Count occurrences of value in a set of variables i

Question

Let's say I have a data frame with 10 numeric variables V1-V10 (columns) and multiple rows (cases).

What I would like R to do is: For each case, give me the number of occurrences of a certain value in a set of variables.

For example the number of occurrences of the numeric value 99 in that single row for V2, V3, V6, which obviously has a minimum of 0 (none of the three have the value 99) and a maximum of 3 (all of the three have the value 99).

I am really looking for an equivalent to the SPSS function COUNT: "COUNT creates a numeric variable that, for each case, counts the occurrences of the same value (or list of values) across a list of variables."

I thought about table() and library plyr's count(), but I cannot really figure it out. Vectorized computation preferred. Thanks a lot!

Answer 1

Here is another straightforward solution that comes closest to what the COUNT command in SPSS does — creating a new variable that, for each case (i.e., row) counts the occurrences of a given value or list of values across a list of variables.

#Let df be a data frame with four variables (V1-V4)
df <- data.frame(V1=c(1,1,2,1,NA),V2=c(1,NA,2,2,NA),
       V3=c(1,2,2,1,NA), V4=c(NA, NA, 1,2, NA))

 #This is how to compute a new variable counting occurences of value "1" in V1-V4.      
    df$count.1 <- apply(df, 1, function(x) length(which(x==1)))

The updated data frame contains the new variable count.1 exactly as the SPSS COUNT command would do.

 > df
      V1 V2 V3 V4 count.1
    1  1  1  1 NA       3
    2  1 NA  2 NA       1
    3  2  2  2  1       1
    4  1  2  1  2       2
    5 NA NA NA NA       0

You can do the same to count how many time the value "2" occurs per row in V1-V4. Note that you need to select the columns (variables) in df to which the function is applied.

df$count.2 <- apply(df[1:4], 1, function(x) length(which(x==2)))

You can also apply a similar logic to count the number of missing values in V1-V4.

df$count.na <- apply(df[1:4], 1, function(x) sum(is.na(x)))

The final result should be exactly what you wanted:

 > df
      V1 V2 V3 V4 count.1 count.2 count.na
    1  1  1  1 NA       3       0        1
    2  1 NA  2 NA       1       1        2
    3  2  2  2  1       1       3        0
    4  1  2  1  2       2       2        0
    5 NA NA NA NA       0       0        4

This solution can easily be generalized to a range of values. Suppose we want to count how many times a value of 1 or 2 occurs in V1-V4 per row:

df$count.1or2 <- apply(df[1:4], 1, function(x) sum(x %in% c(1,2)))

Answer 2

Try

apply(df,MARGIN=1,table)

Where df is your data.frame. This will return a list of the same length of the amount of rows in your data.frame. Each item of the list corresponds to a row of the data.frame (in the same order), and it is a table where the content is the number of occurrences and the names are the corresponding values.

For instance:

df=data.frame(V1=c(10,20,10,20),V2=c(20,30,20,30),V3=c(20,10,20,10))
#create a data.frame containing some data
df #show the data.frame
  V1 V2 V3
1 10 20 20
2 20 30 10
3 10 20 20
4 20 30 10
apply(df,MARGIN=1,table) #apply the function table on each row (MARGIN=1)
[[1]]

10 20 
 1  2 

[[2]]

10 20 30 
 1  1  1 

[[3]]

10 20 
 1  2 

[[4]]

10 20 30 
 1  1  1 

#desired result

Answer 3

I think that there ought to be a simpler way to do this, but the best way that I can think of to get a table of counts is to loop (implicitly using sapply) over the unique values in the dataframe.

#Some example data
df <- data.frame(a=c(1,1,2,2,3,9),b=c(1,2,3,2,3,1))
df
#  a b
#1 1 1
#2 1 2
#3 2 3
#4 2 2
#5 3 3
#6 9 1

levels=unique(do.call(c,df)) #all unique values in df
out <- sapply(levels,function(x)rowSums(df==x)) #count occurrences of x in each row
colnames(out) <- levels
out
#     1 2 3 9
#[1,] 2 0 0 0
#[2,] 1 1 0 0
#[3,] 0 1 1 0
#[4,] 0 2 0 0
#[5,] 0 0 2 0
#[6,] 1 0 0 1

Answer 4

If you need to count any particular word/letter in the row.

#Let df be a data frame with four variables (V1-V4)
             df <- data.frame(V1=c(1,1,2,1,L),V2=c(1,L,2,2,L),
             V3=c(1,2,2,1,L), V4=c(L, L, 1,2, L))

For counting number of L in each row just use

#This is how to compute a new variable counting occurences of "L" in V1-V4.      
df$count.L <- apply(df, 1, function(x) length(which(x=="L")))

The result will appear like this

> df
  V1 V2 V3 V4 count.L
1  1  1  1 L       1
2  1  L  2 L       2
3  2  2  2  1      0
4  1  2  1  2      0