lynnyi - 5 months ago 57

R Question

I want to use R's Matrix library's writeMM to write a sparse matrix to external file in matrix market format.

see: https://stat.ethz.ch/R-manual/R-patched/library/Matrix/html/externalFormats.html

matrix

`4 0`

2 4

library(Matrix)

writeMM(matrix, "./outfile.tsv")

outfile.tsv:

`#rowindex #columnindex #value`

1 1 4

2 1 2

2 2 4

However, I want the printed indices in the output file to be effectively 0 indexed, in contrast to the default in R which is 1-indexed. i.e. I want to subtract 1 from every row and column index that is printed.

How can I do that while using as much of pre-existing functionality as possible?

Answer

Here is a solution using `reshape2`

:

```
m <- matrix(c(4,2,3,4), ncol = 2);
# Reshape
library(reshape2);
m.long <- melt(m);
# 0-based indices
m.long[, 1:2] <- m.long[, 1:2] - 1;
# Optionally sort
m.long <- m.long[order(m.long[, 1], m.long[, 2]) ,]
print(m.long);
Var1 Var2 value
1 0 0 4
3 0 1 3
2 1 0 2
4 1 1 4
```