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Counting the number of elements with the values of x in a vector


Question

I have a vector of numbers:

numbers <- c(4,23,4,23,5,43,54,56,657,67,67,435,
         453,435,324,34,456,56,567,65,34,435)

How can I have R count the number of times a value x appears in the vector?

2016/10/04
1
412
10/4/2016 11:29:19 AM

Accepted Answer

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
...
2009/12/17
516
12/17/2009 5:32:19 PM


I would probably do something like this

length(which(numbers==x))

But really, a better way is

table(numbers)
2009/12/17

There is also count(numbers) from plyr package. Much more convenient than table in my opinion.

2016/08/24

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.

2012/12/13

There is a standard function in R for that

tabulate(numbers)

2012/04/19

numbers <- c(4,23,4,23,5,43,54,56,657,67,67,435 453,435,324,34,456,56,567,65,34,435)

> length(grep(435, numbers))
[1] 3


> length(which(435 == numbers))
[1] 3


> require(plyr)
> df = count(numbers)
> df[df$x == 435, ] 
     x freq
11 435    3


> sum(435 == numbers)
[1] 3


> sum(grepl(435, numbers))
[1] 3


> sum(435 == numbers)
[1] 3


> tabulate(numbers)[435]
[1] 3


> table(numbers)['435']
435 
  3 


> length(subset(numbers, numbers=='435')) 
[1] 3
2017/06/07

Source: https://stackoverflow.com/questions/1923273
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