Vectors can use simple arithmetic expressions (+, -, *, /) to perform basic operations. Let's first look at addition, then discuss a caveat of vector arithmetics.
You can add or subtract the corresponding elements of two or more vectors of the same length together.
> c(1,2,3) + c(99,98,97)  100 100 100 > c(1,2,3) + c(4,5,6)  5 7 9 > c(1,2,3) - c(1,1,1)  0 1 2
But what would happen if all the vectors weren't of the same length? Instead of erroring out, R performs recycling.
Recycling occurs when vector arithmetic is performed on multiple vectors of different sizes. R takes the shorter vector and repeats them until it becomes long enough to match the longer one.
> c(1,2,3,4,5,6) + c(1,3)  2 4 3 7 6 9
As you can see, the
c(1,3) vector repeated itself to form
c(1,3,1,3,1,3) so that it could successfully match the previous term.
If the shorter vector is not a vector of the longer one, then a warning message appears, but the operation still takes place.
> c(1,2,3,4,5) + c(1,3)  2 5 4 7 6 Warning message: In c(1, 2, 3, 4, 5) + c(1, 3) : longer object length is not a multiple of shorter object length
Multiplying or dividing vectors is similar to addition and subtraction in that each corresponding element matches up and a product is formed. When the sizes differ, recycling occurs.
> c(1,2,3) * c(0,3,6)  0 6 18 > c(1,3,5) * c(2,4)  2 12 10Warning message: In c(1, 3, 5) * c(2, 4) : longer object length is not a multiple of shorter object length
One small detail to notice is that these common arithmetic expressions are actually functions. Thus, they can be with a similar function notation.
> "*"(5,6)  30
We can also perform the modulo operator, which outputs the remainder after division of two numbers.
> c(55,54,53) %% c(3)  1 0 2
You can also apply linear algebra on your vectors in R. To calculate the cross product, use
> crossprod(1:3, 4:6) [,1] [1,] 32
You'll notice that the return type isn't a new vector, but instead a one-dimensional matrix. We'll look at matrices in the next lesson.
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