9.6 Data frames

A data frame is a 2-dimensional structure.
It is more general than a matrix.

All columns in a data frame: + can be of different types (numeric, character or logical) + must have the same length

9.6.1 Create a data frame

• With the data.frame function:

# stringsAsFactors: ensures that characters are treated as characters and not as factors
d <- data.frame(c("Maria", "Juan", "Alba"), 
    c(23, 25, 31),
    c(TRUE, TRUE, FALSE),
    stringsAsFactors = FALSE)

• Example why “stringsAsFactors = FALSE” is useful

# Create a data frame with default parameters
df <- data.frame(label=rep("test",5), column2=1:5)
# Replace one value
df[2,1] <- "yes"

## Warning in `[<-.factor`(`*tmp*`, iseq, value = "yes"): invalid factor
## level, NA generated

# Throws an error and doesn't replace the value !

# Create a data frame with 
df2 <- data.frame(label=rep("test",5), column2=1:5, stringsAsFactors = FALSE)
# Replace one value
df2[2,1] <- "yes"
# Works!

• Converting a matrix into a data frame:

# create a matrix
b <- matrix(c(1, 0, 34, 44, 12, 4), 
        nrow=3,
        ncol=2)
# convert as data frame
b_df <- as.data.frame(b)

9.6.2 Data frame manipulation:

Very similar to matrix manipulation.