combination from different vectors

问题:

I have some vectors like this

V1 = c(1,2,3,4,5)
V2 = c(7,8,9,10,11)
V3 = c(15,16,17,18,19)
V4 = c(3,4,5,6,7)

I would like to get all possible combinations from these vectors that look like this [V1[i],V2[j],V3[k],v4[z]] and that sums up to 3+9+17+5. This is a bit different from the duplicate R: sample() command subject to a constraint because I would like to get all combinations with one element from each vector and not get all combination from one vector. A possible solution should have one element from each vector.

For example the following are solutions

[1,11,17,5] 
[2,8,18,6]
[5,9,15,5] 
[3,11,16,3]

and many others are. The solutions can have duplicated values which is fine.

Any ideas on how to implement this in R? I am a beginner in R and I hope really to get some answers.

回答1:

If I understand your questions correctly, you are probably looking for expand.grid:

> expand.grid( V1=V1, V2=V2, V3=V3, V4=V4 )[1:5,]
  V1 V2 V3 V4
1  1  7 15  3
2  2  7 15  3
3  3  7 15  3
4  4  7 15  3
5  5  7 15  3
...

This will yield all a data.frame with one row per combination