将For循环从C ++转换为R.

Question

I am trying to convert this c++ code to R:

 int nrow = df.nrow(), ncol = df.ncol();
  for (int i = 0; i < nrow; i++) {
    for (int j = 0; j < ncol; j++) {
      dat[i * ncol + j] = data[j * nrow + i];
    }
  }

I wrote the following codes in R:

df = cbind(x = c(0.0,0.2,0.4,0.6,0.8,1.0), y = c(0.8,1.0,0.6,0.4,0.0,0.2)))

nRows <- nrow(df)
nCols <- ncol(df)

dat <- vector(mode="numeric", length= nRows * nCols) 

for (i in 1:nRows) {
  for (j in 1:nCols) {
      dat[i * nCols + j] <- df[j * nRows + i]
    }
}

But its not giving me the desired output.

i want to get this:

> dat
  [1] 0.0 0.8 0.2 1.0 0.4 0.6 0.6 0.4 0.8 0.0 1.0 0.2 

but instead im getting this:

> dat
  [1] 0.0 0.0 0.8  NA 1.0  NA 0.6  NA 0.4  NA 0.0  NA 0.2  NA

Can anyone help please?

Answer 1

It looks like you are trying to create a vector that combines two vectors, where the values of the new vector are the values of the old vectors but alternating.

> # Make the vectors
> x = c(0.0,0.2,0.4,0.6,0.8,1.0)
> y = c(0.8,1.0,0.6,0.4,0.0,0.2)
> # Get the column positions and offset them
> x_col_pos = (seq_along(x) * 2) - 1
> y_col_pos = (seq_along(y) * 2)
> # Find the order of the positions to re-arrange the new vector
> z_col_pos = order(c(x_pos, y_pos))
> z = c(x, y)[z_col_pos]
[1] 0.0 0.8 0.2 1.0 0.4 0.6 0.6 0.4 0.8 0.0 1.0 0.2

This can be easily converted into a function.