如何确定等于 r 中某个值的列的长度？

Question

I'm trying to find how many data points are present for each of my categorical variables in the column genotype. So far the following code returns the same values when the first line should return a value roughly 1/3 of the lower line of code.

length(CYP$Genotype == "CYP1B1 KO")
length(CYP$Genotype)

Answer 1

As mentioned in the comments, you want to use sum instead of length to get the frequency of a variable. If you use length on the condition, then it will return the number of items in the vector, which is 8 in this case.

CYP$Genotype == "CYP1B1 KO"
#[1]  TRUE  TRUE  TRUE  TRUE FALSE FALSE FALSE FALSE

length(CYP$Genotype == "CYP1B1 KO")
# [1] 8

Instead, if you use sum , then it will count the number of TRUE statements (which are counted as 1s, whereas, FALSE is a 0).

sum(CYP$Genotype == "CYP1B1 KO")
# [1] 4

As mentioned by @dcarlson, you can use table to get the frequency of the different values in the column, which you could put back into a dataframe.

data.frame(n = cbind(table(CYP$Genotype)))
#          n
#CYP1B1 KO 4
#GRB3C2 F2 1
#RGB2B1 G1 3

Or you can use count from dplyr :

library(dplyr)

CYP %>% 
  count(Genotype)

#   Genotype n
#1 CYP1B1 KO 4
#2 GRB3C2 F2 1
#3 RGB2B1 G1 3

Data

CYP <- structure(list(Genotype = c("CYP1B1 KO", "CYP1B1 KO", "CYP1B1 KO", 
"CYP1B1 KO", "RGB2B1 G1", "RGB2B1 G1", "RGB2B1 G1", "GRB3C2 F2"
)), class = "data.frame", row.names = c(NA, -8L))