How do you make a dummy dataset in R?

How would I make a dataset where each "site", "season", "year", and "species name" is completely crossed (every site was visited during each year/season, and each species could have been caught anytime and place -> i.e. 5 sites x 2 seasons x 2 years x 2 species = df size is 40×5).

df <- data.frame(site = rep(c("1", "2", "3", "4", "5"), each = 2),
                 season = rep(c("dry", "wet"), each = 10), 
                 year = rep(c(2019, 2020), each = 10), 
                 species_name = rep(c("Sailfin molly", "Hardhead silverside"), each = 10),
                 num = sample(x = 0:15, size  = 20, replace = TRUE))

>Solution :

You could use the expand.grid() function:

library(tidyverse)

site <- c("1", "2", "3", "4", "5")
season <- c("dry", "wet")
year <-  c(2019, 2020)
species_name <-  c("Sailfin molly", "Hardhead silverside")
num <-  sample(x = 0:15, size  = 40, replace = TRUE)

df <- data.frame(expand.grid(site, season, year, species_name) %>% mutate(num = num))
colnames(df) <- c("site", "season", "year", "species_name", "num")

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