R - How can I generate difference of all combinations of columns in a data frame

Question

Example:

df <- data.frame(A=1:5, B=seq(0,10,2), C=seq(0,15,3))  
df  
A  B  C  
1  2  3  
2  4  6  
3  6  9  
4  8 12  
5 10 15  

What I want is:

A B C (A-B) (A-C) (B-C)  
1 2 3 -1 -2 -1  
2 4 6 -2 -4 -2  
3 6 9 -3 -6 -3  
4 8 12 -4 -8 -4  
5 10 15 -5 -10 -5  

This is a sample. In my problem I have over 100 columns
Any suggestions on how to do this in R?

Answer 1

We can use the FUN argument in combn

combn(seq_along(df), 2, FUN = function(x) df[,x[1]]- df[,x[2]])
#     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
#[1,]   -1   -2    0   -2   -1    1   -1    2    0    -2
#[2,]   -2   -4   -4   -7   -2   -2   -5    0   -3    -3
#[3,]   -3   -6   -8  -12   -3   -5   -9   -2   -6    -4
#[4,]   -4   -8  -12  -17   -4   -8  -13   -4   -9    -5
#[5,]   -5  -10  -16  -22   -5  -11  -17   -6  -12    -6

Also, combn takes the data.frame as argument, so simply

combn(df, 2, FUN = function(x) x[,1]-x[,2])
#     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
#[1,]   -1   -2    0   -2   -1    1   -1    2    0    -2
#[2,]   -2   -4   -4   -7   -2   -2   -5    0   -3    -3
#[3,]   -3   -6   -8  -12   -3   -5   -9   -2   -6    -4
#[4,]   -4   -8  -12  -17   -4   -8  -13   -4   -9    -5
#[5,]   -5  -10  -16  -22   -5  -11  -17   -6  -12    -6

data

df <- data.frame(A=1:5, B=seq(2,10,2), C=seq(3,15,3), d=seq(1,25,5), e=seq(3,31,6))

Answer 2

Here you go

df <- data.frame(A=1:5, B=seq(2,10,2), C=seq(3,15,3), d=seq(1,25,5), e=seq(3,31,6))

> df
  A  B  C  d  e
1 1  2  3  1  3
2 2  4  6  6  9
3 3  6  9 11 15
4 4  8 12 16 21
5 5 10 15 21 27

z = combn(1:ncol(df),2)

> z
     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
[1,]    1    1    1    1    2    2    2    3    3     4
[2,]    2    3    4    5    3    4    5    4    5     5

y = apply(z,2,function(x){
  df[,x[1]]-df[,x[2]]
})

Result:

> y
     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
[1,]   -1   -2    0   -2   -1    1   -1    2    0    -2
[2,]   -2   -4   -4   -7   -2   -2   -5    0   -3    -3
[3,]   -3   -6   -8  -12   -3   -5   -9   -2   -6    -4
[4,]   -4   -8  -12  -17   -4   -8  -13   -4   -9    -5
[5,]   -5  -10  -16  -22   -5  -11  -17   -6  -12    -6

The matrix z tells you which pair of columns were substracted

You do realize that if df has 100 columns, all the combinations add up to 4950.