How to calculate the percent of column total in R

Problem

We want to calculate the percent of column total in R. In our example, the percent of column freq: 7/397, 23/397, etc.

    x freq
1 Jan    7
2 Feb   23
3 Mar   86
4 Apr  281

Data:

df <- read.table(text = "x    freq
                        Jan   7
                        Feb   23
                        Mar   86
                        Apr   281", 
                        header = TRUE)

Solution

We create the percent of column total using the function prop.table.

df$prob <- prop.table(df$freq)
# Percentages with two decimal places
df$prob <- round(prop.table(df$freq), 4)*100

    x freq  prob
1 Jan    7  1.76
2 Feb   23  5.79
3 Mar   86 21.66
4 Apr  281 70.78

If we'd like to calculate the percent of a specific row, February in our example:

prop.table(df$freq)[df$x == "Feb"] 

 [1] 0.05793451

Alternatives

• Base package

df$prob <- df$freq/sum(df$freq)

• dplyr

library(dplyr)
df %>% mutate(prob = prop.table(freq))
# Or
df %>% mutate(prob = freq / sum(freq))

A specific row:

df %>% filter(x == "Feb")

    x freq       prob
1 Feb   23 0.05793451

References

• Proportion tables in R
• Contingency tables in R
• Transforming contingency tables into frequency tables in R