Sometimes it’s easier to tell R what you don’t want. In other words, exclude columns from the matrix using negative indexing: Here are two alternative ways that both produce the same results: A[, -(1:2)] A[, -seq_len(2)] Results: [,1] [,2] [,3] [,4] [,5] [,6] [1,] 3 4 5 6 7 8 [2,] 3 4 5 6 … Read more
An example, d <- data.frame(x1=rnorm(10), x2=rnorm(10), x3=rnorm(10)) cor(d) # get correlations (returns matrix)
Gavin’s answer is simple and elegant. But if there are many columns, a much faster solution would be: lapply(seq_len(ncol(x)), function(i) x[,i]) The speed difference is 6x in the example below: > x <- matrix(1:1e6, 10) > system.time( as.list(data.frame(x)) ) user system elapsed 1.24 0.00 1.22 > system.time( lapply(seq_len(ncol(x)), function(i) x[,i]) ) user system elapsed 0.2 … Read more
This is a good question. There are many reason you would want to actually transpose the matrix in memory rather than just swap coordinates, e.g. in matrix multiplication and Gaussian smearing. First let me list one of the functions I use for the transpose (EDIT: please see the end of my answer where I found … Read more
here: bool gluInvertMatrix(const double m[16], double invOut[16]) { double inv[16], det; int i; inv[0] = m[5] * m[10] * m[15] – m[5] * m[11] * m[14] – m[9] * m[6] * m[15] + m[9] * m[7] * m[14] + m[13] * m[6] * m[11] – m[13] * m[7] * m[10]; inv[4] = -m[4] * m[10] … Read more
You can resolve this in several ways: Use g++ in stead of gcc: g++ -g -o MatSim MatSim.cpp Add -lstdc++: gcc -g -o MatSim MatSim.cpp -lstdc++ Replace <string.h> by <string> This is a linker problem, not a compiler issue. The same problem is covered in the question iostream linker error – it explains what is … Read more
To make things interesting, let’s try with a bigger matrix: matrix = [ [“Ah!”, “We do have some Camembert”, “sir”], [“It’s a bit”, “runny”, “sir”], [“Well,”, “as a matter of fact it’s”, “very runny, sir”], [“I think it’s runnier”, “than you”, “like it, sir”] ] s = [[str(e) for e in row] for row in … Read more
Copied directly from http://www.lighthouse3d.com/tutorials/glsl-tutorial/data-types-and-variables/. The actual site has much more detailed information and would be worthwhile to check out. Variable Qualifiers Qualifiers give a special meaning to the variable. The following qualifiers are available: const – The declaration is of a compile time constant. attribute – Global variables that may change per vertex, that are … Read more
matrix notation used in opengl documentation does not describe in-memory layout for OpenGL matrices If think it’ll be easier if you drop/forget about the entire “row/column-major” thing. That’s because in addition to row/column major, the programmer can also decide how he would want to lay out the matrix in the memory (whether adjacent elements form … Read more
PyTables and NumPy are the way to go. PyTables will store the data on disk in HDF format, with optional compression. My datasets often get 10x compression, which is handy when dealing with tens or hundreds of millions of rows. It’s also very fast; my 5 year old laptop can crunch through data doing SQL-like … Read more