a linear array as argument of the function yet you are using it as if it was a 2D array, that's not possible.

As it's passed, arr can only be used like arr[i], not like arr[i][j].

edited Jul 2, 2021 at 19:58

answered Jul 2, 2021 at 19:55

anastaciu

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3 Comments

JadaLovelace Over a year ago

ah! so i should put float arr[][] in the arguments?

Dirk is no longer here Over a year ago

No you need proper types for 2d behaviour. See my earlier comment.

anastaciu Over a year ago

@JadaLovelace more or less, the second dimension of the array must be specified in the argument, e.g.: arr[][10].

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