Find the difference between all values of two vectors

5
votes

I am trying to find the difference between many pairs of numbers.

These numbers are in two vectors of unequal length. For example,

    a<- c(1:5)
    b<- c(1:10)

Now, I need some way of calculating the a[[1]] - b and then a[[2]] - b and so on until a[[5]] - b. Each of these calculations should result in a vector, 10 numbers long.

Each of these vectors of differences should be stored together as columns in a dataframe. The first column should be the position of ´b´ subtracted and the subsequent columns should be titled with the position of ´a´ (so there are 5 columns and 10 rows).

         a[1] a[2] ... a[5]
    b[1]
    b[2]
    ...
    b[10]

I am very new to writing functions in R. I am also new to using the *apply group of functions. I have been trying to combine what I have learned about writing functions and the *apply group of functions to solve this but it just isn´t happening yet. Thank you for your help!

P.S. Sorry if this has been asked before. I searched but could not find the answer.

2
rvector
very close: stackoverflow.com/q/30562308/4137985 – Cath

2 Answers

6
votes

Its a job for outer:

t(outer(a, b, '-'))

     # [,1] [,2] [,3] [,4] [,5]
 # [1,]    0    1    2    3    4
 # [2,]   -1    0    1    2    3
 # [3,]   -2   -1    0    1    2
 # [4,]   -3   -2   -1    0    1
 # [5,]   -4   -3   -2   -1    0
 # [6,]   -5   -4   -3   -2   -1
 # [7,]   -6   -5   -4   -3   -2
 # [8,]   -7   -6   -5   -4   -3
 # [9,]   -8   -7   -6   -5   -4
# [10,]   -9   -8   -7   -6   -5
4
votes
sapply(a, "-", b)
#      [,1] [,2] [,3] [,4] [,5]
# [1,]    0    1    2    3    4
# [2,]   -1    0    1    2    3
# [3,]   -2   -1    0    1    2
# [4,]   -3   -2   -1    0    1
# [5,]   -4   -3   -2   -1    0
# [6,]   -5   -4   -3   -2   -1
# [7,]   -6   -5   -4   -3   -2
# [8,]   -7   -6   -5   -4   -3
# [9,]   -8   -7   -6   -5   -4
#[10,]   -9   -8   -7   -6   -5

Explanation

Taking advantage of the fact that a scalar minus a vector in R is an element-wise subtraction between said scalar and each element of the vector, we can simply apply the minus - operator to each value in a against the whole vector b.