Replacing values from a column using a condition in R

# reassign depth values under 10 to zero
df$depth[df$depth<10] <- 0

(For the columns that are factors, you can only assign values that are factor levels. If you wanted to assign a value that wasn't currently a factor level, you would need to create the additional level first:

levels(df$species) <- c(levels(df$species), "unknown") 
df$species[df$depth<10]  <- "unknown" 

I arrived here from a google search, since my other code is 'tidy' so leaving the 'tidy' way for anyone who else who may find it useful

library(dplyr)
iris %>% 
  mutate(Species = ifelse(as.character(Species) == "virginica", "newValue", as.character(Species)))