Hi,
I have a need to loop through all the entries of a DataFrame by row, then
column. I know two different ways:
// Case A: When df.length() is unknown at coding time:
int n = df.nrows();
int m = df.length();
for(int i=1; i<n; i++) {
for(int j=0; j<m; j++) {
NumericVector v = df[j];
// ... do stuff with v[i] ...
}
}
// Case B: If I know the number of columns while writing the C code:
int n = df.nrows();
NumericVector xs = df[0];
NumericVector ys = df[1];
for(int i=1; i<n; i++) {
// ... do stuff with xs[i] and ys[i] ...
}
The second way is less flexible, but it's also quite a bit faster in practice -
I presume this means the "NumericVector ..." expressions are doing a
non-trivial amount of work (perhaps even copying the whole vector?).
Is there a way to have my cake & eat it? Can I efficiently (O[1]) index into a
DataFrame by numeric row index and numeric column index?
I'm also curious why it's a syntax error in Case A to just write `df[j][i]` or
even `((NumericVector) df[j])[i]` - clearly there's magic behind the
"NumericVector" call that I don't understand.
Thanks.
--
Ken Williams, Senior Research Scientist
WindLogics
http://windlogics.com
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