Counting the number of elements with the values of x in a vector
Solution 1:
You can just use table()
:
> a <- table(numbers)
> a
numbers
4 5 23 34 43 54 56 65 67 324 435 453 456 567 657
2 1 2 2 1 1 2 1 2 1 3 1 1 1 1
Then you can subset it:
> a[names(a)==435]
435
3
Or convert it into a data.frame if you're more comfortable working with that:
> as.data.frame(table(numbers))
numbers Freq
1 4 2
2 5 1
3 23 2
4 34 2
...
Solution 2:
The most direct way is sum(numbers == x)
.
numbers == x
creates a logical vector which is TRUE at every location that x occurs, and when sum
ing, the logical vector is coerced to numeric which converts TRUE to 1 and FALSE to 0.
However, note that for floating point numbers it's better to use something like: sum(abs(numbers - x) < 1e-6)
.
Solution 3:
I would probably do something like this
length(which(numbers==x))
But really, a better way is
table(numbers)
Solution 4:
There is also count(numbers)
from plyr
package. Much more convenient than table
in my opinion.
Solution 5:
My preferred solution uses rle
, which will return a value (the label, x
in your example) and a length, which represents how many times that value appeared in sequence.
By combining rle
with sort
, you have an extremely fast way to count the number of times any value appeared. This can be helpful with more complex problems.
Example:
> numbers <- c(4,23,4,23,5,43,54,56,657,67,67,435,453,435,324,34,456,56,567,65,34,435)
> a <- rle(sort(numbers))
> a
Run Length Encoding
lengths: int [1:15] 2 1 2 2 1 1 2 1 2 1 ...
values : num [1:15] 4 5 23 34 43 54 56 65 67 324 ...
If the value you want doesn't show up, or you need to store that value for later, make a
a data.frame
.
> b <- data.frame(number=a$values, n=a$lengths)
> b
values n
1 4 2
2 5 1
3 23 2
4 34 2
5 43 1
6 54 1
7 56 2
8 65 1
9 67 2
10 324 1
11 435 3
12 453 1
13 456 1
14 567 1
15 657 1
I find it is rare that I want to know the frequency of one value and not all of the values, and rle seems to be the quickest way to get count and store them all.