如何计算特定列中满足条件的出现次数（4 个字母）

Question

x = c(1,2,3,4,5)
y = c("AA","BB","CC", "AAAA","BBBB")
data1 = data.frame(x,y)
data1

^^I want the output to be the number of time the 4 letters occur in the y column. Desired output would be 2

I want to count the number of times a 4 letter factor observations occurs in a given column in a dataframe. How do I do this?

Answer 1

If you only want to extract and count factor values that have exactly 4 letters ( any letters, not necessarily the same ), then you can do this:

Step 1--Define a pattern to match:

pattern <- "\\w{4}"

Step 2--Define a function to extract only the raw matches:

extract <- function(x) unlist(regmatches(x, gregexpr(pattern, x, perl = T)))

Step 3--Apply the function to the data of interest:

extract(data1$y)

And that's the result:

[1] "AAAA" "BBBB"

Step 4--To count the number of matches you can use length :

length(extract(data1$y))
[1] 2

EDIT : Alternatively you can use str_extract from the package stringr :

STEP 1: store the result in a vector extr :

extr <- str_extract(data1$y, "\\w{4}")

STEP 2: using length , the negation operator ! and is.na , a function that tests for NA and evaluates to TRUE and FALSE, you can count the number of times that test evaluates to FALSE:

length(extr[!is.na(extr)])
[1] 2

Answer 2

Maybe you can try nchar if you have strings in column y always consisting of letters

sum(nchar(as.vector(data1$y))==4)

# > sum(nchar(as.vector(data1$y))==4)
#   2