Deleet - 1 year ago 61
R Question

# Generate matrix of combinations with rules, repeated binary choice

I am trying to do sampling of variables for a statistical analysis. I have 10 variables, and I want to examine every possible combination of 5 of them. However, I only want those that follow certain rules. I only want those with 1 xor 2, 3 xor 4, 5 xor 6, 7 xor 8 and 9 xor 10. In other words, all combinations given 5 binary choices (32).

Any idea how to do this efficiently?

A simple idea is to find all the 5 out 10 using:

``````library(gtools)
sets = combinations(10,5) # choose 5 out of 10, all possibilities
sets = split(sets, seq.int(nrow(sets))) #so it's loopable
``````

And then loop over these keeping only the ones that meet the criteria and thus ending up with the 32 ones desired.

But surely there is a more efficient way than this.

This will construct a matrix whose 32 rows enumerate all the possible combinations satisfying your contraint:

``````m <- as.matrix(expand.grid(1:2, 3:4, 5:6, 7:8, 9:10))

## Inspect a few of the rows to see that this works:
m[c(1,4,9,16,25),]
#      Var1 Var2 Var3 Var4 Var5
# [1,]    1    3    5    7    9
# [2,]    2    4    5    7    9
# [3,]    1    3    5    8    9
# [4,]    2    4    6    8    9
# [5,]    1    3    5    8   10
``````
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